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Abstract
Semiconductor saturable absorber mirrors (SESAMs) are key devices for passive mode locking of numerous laser types and have been implemented for a variety of operational wavelengths ranging from 800 nm to 2400 nm. However, for 1560 nm the fabrication of SESAMs based on the standard AlAs/GaAs material system requires highly strained InGaAs absorber layers, which reduce the device efficiency and compromise fragile long-term performance. Here, we present SESAMs for ultrashort pulse generation at 1560 nm that are grown entirely lattice-matched to InP and thus have the potential for less structural defects and a higher operational lifetime. A highly reflective InGaAlAs-InAlAs Bragg mirror is capped with a heavily iron doped InGaAs:Fe absorber layer, which facilitates an unprecedented combination of sub-picosecond carrier lifetime and high optical quality. Therefore, the presented SESAMs show ultrafast response (τA < 1 ps), low non-saturable losses and high effective modulation depth (ΔReff ≥ 5.8%). Moreover, a nearly anti-resonant SESAM design provides high saturation and roll-over fluence (Fsat ≥ 17 µJ/cm2, F2 ≥ 21 mJ/cm2). With these SESAMs, we show self-starting and stable mode locking of an erbium doped fiber laser at 80 MHz repetition rate, providing ultrashort optical pulses at 17.5 mW average power.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ultrafast fiber lasers operating at 1560 nm find wide application in industry [1], science [2,3,4] and optical telecommunication [5] thanks to their high peak intensities, broad spectra and ultrashort, i.e. sub-picosecond, pulse duration. Their widespread use is nowadays enabled by the unique properties of fiber optic technology, such as good spatial beam quality, compactness and ease of use.
The invention of the semiconductor saturable absorber mirror (SESAM) was a major step towards the development of stable and compact ultrafast laser systems [6]. SESAMs provide the required ultrashort response, large bandwidth, and low non-saturable losses while also facilitating reliable and self-starting passive mode locking [7]. Moreover, the linear and nonlinear optical properties of a SESAM can be optimized independently of the laser design by means of sophisticated epitaxial growth [8]. Competing techniques, such as Kerr lens or additive-pulse mode locking, do not have comparable degrees of freedom.
A SESAM is composed of two main parts: A distributed Bragg reflector (DBR), which acts as bottom mirror, and one or multiple ultrafast saturable absorber layers, which govern the nonlinear SESAM response. The current state-of-the-art fabrication of SESAMs for 1560 nm faces two major challenges: The first challenge is to grow the absorbing InGaAs layer with the required ultrafast non-radiative recombination times ≤ 1 ps, while also maintaining high crystalline quality and small non-saturable losses. So far, this was achieved with low-temperature (LT) molecular-beam epitaxy (MBE) growth. This technique can facilitate ultrashort recombination times but results in impaired crystalline quality and increased defect densities due to the incorporation of excess arsenic at low growth temperatures [9]. This obstacle can be overcome by employing highly iron doped InGaAs (InGaAs:Fe), which is grown at higher temperatures and hence can show superior crystalline quality when compared to LT-grown InGaAs. In a previous publication we optimized InGaAs:Fe for ultrashort recombination times and high mobility and fabricated ultrafast photoconductive switches from this material [10].
The second challenge is to grow a DBR with high and broadband reflectivity. Until now, SESAMs for 1560 nm have mainly been grown on GaAs [11,12,13,14], which allows for nearly lattice-matched growth of AlAs-GaAs DBRs with high refractive index contrast (Δn ≈ 0.5 [15]) and broad stopbands around 1560 nm. However, indium rich InGaAs absorber layers for 1560 nm are highly strained when grown on AlAs-GaAs DBRs due to the significant lattice mismatch (Δa/a ≈ 3.6% for In0.53Ga0.47As). The resulting excessive defect formation leads to significant non-saturable losses. In addition, detailed work on laser diodes has shown that increased strain results in less robust devices and reduced device lifetime [16].
