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Abstract
A GaSb-based SEmiconductor Saturable Absorber Mirror (SESAM) enables continuous-wave picosecond mode-locked operation with excellent stability of a polarization-maintaining mid-infrared Er:ZBLAN fiber laser. The GaSb-based SESAM mode-locked fiber laser delivers an average output power of 190 mW at 2.76 µm at a repetition rate of 32.07 MHz (corresponding to a pulse energy of ∼6 nJ) and exhibits a high signal-to-noise ratio of ∼80 dB. The polarization extinction ratio is more than 23 dB. By employing an intracavity diffraction grating, the laser wavelength is continuously tunable across 2.706–2.816 µm. Passively Q-switched operation of this laser is also demonstrated.
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1. Introduction
Presently, there is a significant need for efficient saturable absorbers for ultrafast lasers emitting at 3 µm and beyond. This range of wavelengths holds practical importance for applications such as laser surgery, molecular spectroscopy, material processing and pumping of mid-infrared supercontinuum sources [1]. SEmiconductor Saturable Absorber Mirrors (SESAMs) based on InxGa1−xAs quantum wells (QWs) revolutionized the field of ultrafast oscillators in the near-infrared by solving the Q-switching instability problem [2]. Still, their utilization in the short-wave infrared is limited.
GaAs exhibits a bandgap of 1.42 eV at room temperature, rendering it suitable for near-IR applications. Conversely, GaSb has a bandgap of 0.72 eV at room temperature, which is more appropriate for short-wave infrared to mid-infrared applications, and it exhibits a larger lattice constant. A potential material for distributed Bragg reflectors (DBRs) is AlSb, as it provides an enhanced refractive index contrast compared to arsenides. The latter allows for production of highly reflective DBRs with wide stop bands that extend beyond ∼200 nm. The band gap and offset of InGaAsSb / AlGaAsSb QWs can be tailored to encompass emissions ranging from 1.9 µm to over 3 µm. Furthermore, another noteworthy aspect is that at their operation wavelengths, GaSb-based SESAMs with type-I quantum wells (QWs) exhibit sub-picosecond absorption recovery time owing to Auger recombination which dominates in semiconductors with a low bandgap energy like GaSb-based materials, an effect initially observed in connection with SESAM used for semiconductor lasers [3,4] and subsequently validated through pump-probe nonlinear measurements [4]. Note that for SESAMs based on InxGa1−xAs QWs tailored for the near-infrared spectral range, to achieved sufficiently fast absorption recovery times, additional defects have been introduced into and around the absorber layers to shorten the recovery time through recombination over nonradiative defect trapping. Such defect engineering requires a meticulous balance between fast recovery times and minimal nonsaturable losses. In contrast, for GaSb-based SESAMs, there is no need for defect engineering. Pajaeste et al. [3] also showed that the recovery dynamics of such SESAMs are not significantly affected by low-temperature growth, despite the introduction of additional defects. This is clearly a distinct advantage of this material for saturable absorber applications in mode-locked lasers.
GaSb-based SESAMs have been successfully applied in solid-state and semiconductor lasers emitting around 2 µm [5–7] and 2.4 µm [8]. However, there has been relatively less focus on their use for longer wavelengths approaching 3 µm. In general, this spectral range is effectively covered by Erbium (Er), Holmium (Ho), and Dysprosium (Dy) lasers which operate on the 4I11/2 → 4I13/2, 5I6 → 5I7 and 6H13/2 → 6H15/2 transitions, respectively. Fiber laser technology offers inherent advantages, including nearly diffraction-limited beam quality, power stability, good thermal management, a small footprint, and flexible configuration. Fluoride glasses, such as ZrF4–BaF2–LaF3–AlF3–NaF (ZBLAN), feature low phonon energies which play a crucial role in minimizing the non-radiative path from the upper laser levels of Er3+, Ho3+ and Dy3+ ions [9]. Compact, high-power, and efficient 2.8 µm Er:ZBLAN fiber lasers, which are pumped by commercially available InGaAs laser diodes at 0.97 µm, have been demonstrated [10,11].
