We present a detailed experimental study on the influence of dispersion on the pulse formation in passively modelocked vertical-external-cavity surface-emitting lasers (VECSELs). We have demonstrated that the shortest pulse duration requires slightly positive dispersion to balance the nonlinear phase shift induced by strong semiconductor gain and absorber saturation. This is in contrast to soliton modelocking in ion-doped solid-state lasers – but similarities remain and will be discussed. Our results are in good qualitative agreement with numerical simulations and confirm the quasi-soliton modelocking mechanism of ultrafast VECSELs.
©2010 Optical Society of America
Soon after the first demonstration of a passively modelocked vertical-external-cavity surface-emitting laser (VECSEL , ) with a semiconductor saturable absorber mirror (SESAM [2,3], ) in 2000  their performance has surpassed other ultrafast semiconductor lasers in average power, pulse quality and pulse duration. Without external pulse compression, they offer the shortest pulses directly from a semiconductor laser [5,6], and recently even sub-100 fs pulses have been achieved . Moreover, ultrafast optically pumped VECSELs can generate substantially higher average output powers than other modelocked semiconductor lasers with currently up to 2.1 W in 4.7-ps pulses . The vertical-emitting geometry has a clear power scaling advantage, which has enabled continuous-wave (cw) power of up to 20 W in fundamental transverse mode operation . Many different operation wavelengths can be easily achieved with band gap engineering and a more detailed review is given in . Furthermore, the SESAM and the VECSEL gain structure can be integrated into a single semiconductor structure, which is referred to as modelocked integrated external-cavity surface emitting laser (MIXSEL , ). So far, MIXSELs have generated an average power of up to 660 mW in 23-ps pulses and operated at repetition rates up to 10 GHz . The compactness and the simplicity of the MIXSEL platform appears well-suited for cost-efficient mass production  and large-scale applications such as optical interconnects or optical clocking of future high frequency multi-core microprocessors .
Many other applications such as broadband continuum generation for illumination and optical coherence tomography , self-referenced frequency combs  for metrology applications [17,18], and biomedical imaging need high power femtosecond pulses. Therefore, one of the next milestones in ultrafast VECSELs and MIXSELs is the power scaling in the femtosecond regime. To date femtosecond VECSELs have generated only limited average output powers well below 100 mW. Femtosecond high-power pulse generation will require a more detailed understanding of the pulse forming mechanism in modelocked VECSELs and further optimization of the experimental key parameters such as dispersion and SESAM recovery time .
Motivated by the successful soliton modelocking model [20,21] in SESAM modelocked ion-doped solid-state lasers, we have performed a theoretical study on soliton-like pulse-shaping mechanism for SESAM-modelocked VECSELs . For soliton modelocking in ion-doped solid-state lasers self-phase modulation (SPM) and negative group delay dispersion (GDD) is balanced to form soliton pulses (with sech2-shaped temporal profile), where k is the wave number in vacuum, the nonlinear refractive index and the pulse intensity. For normal materials the is positive and soliton formation then requires negative GDD. In the femtosecond to few picosecond domain this pulse formation process becomes dominant and the SESAM only starts and stabilizes this pulse formation process. This relaxes the requirements on the SESAM parameters. Passive modelocking at gigahertz repetition rates typically has negligible SPM and therefore soliton modelocking becomes less effective at these high pulse duty cycles and correspondingly lower peak powers. Furthermore, in contrast to ion-doped solid-state lasers semiconductor lasers exhibit strong dynamic gain saturation . The combination of both the nonlinear phase shift introduced by the dynamic gain saturation and the fast absorber saturation leads mathematically to a similar effect as SPM but with a negative . We therefore referred to this modelocking regime in VECSELs as quasi-soliton modelocking  for which in mathematical analogy to the soliton modelocking model the interplay of a nonlinear phase shift with positive intra-cavity GDD becomes the dominant pulse forming mechanism. Thus, this nonlinear phase shift is not based on SPM but is induced by significant VECSEL gain and SESAM saturation. The combination of both effects behaves approximately like a negative SPM material (i.e. negative ). Therefore, soliton-like pulse formation is obtained with positive instead of negative GDD and this pulse-shaping theory is referred to as quasi-soliton modelocking. In , however, only a qualitative experimental agreement could be found, since various important parameters were changed simultaneously during the experiments.