To reduce the strain and redshift the absorption, it was proposed to add a few percent of nitrogen to InGaAs [17,18]. However, the lattice mismatch to GaAs remains significant (Δa/a ≈ 2% for In0.36Ga0.64N0.04As0.96). Moreover, InGaNAs saturable absorbers suffer from poor temporal response that can only be counteracted by post-growth ion implantation. This in turn comes at the expense of high non-saturable losses [19].
To annihilate the strain completely, SESAMs grown on InP have been investigated, which allows for lattice-matched growth of the DBR and the absorber layers. Here, the challenge arises from the materials available for the DBR, which suffer from either high growth complexity or relatively low refractive index contrast. For example, DBRs with high Δn > 0.4 have been demonstrated based on antimonide compounds, such as AlAsSb–InGaAs(P) [20], AlGaAsSb-InP [21], and (Al)GaAsSb–AlAsSb [22], but the growth of these compounds is difficult due to a wide miscibility gap and poor interface control, which leads to high defect densities and rough surfaces.
In contrast, the growth of InGaAlAs and InGaAsP lattice-matched to InP is well established, but these offer lower Δn and therefore require the growth of more Bragg pairs to achieve high DBR reflectivity. Thus, precise control of the growth conditions is needed to achieve uniform and accurate layer thicknesses over long growth times. Nowadays this can be achieved by employing advanced in-situ growth monitoring techniques [23,24,25].
InGaAlAs-InAlAs heterostructures are particular attractive for DBRs at 1560 nm, since they offer a larger refractive index contrast (Δn = 0.3 [26]) than InGaAsP-InP (Δn = 0.26 - 0.27 [26]), and do not require repeated switching of the group V element as in InGaAlAs-InP (Δn = 0.34 [27]), which is hard to control precisely.
In this paper, we report on the monolithic growth of InP-based strain-free SESAMs for 1560 nm, which include for the first time InGaAs:Fe absorber. These SESAMs consist of an InGaAlAs-InAlAs DBR and an InAlAs-InGaAs:Fe-InAlAs spacer-absorber-cap array and are optimized for operation within a compact erbium doped ultrafast fiber laser.
The paper is organized as follows: In section two we discuss the design and fabrication of the presented SESAMs, while their optical performance is examined in detail in section three. Finally, in section four we demonstrate the SESAM`s capability for ultrashort pulse generation within the laser oscillator.
2. SESAM design and fabrication
2.1 Design procedure
The SESAM design is crucial in order to obtain stable continuous-wave mode locking and a reliable laser self-start [28,29] as well as to prevent unwanted effects such as Q-switching instabilities [30,31] and multiple pulsing [32]. Therefore, SESAM structures are customized specifically for each laser system in order to obtain the required nonlinear feedback. In particular, this demands a fine adjustment of the SESAM saturation curve, i.e. the dependence of its reflectivity R on the pulse fluence F. The important parameters describing this curve are saturation fluence Fsat and roll-over fluence F2, as depicted by [33]
During the design process, we calculate the SESAM parameters above in order to meet the laser-specific requirements. For this purpose, Eq. (1) can be rewritten in terms of incident and reflected pulse fluence, Fin and Fout, as
Note, that R and I are also functions of the wavelength λ, which is omitted here for clarity. Equation (2) allows to calculate the nonlinear reflectivity due to saturable absorption by modelling the time-dependent reflectivity R(t) = R(α(t)) for the transient time T of a pulse with intensity Iin(t). This is done in a time-iterative procedure with t = (ti)i = 0,1,..,N and N = T/Δt: First, the effective intensity in the absorber IA,eff is calculated via a transfer-matrix formalism, where the intensity of incoming light is normalized to |E|2 = 1, so that [31]
In a second step, the bleaching of absorption α(ti) is analytically determined based on a slow absorber model, for which the pulse duration τP is assumed much shorter than the absorber recovery time τA, so that [36]
2.2 Target design
Figure 1 (top left) shows a schematic of our SESAM design. It consists of a 600 nm thick InAlAs buffer layer followed by a Bragg mirror with 25½ Bragg pairs of alternating InAlAs and InGaAlAs layers, all lattice-matched to InP. This requires a ternary alloy composition of In0.52Al0.48As (bandgap Eg = 858 nm, β = 8.0 cm/GW), and the quaternary alloy composition is chosen to be In0.53Ga0.39Al0.08As (Eg = 1475 nm, β = 61.9 cm/GW). This combination leads to a sufficiently large Δn = 0.3 and ensures that the quaternary alloy is still non-absorbing at 1560 nm. An InAlAs spacer separates the DBR from the absorbing bulk InGaAs:Fe layer, which is likewise lattice-matched to InP with a composition of In0.53Ga0.47As (Eg = 1685 nm, β = 63.2 cm/GW). The structure is terminated by an InAlAs cap layer. Two SESAMs with the same design but different layer thicknesses were fabricated. Their details are shown in Table 1.