Table 1 presents an overview of passively mode-locked Er:ZBLAN fiber lasers reported recently [12–20]. The first stable continuous-wave mode-locking (CW ML) of an Er:ZBLAN fiber laser was demonstrated in 2014 by Haboucha et al. employing a commercial SESAM and a fiber Bragg grating in a linear cavity configuration [13]. The laser generated stable, self-starting pulse trains at 2.80 µm with a repetition rate of 51.8 MHz and a pulse duration of 60 ps. Power scaling of this type of lasers and certain shortening of the pulse duration (still remaining in the range of a few tens of ps) was later achieved [14]. It was shown that the structured water vapor absorption in the atmospheric air in the free-space sections of fiber laser cavities is the key limiting factor for achievable pulse durations [21]. Shen et al. reported on a SESAM mode-locked Er:ZBLAN fiber laser delivering pulses as short as 6.4 ps at a repetition rate of 28.9 MHz using an intracavity blazed diffraction grating ensuring relatively high, controlled and spectrally selective reflection improving the mode-locking stability [15].
Table 1. Passively Mode-Locked 2.8-µm Er:ZBLAN Fiber Lasers Reported So Fara
We should note that the commercial SESAM technology used in these demonstrations are based on a lattice-mismatch GaAs-heterostructure structure employing Au-based mirrors, rather than monolithic DBRs. This brings limitation in the ability to engineer the thickness of the absorbing region, as well as additional non-saturable losses linked to the use of metal reflectors and lattice-mismatched structures, ultimately impacting the lifetime and ability to design for optimal nonlinear properties.
Picosecond mode-locking of Er:ZBLAN fiber lasers was also demonstrated using saturable absorbers based on 2D materials such as graphene, black phosphorus, or tungsten diselenide [16]. However, such saturable absorbers cannot outperform SESAMs in terms of robustness, thermal management, long-term stability, and ability to accurately control the linear and nonlinear optical properties.
Moreover, sub-picosecond pulse generation in the vicinity of 3 µm wavelength is routinely obtained using nonlinear polarization evolution (NPE) as firstly demonstrated by Duval et al. with a fiber ring Er:ZBLAN laser delivering 207 fs pulses at 2.81 µm at 55.2 MHz [17]. The ultrashort pulse generation was attributed to a combination of an ultrafast response of NPE and the soliton shaping effect. By combining NPE and semiconductor saturable absorber, Yu et al. reported on a hybrid mode-locked femtosecond Er:ZBLAN fiber oscillator at 2.8 µm [22]. The NPE technique is widely used in ultrafast fiber oscillators due to its high damage threshold, ultrafast recovery time, and flexibility in cavity design. However, its most successful implementation is in non-polarization-maintaining (PM) fibers and sub-picosecond operation, making such lasers sensitive to environmental changes.
There also exist reports on passive mode-locking of Ho3+/Pr3+ [23] and Dy3+ [24] fluoride fiber lasers emitting around 3 µm. The recent review summarized the relevant results on mode-locked short-wave infrared and mid-infrared lasers [25].
Recently, Qin et al. reported on the fabrication of engineered InAs/GaSb type-II superlattice SESAMs, their nonlinear optical properties and application in a mode-locked Er:ZBLAN fiber laser emitting at 3.5 µm (the 4F9/2 → 4I9/2 transition) [26], yet one may expect that such structure would exhibit a slow absorption recovery time, as it has been reported for the 2.4 µm SESAMs on type-II QWs.
In the present work, we report on the first polarization-maintaining Er:ZBLAN fiber laser tunable around 2.8 µm passively mode-locked with a monolithic GaSb-based SESAM. The novelty of this study consists of the use of a novel SESAM structure to achieve self-starting continuous-wave mode-locking with excellent stability of a polarization-maintaining mid-infrared fluoride fiber laser. This proof-of-principle demonstration paves the way towards applications of GaSb-based SESAMs in picosecond and (potentially) femtosecond fiber lasers emitting around 3 µm (based on Er3+, Ho3+/Pr3+ and Dy3+ ions) and at even longer wavelengths. This work continues our initial study of a similar laser based on a conventional GaAs-based SESAM [27]. Shortly after this initial result, Luo et al. [20] reported on further development of a similar picosecond mid-infrared Er fiber laser employing a similar double-clad polarization-maintaining Er:ZBLAN active fiber and a commercial GaAs-based SESAM mainly leading to output power scalability.