Here, we present the results of the first systematic experimental investigation of the influence of intra-cavity GDD on the pulse duration for SESAM modelocked VECSELs. For this study, we developed a versatile new type of dispersive mirror based on a hybrid semiconductor/dielectric coating approach. It allowed us to discretely vary the overall intra-cavity GDD between −14000 fs2 and 18000 fs2 by simply introducing different dispersive mirrors into the laser cavity. Our experiments confirm the quasi-soliton pulse-shaping mechanism for VECSELs, show good qualitative agreement with numerical simulations and confirm that indeed positive GDD is required for the shortest pulses. In addition, we found an additional benefit with a high tolerance in positive GDD values for the pulse duration.
2. Design of hybrid semiconductor/dielectric coating dispersive mirrors
In order to experimentally control the GDD of our modelocked VECSELs, we developed a new type of dispersive mirrors. We produced a set of different mirrors with GDD values ranging between −7000 fs2 to 9000 fs2 for a single reflection. We based the devices on a standard Gires-Tournois interferometer (GTI , ) design [see Fig. 1(a) ]. A standard GTI design consists of a highly reflective (HR) and a partially reflective (PR) mirror section, which are separated by a spacer layer. An incoming beam is typically sent at nearly perpendicular incidence onto the PR section. The transmitted light that enters the spacer layer will be reflected by the HR section, which ideally has a reflectivity of 100%. In the case of no losses, the GTI will reflect everything but with a wavelength-dependent phase-shift that can generate either positive or negative GDD. The amount of GDD depends on the wavelength, the reflectivity of the PR mirror section, the spacer material and the spacer thickness.
In Fig. 2(a) , we show the calculated dispersion for a GTI consisting of a standard HR mirror section [realized by a GaAs/AlAs distributed Bragg reflector (DBR)] and the Fresnel reflection of the spacer/air interface as PR mirror section. We chose two different spacer materials, GaAs and fused silica, and plotted the resulting GDD as a function of the spacer thickness for a center wavelength of 960 nm. For the GaAs spacer, we can obtain sufficiently large values for the GDD, however it would require very precise layer thickness control. For example, for a design value of 4500 fs2, a difference in spacer-thickness of 1 nm would correspond to an error in GDD of ≈223 fs2. This accuracy is difficult to achieve with standard processing methods.
In comparison, fused silica as a spacer material provides a much smoother GDD dependence on the material thickness. In addition, fused silica can easily be deposited on multiple semiconductor samples with different and accurately controllable thicknesses using plasma enhanced chemical vapor deposition (PECVD). The only drawback is the limited amount of GDD that we can obtain for typical layer thicknesses below 1 µm.
Because fused silica or GaAs spacer materials did not provide the required dispersion properties, we used a hybrid approach for our dispersive mirror design, which benefits from the advantages of both materials. Its main part consists of a bottom DBR and a 3.1-µm thick GaAs layer, which can provide a sufficiently large amount of dispersion (in the order of 104 fs2). For the fine adjustment of the final GDD, we insert an anti-reflective (AR) section on the top, and add an additional layer of fused silica. The thickness of this fused silica layer needs to be controlled very accurately in order to obtain a specific value for the GDD of the final dispersive mirror. The AR section consists of a 10-layer AlAs/GaAs system to reduce internal reflections between the 3.1-µm thick GaAs layer and the fused silica layer to less than 0.1% for the entire wavelength range from 950 to 970 nm.
We grew the semiconductor part of the design using molecular beam epitaxy (MBE) on a 650 µm GaAs wafer (using a VEECO GEN III MBE of the ETH FIRST lab). After cleaving the wafer into 3x3 mm2 pieces, we added the fused silica coatings with different thicknesses to obtain a set of dispersive mirrors. The thicknesses of the fused silica layers on the individual samples ranged from 80 nm to 350 nm which correspond to GDD values between –7000 fs2 and 9000 fs2 [see Fig. 2(c)].
The GDD curves for some of the fabricated devices are plotted in Fig. 2(c) as a function of the wavelength. The measurement data is plotted as dots and a corresponding spline interpolation is plotted as a solid line for each of the dispersive mirrors. Figure 2(d) shows the comparison of our GDD simulation (red line) with the GDD measurement results (points with a black line spline fit) for one of our devices, in this case for that with a fused silica layer thickness of 285 nm. All GDD measurements were done with a home-built GDD measurement setup based on white-light interferometry .
3. Experimental laser setup
For our experiment we used a standard z-shaped cavity (Fig. 3 ) with an output coupler, an optically pumped VECSEL gain-structure, one of the dispersive mirrors, a SESAM and a 20- µm thick etalon for wavelength tunability. To change the overall GDD of the laser cavity, we use the set of dispersive mirrors as described in Section 2. Thus, the total intra-cavity GDD can be changed without changing the emission wavelength which can be controlled by the 20-μm thick fused silica etalon.