Fig. 1. (top left) SESAM structure consisting of a bottom Bragg mirror with 25 ½ Bragg pairs of alternating InAlAs and InGaAlAs layers and an InAlAs-InGaAs:Fe-InAlAs spacer-absorber-cap stack. (bottom left) Refractive index profile of SESAM A and simulated field enhancement at 1560 nm. (right) Cross-sectional view of the grown SESAM A as recorded with scanning electron microscopy, showing the entire structure and enlarged images of the bottom and top end of the structure.
Table 1. Target layer thicknesses and absorber Fe-doping concentration for the presented SESAM samples.
Figure 1 (bottom left) exemplifies the refractive index profile and calculated field enhancement |E|2 within SESAM A for incident light with λ0 = 1560 nm. For both SESAMs the spacer thickness was chosen to place the anti-node of the field intensity in the InGaAs:Fe absorber. The resonance of the Fabry-Perot cavity, which is formed by the entire spacer-absorber-cap stack, was adjusted by the cap thickness to be near anti-resonant with respect to λ0. This reduces the field enhancement within the devices significantly, leading to lower non-saturable losses, higher saturation fluence and increased roll-over fluence [39]. The absorber thicknesses were adjusted to 100 nm, and 110 nm respectively, in order to counteract the small modulation depth that is expected for an anti-resonant SESAM design.
2.3 MBE growth
The SESAMs were grown in a two-step growth procedure by gas-source molecular-beam epitaxy (RIBER 412) on Sn-doped InP (100) substrates, rotating at a speed of 10 rpm. Deoxidation of the substrate was conducted for 10 minutes at 530 °C and under high arsenic pressure to stabilize the surface. Wafer surface temperature and layer growth rate were tracked during the growth using a commercial in-situ reflectometer (Laytec EpiTT). In the first growth step the InAlAs buffer layer and the Bragg mirror were grown at a growth temperature of 490 °C. Thereafter, the sample was unloaded from the growth chamber but kept under ultra-high-vacuum (p < 10−9 Torr) to avoid sample contamination. The beam fluxes were then newly calibrated in order to obtain precise layer thicknesses in the second growth step, which adds the device critical spacer-absorber-cap stack. Here, the growth temperature is slightly reduced to 450 °C to allow for homogeneous Fe-doping of the InGaAs absorber layer [10]. Growing InGaAs:Fe at higher temperatures compared to the commonly employed low-temperature grown InGaAs can provide superior crystalline quality. Iron is incorporated as deep acceptor in the middle of the bandgap and acts as a recombination center [40]. Hence, the Fe-doping concentration determines the decisive absorber dynamics, specifically τA. The nominal Fe-doping concentration was calibrated using secondary ion mass spectroscopy (SIMS) measurements and amounts to 8 × 1018 cm−3 for SESAM A and 2 × 1019 cm−3 for SESAM B.