2. Laser set-up
As a saturable absorber to initiate and sustain the CW ML operation regime of the Er:ZBLAN fiber laser, we employed a specially designed antireflection (AR) coated GaSb-based SESAM (RefleKron). A set of SESAMs was fabricated and tested featuring different linear reflectivity around 2.8 µm in the range of 62% to 87% and different types of AR coatings.
The SESAM which enabled the best mode-locking performance featured a relatively broad stopband of 2.65–2.97 µm as seen on the calculated and measured linear reflectivity curves in Fig. 1. They are in good agreement with each other indicating a well-controlled SESAM design. The AR coating of this SESAM was designed to let an intermediate E-field inside the SESAM so that it was less prone to damaging. The SESAM is expected to exhibit a fast recovery time of <1 ps due to Auger recombination. An estimate for non-saturable losses (about 1/5 of the total linear absorption) was provided by the manufacturer. The modulation depth of the GaSb-SESAM used in following is estimated (from the linear reflectivity measurement) to be about 14% at 2.76 µm.
It is worth comparing the characteristics of commercially available GaAs-based SESAMs (Batop, Gmbh) and custom-made GaSb-based ones. In the first case, the parameters provided by the supplier are as following: a high reflection band of 2.73–2.87 µm, a linear absorbance of 14% to 34%, a fraction of non-saturable losses of about 2/5, a saturation fluence of 40 µJ/cm2 and a recovery time of 10 ps [28]. In fact, such a SESAM was tested in our previous study [27]. Thus, the studied GaSb-based SESAMs are very attractive because they allow a much faster recovery time as well as a low saturation fluence. According to the previous study of GaSb-based SESAMs at the wavelengths around 2.4 µm, their relaxation time is shorter than 1 ps [29], and faster recovery times are expected around 2.8 µm, representing a clear advantage over GaAs-based structures. Indeed, the development of SESAMs combining fast recovery time, low saturation fluence and high modulation depth is highly desired to enable the generation of sub-picosecond pulses at this wavelength which is subject to strong water vapor absorption.
The layout of the SESAM mode-locked polarization-maintaining Er:ZBLAN fiber setup is shown in Fig. 2. The gain medium was a 2.4 m long double-clad heavily doped 7 mol% Er:ZBLAN fluoride fiber (Le Verre Fluoré) with a core diameter of 15 µm. The inner cladding had a truncated circular (double D-shaped) profile (250 µm diameter) to enhance the pump absorption. The polarization maintaining fiber property was ensured by two air gaps located near the fiber core enhancing its birefringence (measured beat-length: 38 mm at 2.88 µm). Both fiber end-facets were cleaved at an angle of 8° to avoid parasitic lasing from the Fresnel reflections. The pump source was a fiber-coupled (fiber core diameter: 105 µm, N.A. = 0.15) multimode InGaAs laser diode (Lumics) with a central wavelength of 975 nm (emission bandwidth: 3 nm). Its output was collimated using an objective and focused into the fiber through a dichroic mirror (DM, coated for high transmission (HT) at 0.98 µm and high reflection (HR) at 2.6–3 µm) using an uncoated spherical ZnSe lens (L2, f = 12 mm) resulting in cladding pumping. Another ZnSe lens (L3, f = 12 mm) was used to collimate the laser emission after the output fiber facet. The pump wavelength addressed the 4I15/2 → 4I11/2 Er3+ transition in absorption. One cavity arm contained a dichroic mirror installed for 22.5° incidence, an antireflection (AR) coated half-wave plate and a Glan-Taylor polarizer for matching the polarization state of the laser emission with one of the eigenstates of the birefringent fiber, as well as a Black Diamond short focal length lens (f = 6 mm) creating a beam waist on the SESAM ensuring its efficient bleaching. Another cavity arm was terminated by a plane-wedged output coupler (OC) providing a transmission TOC of 50% at 2.6–3 µm and HT at the pump wavelength and contained a gold-coated HR folding mirror and a ruled reflective diffraction grating (Thorlabs, 450 lines/mm) oriented for the 1st order of diffraction for wavelength tuning. Another AR coated half-wave plate was inserted to control the polarization state of laser emission. The residual (non-absorbed) pump after the OC was filtered out using another DM.
