Peak power scaling of semiconductor disk lasers is important for many applications, but their complex pulse formation mechanism requires a rigorous pulse characterization to confirm stable fundamental modelocking. Here we fully confirm sub-300-fs operation of Modelocked Integrated eXternal-cavity Surface Emitting Lasers (MIXSELs) with record high peak power at gigahertz pulse repetition rates. A strain-compensated InGaAs quantum well gain section enables an emission wavelength in the range of Yb-doped amplifiers at ≈1030 nm. We demonstrate the shortest pulses from a MIXSEL with a duration of 253 fs with 240 W of peak power, the highest peak power generated from any MIXSEL to date. This peak power performance is comparable to conventional SESAM-modelocked VECSELs for the first time. At a 10-GHz pulse repetition rate we still obtained 279-fs pulses with 310 mW of average output power, which is currently the highest output power of any femtosecond MIXSEL. Continuous tuning of the pulse repetition rate has been demonstrated with sub-400-fs pulse durations and >225 mW of average output power between 2.9 and 3.4 GHz. The strain-compensated MIXSEL chip allowed for more detailed parameter studies with regards to different heat sink temperatures, pump power, and epitaxial homogeneity of the MIXSEL chip for the first time. We discuss in detail, how the critical temperature balance between quantum well gain and quantum well absorber, the partially saturated absorber and a limited epitaxial growth quality influence the overall device efficiency.
In this paper we present the first semiconductor disk laser (SDL) emitting simultaneously two collinearly overlapping cross-polarized gigahertz modelocked pulse trains with different pulse repetition rates. Using only a simple photo detector and a microwave spectrum analyzer directly down-converts the frequency comb difference from the optical to the microwave frequency domain. With this setup, the relative carrier-envelope-offset (CEO) frequency can be accessed directly without an f-to2f interferometer. A very compact design is obtained using the modelocked integrated external-cavity surface emitting laser (MIXSEL) which is part of the family of optically pumped SDLs and similar to a vertical external cavity surface emitting laser (VECSEL) but with both gain and saturable absorber integrated into the same semiconductor wafer (i.e. MIXSEL chip). We then simply added an additional intracavity birefringent crystal inside the linear straight cavity between the output coupler and the MIXSEL chip which splits the cavity beam into two collinear but spatially separated cross-polarized beams on the MIXSEL chip. This results in two modelocked collinear and fully overlapping cross-polarized output beams with adjustable pulse repetition frequencies with excellent noise performance. We stabilized both pulse repetition rates of the dual comb MIXSEL.
We present a 1.75-GHz self-referenceable frequency comb from a vertical external-cavity surface-emitting laser (VECSEL) passively modelocked with a semiconductor saturable absorber mirror (SESAM). The VECSEL delivers 231-fs pulses with an average power of 100 mW and is optimized for stable and reliable operation. The optical spectrum was centered around 1038 nm and nearly transform-limited with a full width half maximum (FWHM) bandwidth of 5.5 nm. The pulses were first amplified to an average power of 5.5 W using a backward-pumped Yb-doped double-clad large mode area (LMA) fiber and then compressed to 85 fs with 2.2 W of average power with a passive LMA fiber and transmission gratings. Subsequently, we launched the pulses into a highly nonlinear photonic crystal fiber (PCF) and generated a coherent octave-spanning supercontinuum (SC). We then detected the carrier-envelope offset (CEO) frequency (f(CEO)) beat note using a standard f-to-2f-interferometer. The f(CEO) exhibits a signal-to-noise ratio of 17 dB in a 100-kHz resolution bandwidth and a FWHM of ≈10 MHz. To our knowledge, this is the first report on the detection of the f(CEO) from a semiconductor laser, opening the door to fully stabilized compact frequency combs based on modelocked semiconductor disk lasers.
The high-power semiconductor laser studied here is a modelocked integrated external-cavity surface emitting laser (MIXSEL), which combines the gain of vertical-external-cavity surface-emitting lasers (VECSELs) with the saturable absorber of a semiconductor saturable absorber mirror (SESAM) in a single semiconductor layer stack. The MIXSEL concept allows for stable and self-starting fundamental passive modelocking in a simple straight cavity and the average power scaling is based on the semiconductor disk laser concept. Previously record-high average output power from an optically pumped MIXSEL was demonstrated, however the long pulse duration of 17 ps prevented higher pulse repetition rates and many interesting applications such as supercontinuum generation and broadband frequency comb generation. With a novel MIXSEL structure, the first femtosecond operation was then demonstrated just recently. Here we show that such a MIXSEL can also support pulse repetition rate scaling from ≈5 GHz to >100 GHz with excellent beam quality and high average output power, by mechanically changing the cavity length of the linear straight cavity and the output coupler. Up to a pulse repetition rate of 15 GHz we obtained average output power >1 W and pulse durations <4 ps. Furthermore we have been able to demonstrate the highest pulse repetition rate from any fundamentally modelocked semiconductor disk laser with 101.2 GHz at an average output power of 127 mW and a pulse duration of 570 fs.
Novel surface-emitting optically pumped semiconductor lasers have demonstrated >1 W modelocked and >100 W continuous wave (cw) average output power. The modelocked integrated external-cavity surface emitting laser (MIXSEL) combines the gain of vertical-external-cavity surface-emitting lasers (VECSELs) with the saturable absorber of a semiconductor saturable absorber mirror (SESAM) in one single semiconductor structure. This unique concept allows for stable and self-starting passive modelocking in a simple straight cavity. With quantum-dot based absorbers, record-high average output power was demonstrated previously, however the pulse duration was limited to 17 ps so far. Here, we present the first femtosecond MIXSEL emitting pulses with a duration as short as 620 fs at 4.8 GHz repetition rate and 101 mW average output power. The novel MIXSEL structure relies on a single low temperature grown quantum-well saturable absorber with a low saturation fluence and fast recovery dynamics. A detailed characterization of the key modelocking parameters of the absorber and the challenges for absorber integration into the MIXSEL structure are discussed.
In this paper a study of waveguide photodetectors based on InAs/InP(100) quantum dot (QD) active material are presented for the first time. These detectors are fabricated using the layer stack of semiconductor optical amplifiers (SOAs) and are compatible with the active-passive integration technology. We investigated dark current, responsivity as well as spectral response and bandwidth of the detectors. It is demonstrated that the devices meet the requirements for swept-source optical coherent tomography (SS-OCT) around 1.7 µm. A rate equation model for QD-SOAs was modified and applied to the results to understand the dynamics of the devices. The model showed a good match to the measurements in the 1.6 to 1.8 µm wavelength range by fitting only one of the carrier escape rates. An equivalent circuit model was used to determine the capacitances which dominated the electrical bandwidth.