Figure 1 (right) shows cross-sectional views of the fabricated SESAM A as recorded with scanning electron microscopy (SEM). The inter-layer interfaces are sharp and planar across the overall device. This results in a homogeneously planar surface, as verified with differential interference contrast microscopy. The SESAM structures were further examined by x-ray diffractometry measurements using a commercial diffractometer (Bruker D8). Figure 2 exemplifies the measured and simulated ω−2θ scan for SESAM A, whose sharp peaks demonstrate very high structural quality. The main peak in the center at 31.67° arises from the InP substrate. The fact that all other peaks with relatively high intensity lie very close to it (within ± 0.06° = 216 arcsec) illustrates the high degree of lattice matching achieved. While any lattice mismatch Δa/a below 10−3 can be considered strain-free for most InP-based processes, the largest lattice mismatch here accounts to only 5.5×10−4, i.e. 0.055% for the top two InAlAs layers. All other layers even have Δa/a below 2×10−4, i.e. 0.02%.
Fig. 2. X-ray diffractometry measurement of SESAM A and corresponding fit. All layers are found to be almost strain-free with minimum remaining lattice mismatches Δa/a of 5.5×10−4, i.e. 0.055% for the top two InAlAs layers and below 2×10−4, i.e. 0.02% for all other layers.
3. Optical SESAM characterization
3.1 Linear spectral reflectivity
The linear spectral SESAM reflectivity Rlin(λ) is measured with a commercial reflectivity measurement unit (Filmetrics F20-EXR), which is equipped with an integrated spectrometer and a white light source. Its calibration is done with a silver and a gold reference. Figure 3 shows the measured and simulated Rlin(λ) of the presented SESAM samples: Both SESAMs have a broad stopband with a full-width-half-maximum (FWHM) of 103 nm and 107 nm, respectively. For SESAM A, the maximum linear reflectivity is 90% and lies at about 1560 nm, while the spectral center of the stopband lies at λc = 1540 nm. SESAM B has an equal high maximum reflectivity, which however lies at 1600 nm due to the larger DBR layer thicknesses when compared to SESAM A (see Table. 1). Accordingly, the spectral center of the stopband lies at λc = 1580 nm. For both SESAMs, a good agreement is found between measurement and simulation in terms of spectral shape and position of the stopband, as well as absolute reflectivity.
Fig. 3. Measured and simulated spectral reflectivity Rlin(λ) for SESAM A (left) and B (right). Both SESAMs have comparable high reflectivity but their stopband position is shifted to each other due to their differing DBR layer thicknesses.
3.2 Nonlinear reflectivity
The fluence-dependent reflectivity is the central SESAM characteristic, since it provides the required nonlinear feedback for passive mode locking. Here it is measured within the optical setup depicted in Fig. 4. A femtosecond fiber laser centered at 1560 nm provides 100 mW average power at 80 MHz repetition rate. The optical power is adjusted by a continuously variable neutral density filter and subsequently split into two beams. One beam is used to measure the reference power P1, while the other beam P3 is focused on the sample. The reflectivity, i.e. R(F) = P2(F)/P1, can be measured in a pulse fluence range of four orders of magnitude, reaching from 0.1 µJ/cm2 up to a maximum fluence of 1.3 mJ/cm2. In order to calibrate the reflectivity over the entire pulse fluence range, we used a highly reflective silver mirror. Measurements were conducted with optical pulses with a duration of 165 fs, which was measured with an autocorrelator.
Fig. 4. Nonlinear reflectivity measurement setup: C: Collimator, ND: (variable) neutral density filter, BS: Beam splitter, L: Lens PD: Photodiode.