Fig. 2. Scheme of the laser setup. LD – laser diode; M – gold folding mirror; DM – dichroic mirrors; L1 – Black Diamond lens (f = 6 mm); L2 and L3 – ZnSe lenses (f = 12 mm); OC – output coupler.
The laser spectra were characterized using an optical spectrum analyzer (Yokogawa, AQ6376E) with a resolution of 0.1 nm. The temporal dynamics of the laser was studied using a liquid N2 cooled HgCdTe photodetector with a fast amplifier (1 GHz) (Kolmar) and a 500 MHz bandwidth digital oscilloscope (Tektronix). The radio frequency (RF) spectra were measured using an RF analyzer (Rohde & Schwarz FSV 7 GHz).
3. Laser performance
3.1 Mode-locked fiber laser
First, the continuous-wave (CW) laser performance was studied by replacing the SESAM with a HR mirror. The CW Er:ZBLAN fiber laser operated at 2.76 µm with a slope efficiency of 14.3% (vs. the incident pump power) and a laser threshold of 0.20 W. In the free-running regime (without the diffraction grating), by implementing the SESAM whose linear reflectivity spectrum is depicted in Fig. 1 (measured linear reflectivity at 2.76 µm: ∼62%) and adjusting the position of the beam waist on it, stable CW mode-locking was readily achieved by increasing the incident pump power >0.9 W (at lower pump powers, the laser operated in the Q-switched mode-locked regime). The average output power of the ML Er:ZBLAN fiber laser reached 190 mW at 2761 nm for an incident pump power of 3.35 W, so that the optical efficiency was 5.7%, Fig. 3(a). The output power was varying linearly with the pump power. The CW ML laser operation was stable for hours and showed weak sensitivity to external vibrations and temperature variations which is due to the polarization maintaining design of the Er:ZBLAN fiber. No optical damage of the SESAM was observed during the ML experiments. With other tested SESAMs, the mode-locked operation regime was not stable either due to an insufficient modulation depth (measured linear reflectivity at 2.8 µm: ∼85%) or another type of the applied AR coating maximizing the E-field thus making such structures more prone to damaging.
Fig. 3. SESAM mode-locked polarization-maintaining Er:ZBLAN fiber laser (without the diffraction grating): (a) input-output dependence, η – slope efficiency; (b) a typical laser spectrum in the CW ML regime.
The typical spectrum of the ML Er:ZBLAN fiber laser without using the diffraction grating is shown in Fig. 3(b). It exhibited weak intensity modulation attributed to the structured water vapor absorption in the air. The spectral bandwidth (full width at half maximum, FWHM) was measured to be 0.4 nm being weakly dependent on the pump power. The pulse duration was estimated to be around 30 ps, calculated by Fourier-transform of the spectrum.
By using the diffraction grating, the wavelength tunability of the Er:ZBLAN fiber laser in the CW ML regime was investigated. The incident pump power was fixed to 1.0 W and the laser wavelength was continuously tuned across 108 nm (2708 – 2816 nm) at the zero-power level, with a maximum at 2736 nm, see Fig. 4(a). The stability of the CW ML operation regime was well preserved over this range. The corresponding normalized laser spectra are shown in Fig. 4(b). The spectra exhibited similar modulation and the spectral FWHM was in the range of 0.4–0.5 nm and it slightly increased for longer laser wavelengths. The latter behavior is attributed to a weaker effect of the structured water vapor absorption above 2.79 µm.
Fig. 4. Wavelength tunability of the Er:ZBLAN fiber laser in the CW ML regime (Pinc = 1.0 W): (a) circles - wavelength tuning curve; grey lines – structured water absorption in the air (HITRAN database data); (b) the corresponding laser spectra.