The VECSEL was grown in the FIRST clean room facility at ETH Zurich using metal-organic vapor-phase epitaxy (MOVPE i.e. a AIXTRON AIX 200/4 MOVPE) with a design similar to . The VECSEL contains an AlAs/AlGaAs DBR which reflects both the laser and the pump wavelength using a super lattice mirror structure design. The calculated reflectivity for the laser wavelength at 10° angle of incidence is >99.95% and for the pump radiation at an angle of incidence of 45° it is >99.5%. The active region of the VECSEL consists of seven InGaAs quantum wells (QWs) with a thickness of 5 nm, placed at the peaks of the standing wave pattern of the electric field inside the VECSEL structure. The QWs are spaced by GaAs spacer layers which also serve as absorbing medium for the pump radiation. From these spacer layers the excited carriers can drift into the QWs. On top of the active region there is an AR-section which controls the field enhancement in the gain region.
Self-starting and reliable modelocking is achieved with a quantum-dot (QD) SESAM with moderate saturation fluence . It consists of an AlAs/GaAs DBR, a single InAs QD saturable absorber layer which is centered at a maximum of the standing wave pattern of the electric field and several AlAs/GaAs layers on top of the structure to control the field enhancement. The SESAM was grown using MBE at the FIRST clean room facility at ETH in Zurich and it has a resonant design similar to the one described in . We measured its nonlinear reflectivity with a homebuilt high dynamic range setup , which was driven by 180-fs pulses at 956 nm from a commercial 80-MHz repetition rate Ti:sapphire laser. The measured saturation fluence is 4.2 µJ/cm2, the modulation depth 1.3%, and the nonsaturable losses are < 1%. Due to the resonant design, both the modulation depth and the saturation fluence are wavelength dependent. With increasing wavelength, the saturation fluence increases and the modulation depth decreases.
The laser mode radius on the VECSEL was ≈90 µm and on the SESAM ≈45 µm. The VECSEL gain structure was pumped optically using an 808 nm diode laser at a pumping angle of about 45°. The output coupling mirror had a radius of curvature of 38 mm and a reflectivity of 99.3% (i.e. an output coupling of 0.7%). The temperature of the VECSEL heat sink was set to 5°C. The typical output power was about 30 mW for all GDD dependent measurements.
In Fig. 4 , we show the autocorrelation and the optical spectrum of the pulses with the shortest pulse duration of 1.52 ps with 30 mW of average output power, at 4.2 GHz repetition rate and with a positive total intra-cavity dispersion of 5300 fs2. The FWHM spectral width of the pulses was 0.86 nm, resulting in a time bandwidth product (TBP) of 0.43 which is 1.35 times the transform limit of an ideal sech2 soliton pulse shape.
4. GDD dependent results
Both VECSELs and SESAMs provide a significant amount of dispersion (in the range of ± 104 fs2), which strongly depends on the operation wavelength. The typical MBE growth accuracy leads to a relatively large uncertainty, and measurements of the real GDD are needed. Therefore, we first measured the wavelength-dependent GDD of the VECSEL, the SESAM and all fabricated dispersive mirrors with our home-built GDD measurement setup based on white-light interferometry . An example of the obtained GDD curves for the whole cavity, as well as the individual cavity elements for one specific cavity setup, is given in Fig. 5(a) The GDD of the entire cavity was then determined according to Eq. (1)Fig. 3).
For every dispersive mirror we changed the center wavelength of the laser emission using the etalon, tuning the wavelength between 950 nm and 960 nm in increments of 0.5 nm. We determined the optical spectrum, the microwave spectrum, and the autocorrelation (AC) trace using a standard second harmonic generation (SHG) autocorrelator. We took special care during those measurements not to alter other parameters, such as the repetition rate or the pump power, to insure that only the total intra-cavity GDD dependence was investigated. Therefore, we can determine the pulse duration as a function of the total intra-cavity GDD for any given emission wavelength from these measurements because every dispersive mirror has a different wavelength-dependent GDD [Fig. 2(c)].
In Fig. 5(b), the measured pulse duration is given as a function of the total intra-cavity GDD for several different laser wavelengths.
We clearly see that the pulse duration strongly depends on the amount of total intra-cavity GDD. For positive GDD values the pulse duration becomes shorter compared to negative GDD values. The minimum duration is observed at low positive values between 2000 and 6000 fs2. The pulse duration for a given value of intra-cavity GDD depends strongly on the operation wavelength, which is mainly due to the resonant SESAM design, which introduces a significant wavelength dependent modulation depth and saturation fluence. This effect, however, is much more pronounced in the negative GDD regime compared to the positive [see Fig. 5(b)].