Figure 5 shows the measured nonlinear reflectivity curves of the presented SESAMs. The initial rise of reflectivity results from the increased saturation of the InGaAs:Fe absorber, while at higher pulse fluence the reflectivity drops due to two-photon absorption in the entire device. The curves were fitted using Eq. (1). For SESAM A, this provides a saturation fluence of Fsat = 24 µJ/cm2 and a roll-over fluence of F2 = 38 mJ/cm2, which limits the modulation depth (ΔR = 7.2%) to an effective value of ΔReff = 5.8%. SESAM B shows a smaller saturation and roll-over fluence (Fsat = 17 µJ/cm2, F2 = 21 mJ/cm2), but a larger (effective) modulation depth (ΔR = 12.1% and ΔReff = 10.1%). This is due to its thicker absorber layer and the spectral position of its stopband. On the one hand, the thicker absorber layer increases the optical thickness of the cavity and therefore implements a slightly more resonant design. Consequently, SESAM B encounters a higher field enhancement, which results in a higher ΔR but lower Fsat and F2. On the other hand, the spectral position of the stopband of SESAM B is shifted to higher wavelengths when compared to SESAM A (Fig. 3), allowing for additional increment of the modulation depth at 1560 nm. Lastly, both SESAMs show relatively low non-saturable losses when compared to the modulation depth, accounting to ΔRns = 3.8% and 4.2% for SESAM A and B respectively.
Fig. 5. Nonlinear reflectivity R(F) for SESAM A (left) and B (right). The measured R(F) (black dots) were fitted with Eq.(1) and compared to the simulation (blue). Dashed lines indicate R(F) without roll-over.
The simulation predicts the discussed parameters well. However, the measured linear reflectivity Rlin (89.0% for SESAM A and 83.7% for SESAM B) and maximum reflectivity Rmax (96.2% for SESAM A and 95.8% for SESAM B) fall slightly behind the simulation for both SESAMs. This is probably due to scattering losses at imperfections that are not considered by the simulation.
3.3 Time-differential reflectivity
The saturable absorber recovery time τA is yet another key parameter for SESAM-based passive mode locking of a laser. It determines the duration of net gain in the laser and hence significantly affects the shortest possible pulse duration, that can be achieved without using further pulse compression techniques [41]. Here, we analyze the carrier lifetime in our SESAMs after transient optical excitation by differential reflectivity measurements. This is a standard technique for investigating ultrashort carrier dynamics in semiconductors [42]. A femtosecond pump pulse excites electrons in the absorber from the valence to the conduction band, while the change of reflectivity for a time-delayed probe pulse is measured. The reflectivity for the probe pulse is increased right after excitation by the pump pulse, as a result of Pauli-blocking. Afterwards, the reflectivity decreases steadily, as electrons and holes are being trapped and finally recombine. In our setup, a femtosecond fiber laser provides 90 fs pulses with a central wavelength of 1560 nm at 100 MHz repetition rate. The pump and probe beam are cross polarized to avoid interference on the sample. Their power is controlled with in-fiber attenuators and additionally modulated with a chopper wheel. The relative change of reflectivity ΔR/R0 for the time-delayed probe pulse is measured via lock-in detection.
Figure 6 shows the measured differential reflectivity signals of the two SESAMs. The decays ΔR/R0 were fitted with a bi-exponential function, where the two time constant τ1 and τ2 can be attributed to non-radiative recombination channels of carriers via Fe-related defects [43]. One observes that these mechanisms ultimately depend on the density of incorporated iron dopants, since both, τ1 and τ2 are decreased for increased doping concentration. Accordingly, SESAM B shows a quicker absorber recovery. The absorber relaxation time τA is here regarded as the time, at which the reflectivity has recovered to 1/e of (ΔR/R0)max. It accounts to τA = 1.6 ps for SESAM A and τA = 700 fs for SESAM B. Table 2 summarizes the measured optical parameters for both SESAMs.
Fig. 6. Measured differential reflectivity for SESAM A and B. The signals were normalized and are shown on a logarithmic scale. The relaxation constants were determined by fitting the signals with a bi-exponential function. The absorber recovery time τA is 1.6 ps for SESAM A and 700 fs for SESAM B.