The typical oscilloscope traces for the SESAM ML Er:ZBLAN fiber laser measured for different time spans are shown in Fig. 5(a). The time interval between two pulses, 31.2 ns, well matched the total optical cavity length. The stability of the ML laser was confirmed by measuring the RF spectra. The fundamental beat note at 32.07 MHz exhibited a high extinction ratio of 80 dB, Fig. 5(b). A wide-range span revealed uniform harmonics over the 1 GHz-range, Fig. 5(c), then confirming CW mode-locking without any Q-switching instabilities. As compared to the previous work on a similar Er:ZBLAN fiber laser employing a commercial GaAs-based SESAM, we report on a notable improvement of the mode-locking stability (note a noticeable decline in the harmonic intensity over a 1 GHz-wide span in [20]) assigned to the monolithic design of the employed GaSb-based SESAM.
Fig. 5. SESAM mode-locked polarization-maintaining Er:ZBLAN fiber laser at 2.76 µm: (a) oscilloscope traces of typical pulse trains; (b,c) radio frequency (RF) spectra: (b) fundamental beat node; (c) harmonics on a 1 GHz-wide span, SBW, resolution bandwidth. Pinc = 1.45 W.
The emission from the ML Er:ZBLAN laser was linearly polarized over the whole studied tuning range and the measured polarization extinction ratio (PER) was better than 23 dB.
3.2 Passively Q-switched fiber laser
The operation of the laser in the passively Q-switched regime (PQS) was also investigated. For this, the SESAM was moved slightly out of the beam waist. The PQS Er:ZBLAN fiber laser generated a maximum average output power of 172 mW at 2761 nm with a slope efficiency of 6.3%, as shown in Fig. 6(a). Stable PQS operation started slightly above the laser threshold, at Pinc > 0.7 W. The single Q-switched pulses had a nearly Gaussian temporal profile. When increasing Pinc from 0.7 to 3.2 W, the pulse duration (FWHM) slightly decreased from 350 to 280 ns, the pulse energy increased in the range 1.8 to 2.6 µJ and the pulse repetition rate (PRR) increased from 10 to 65 kHz. Wavelength tuning of the PQS Er:ZBLAN laser was also demonstrated resulting in a tuning range of 119 nm (2704 – 2823 nm). The laser exhibited high PER in all cases.
Fig. 6. Passively Q-switched polarization-maintaining Er:ZBLAN fiber laser: (a) input-output dependence, inset – oscilloscope trace of a single Q-switched pulse; (b) pulse duration, pulse energy, and pulse repetition rate (PRR) as a function of the incident pump power.
4. Conclusions
To conclude, GaSb-based SESAMs represent a viable option for the development of ultrafast fiber oscillators emitting around 3 µm (potentially including fiber lasers based on Er3+, Ho3+ and Dy3+ ions) due to their wide stopbands enabling broadband wavelength tuning preserving excellent mode-locking stability and potentially supporting fs pulse generation, intrinsically short recovery times and easy engineering of the saturable absorption level at 3 µm. Benefiting from the monolithic fabrication approach to integrate the DBR and QWs in a single-epitaxy run, the approach is highly scalable to volume applications using established epitaxial technology in semiconductor industry. In the present work, we report for the first time on the application of a GaSb-based SESAM in a picosecond passively mode-locked mid-infrared fiber laser based on a double-clad polarization maintaining heavily doped Er:ZBLAN fluoride fiber. Pumped by a fiber-coupled 976-nm InGaAs laser diode, this laser delivered an average output power of 190 mW at 2761 nm for an incident pump power of 3.35 W. The mode-locked laser exhibited a high signal-to-noise ratio of 80 dB at the fundamental frequency of 32.07 MHz. The laser had a full tuning range of 110 nm in the CW ML regime, ranging from 2706 nm to 2816 nm. The feasibility of the passively Q-switched laser operation with the same SESAM was also demonstrated resulting in generation of 280 ns / 2.6 µJ pulses at a repetition rate of 65 kHz. We believe that the application of GaSb-based SESAMs with increased modulation depths (a few tens of percents) could lead to generation of a sub-ps pulses from 2.8 µm Er:ZBLAN fiber lasers. Another promising development is to consider the mode-locking mechanism involving both SESAM for reliable self-starting and NPE for reducing the pulse duration.
Funding
Agence Nationale de la Recherche (NEOPMI (10-LABX-00099), ANR-10-LABX-09-01, ANR-19-CE08-0028, labex EMC3, SPLENDID2); Horizon 2020 Framework Programme (101034329); European Regional Development Fund (Chaire RELANCE, RIN NovaMAT).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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