5. Numerical Simulations
We compared our experimental data with numerical simulations done in MATLAB. We simulated the pulse propagation by implementing numerical iterations of a circulating pulse inside a VECSEL cavity, similar to . Within a cavity round trip the pulse interacts with the cavity elements such as gain, absorber, output coupler and cavity dispersion. These elements are represented by operators either in time or frequency domain. The slowly varying envelope approximation is used to describe the temporal evolution of the pulse shape and fast Fourier transformation (FFT) converts the pulse between the time and the frequency domain. The effects our simulation takes into account are shown in Fig. 6 .
The gain filter simulates the optical gain with respect to the center wavelength and the bandwidth. Effects of dynamic saturation of the gain and the absorber are described in the time domain using the differential equation Eq. (2)Eq. (3) 28]. The values we use in our simulation were α absorber = 2 and α gain = 3. The GDD is included in the model by applying Eq. (4) Fig. 7 .
Although a pulse build-up from noise is possible, we usually started the simulations with a sech2-pulse shape, which significantly reduces the number of iterations for a stable solution.
The combination of saturable gain and saturable absorption with negligible intra-cavity SPM results in quasi-soliton modelocking  as discussed in Section 1. According to the Kramers-Kronig relation, the saturation directly influences the phase when the laser pulse hits the VECSEL gain structure or the SESAM. The overall phase change is approximately comparable with the phase change from SPM for soliton modelocking as typically observed in ion-doped low repetition rate solid-state lasers. However, the sign is opposite. This behavior is illustrated in Fig. 8(a) . For soliton modelocked solid-state lasers, shorter pulses are obtained if SPM is balanced with an appropriate amount of negative cavity GDD. In case of SESAM-modelocked VECSELs, however, the total nonlinear phase change has the opposite sign [Fig. 8(a)] and therefore positive GDD is required for the generation of short pulses. We obtained an asymmetric curve for the pulse duration as a function of the GDD, where positive GDD values are better suited for the generation of short pulses. As one might expect, if saturation effects are neglected, the curve becomes perfectly symmetric which also was reproduced using our theoretical model [Fig. 8(b)].
In comparison to our experimental results our simulations give a good qualitative agreement [see Fig. 8(b) and 8(d)]. For the simulation parameters we used the experimental values from Section 3 and we assumed a gain bandwidth ∆λ gain = 10 nm, F sat,gain = 500 µJ/cm2, τ gain = 3 ns and τ absorber = 8 ps. According to our simulations these assumed parameters only have a small influence on the pulse duration. In Fig. 8(d), the measurement results for an emission wavelength of 956 nm are shown together with the corresponding simulation, and in Fig. 8(b) for 957 nm. We clearly see that positive GDD values are favored over negative ones to achieve short pulse durations. This also verifies that the presumed modelocking mechanism for SESAM-modelocked VECSELs is indeed very well described by the quasi-soliton modelocking model.
6. Discussion and conclusion
We present the first detailed experimental analysis of the influence of intra-cavity GDD on the pulse duration for passively modelocked VECSELs. In order to be able to change the intra-cavity GDD without changing the laser wavelength we developed a versatile dispersive mirror based on a hybrid semiconductor/dielectric coating approach. Using these devices in our experiments, we confirm that negative GDD results in significantly longer pulses compared to positive GDD. Thus, operation in the low positive GDD regime is ideal for the generation of the shortest pulses. Furthermore, the influence of positive GDD is much reduced compared to negative GDD, resulting in a more relaxed tolerance on the growth accuracy in that regime. The experimentally observed characteristics could also be reproduced in our simulations based on the quasi-soliton theory. We found a good qualitative agreement of the experimental results with our simulations which not only confirms the simplified theory, but also allows for predictions on the pulse duration for specific SESAM-VECSEL combinations. We can now adjust SESAM and VECSEL designs to push the total intra-cavity GDD to slightly positive values. We believe that together with optimized SESAM recovery dynamics and improved gain bandwidth engineering it will be possible to demonstrate power scaling in the femtosecond pulse generation regime as well.
This work was supported by ETH Zurich with the FIRST clean room facility and was financed by the Intel Corporation through a university sponsored research agreement, by the Swiss Confederation Program Nano-Tera.ch, which was scientifically evaluated by the SNSF and by the European Community's Seventh Framework Program FAST-DOT under grant agreement 224338.
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