Table 2. Measured SESAM parameters and laser test results. Uncertainties were derived from consecutive measurements on the same sample.
4. Laser performance
The fabricated SESAMs were tested for ultrashort pulse generation within a fiber ring oscillator, which is part of a commercial available fiber laser system (TOPTICA FemtoFiber smart [44]). The ring oscillator is designed to operate in the soliton regime at 80 MHz repetition rate and 1560 nm central wavelength. Its active region is erbium doped and pumped using an external diode.
We achieved reliable self-start and stable continuous-wave mode locking (CWML) of the oscillator using SESAM B. In contrast, the oscillator only showed Q-switched mode locking (QML) when operated with SESAM A. We believe that this is mainly caused by the much higher roll-over fluence of SESAM A. In previous studies it has been shown, that a large F2, i.e. a late roll-over, has a low power-limiting effect, which may cause relaxation oscillations of the pulse energy to turn into QML [34]. The longer recovery time of SESAM A should not impair the laser self-start, since the upper limit of τA for stable CWML is generally large when compared to the desired pulse duration [45,41]. However, the laser self-start may be affected by the group delay dispersion induced by the SESAMs. According SESAM design optimizations as well as SESAM device lifetime experiments are currently underway.
Figure 7 shows the measured interferometric autocorrelation (IAC) of the laser oscillator when operated with SESAM B. It was taken with a commercial autocorrelator (APE PulseCheck). The autocorrelation (AC) function is extracted by averaging the IAC signal, and fitted using a sech2 function. This provides an AC pulse duration of 510 fs, which corresponds to an actual pulse duration (FWHM) of 330 fs.
Fig. 7. Measured interferometric autocorrelation (IAC) (left) and spectrum (right) of the fiber ring oscillator when operated with SESAM B. The pulse duration is determined by fitting the extracted intensity autocorrelation (AC) with a sech2 function, which provides τP = 330 fs. The pulse spectrum is centered at 1558 nm and has a spectral width of FWHM = 7.2 nm.
From measuring the optical pulse spectrum (Fig. 7) we find that the pulses are bandwidth-limited. The spectrum is centered at 1558 nm and has a spectral width of FWHM = 7.2 nm, whereas the spectral wings provide the required additional bandwidth for 330 fs pulse duration. The narrow pulse bandwidth is well covered by the stopbands of both SESAMs, as expected.
The average oscillator output power was measured to be 17.5 mW. In the entire fiber laser, the power and pulse duration are further improved. A fiber amplifier increases the oscillator output power to 300 mW, which also results in spectral pulse broadening. Afterwards, the pulses are compressed within a nonlinear fiber to durations of about 100 fs.
5. Summary and outlook
We have presented design, growth and optical characterization of entirely strain-free SESAMs for the delicate 1560 nm spectral regime based on the InP/InGaAlAs material system. In particular, we have demonstrated for the first time that heavily iron doped InGaAs can provide adjustable, ultrafast SESAM response combined with high optical quality. Moreover, high SESAM saturation and roll-over fluence were achieved with an anti-resonant design, without compromising on sufficient modulation depth. This design was shown to enable successful and self-starting passive mode locking of an erbium doped fiber ring laser, which provided ultrashort 330 fs pulses with 17.5 mW average power. In the future, the non-saturable losses of our SESAMs can be further decreased by adding more DBR mirror pairs. This will also make them attractive candidates for mode locking of other laser types that require lower loss, such as VECSELs. Additional performance improvements may also be achieved by using alternative absorber dopants like rhodium, which has already shown excellent results when employed as ultrafast photoconductor for THz antennas [46]. To conclude, we believe that the proven structural advantage of InP-based SESAMs paired with their demonstrated excellent optical performance can resolve the long-lasting strain-related durability issues of GaAs-based SESAMs for the 1560 nm spectral regime.
Acknowledgement
We acknowledge fruitful discussions with Franz X. Kärtner.
Disclosures
The authors declare no conflict 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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