The modelocked integrated external-cavity surface emitting laser (MIXSEL) is the most compact technology of ultrafast semiconductor disk laser, combining in the same epitaxial structure an active region and a saturable absorber for stable and self-starting passive modelocking in a linear straight cavity. Here we present the first MIXSEL structure able to produce sub-300-fs pulses at an average output power of 235 mW and 3.35 GHz pulse repetition rate, resulting in a record-high peak power of 240 W. At 10 GHz repetition rate the same MIXSEL generated 279-fs pulses with 310 mW of average output power. An optimized antireflection coating for dispersion minimization together with a reduced field enhancement inside the structure enabled the sensible improvement and the record performances of this novel MIXSEL. Furthermore, thanks to the development of suitable saturable absorbers with fast recovery dynamics and low saturation fluence, we demonstrate the first entirely MOVPE-grown MIXSEL.
Ultrafast, optically pumped, passively modelocked vertical external-cavity surface-emitting lasers (VECSELs) are excellent sources for industrial and scientific applications that benefit from compact semiconductor based high-power ultrafast lasers with gigahertz repetition rates and excellent beam quality. Applications such as self-referenced frequency combs and multi-photon imaging require sub-200-fs pulse duration combined with high pulse peak power. Here, we present a semiconductor saturable absorber mirror (SESAM) modelocked VECSEL with a pulse duration of 147 fs and 328 W of pulse peak power. The average output power was 100 mW with a repetition rate of 1.82 GHz at a center wavelength of 1034 nm. The laser has optimal beam quality operating in a fundamental transverse mode with a M<sup>2</sup> value of <1.05 in both orthogonal directions. The VECSEL was grown by metal-organic vapor phase epitaxy (MOVPE) with five pairs of strain-compensated InGaAs quantum wells (QWs). The QWs are placed symmetrical around the antinodes of the standing electric field at a reduced average field enhancement in the QWs of ≈ 0.5 (normalized to 4 outside the structure). These results overcome the trade-off between pulse duration and peak power of the state-of-the-art threshold values of 4.35 kW peak power for a pulse duration of 400 fs and 3.3 W peak power for a pulse duration of 107 fs.
We present a single semiconductor disk laser simultaneously emitting two different gigahertz modelocked pulse trains. A birefringent crystal inside a modelocked integrated external-cavity surface-emitting laser (MIXSEL) separates the cavity beam into two spatially separated beams with perpendicular polarizations on the MIXSEL chip. This MIXSEL then generates two orthogonally polarized collinear modelocked pulse trains from one simple straight cavity. Superimposing the beams on a photo detector creates a microwave beat signal, representing a strikingly simple setup to down-convert the terahertz optical frequencies into the electronically accessible microwave regime. This makes the dual-comb MIXSEL scheme an ultra-compact and cost-efficient candidate for dual-comb spectroscopy applications.
Ultrafast VECSELs with high peak power are of great interest for gigahertz frequency combs, as they provide a high power per comb-line and large comb-tooth spacing. However, the detection and stabilization of the carrier-envelope-offset frequency (<i>f</i><sub>CEO</sub>) using an <i>f</i>-to-2<i>f</i> detection scheme, crucial for most frequency comb applications, requires short pulse durations around 100 fs combined with kilowatt peak power to generate a coherent octave-spanning supercontinuum. We present the detection of the <i>f</i><sub>CEO</sub> beat notes from an ultrafast semiconductor laser. The laser is a SESAM-modelocked VECSEL which generates 231-fs pulses in 100-mW average output power at a repetition rate of 1.75 GHz and a wavelength of 1040 nm. As the performance of the oscillator is not sufficient for direct <i>f</i><sub>CEO</sub> detection the pulses were amplified in an Yb-doped fiber amplifier and subsequently broadened by self-phase modulation in a large mode area fiber. The amplified pulses were compressed to a pulse duration of 85 fs at 2.2 W of average output power and launched into a highly nonlinear photonic crystal fiber. A coherent octave-spanning supercontinuum covering 680 nm to 1360 nm was generated, which supported for the first time <i>f</i><sub>CEO</sub> detection from a femtosecond VECSEL in a standard <i>f</i>-to-2<i>f</i> interferometer.
The MIXSEL combines the gain of a VECSEL with the saturable absorber of a SESAM in one semiconductor structure to achieve fundamental modelocking in a simple straight cavity. We present a high-power MIXSEL with sub-10-ps pulse durations that can be scaled easily in repetition rate from a few GHz to <100 GHz. At 5.1 GHz repetition rate an average output power of 1.05 W in 2.4-ps-pulses was achieved. By scaling the repetition rate to 10 GHz (3.9-ps-pulses at 1.29 W), then to 20.7 GHz (2.35-ps-pulses at 607 mW) and most recently to even more than 100 GHz makes this high-power MIXSEL an attractive source suitable for applications such as optical clocking or optical sampling.
The modelocked integrated external-cavity surface emitting laser (MIXSEL) combines the active region and basic layout 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. This concept allows stable and self-starting passive modelocking in a simple straight cavity. Record-high average output power has been demonstrated previously with a MIXSEL based on a quantum dot saturable absorber, but with a minimum pulse duration of only 17 ps up to now. Here we present a femtosecond MIXSEL emitting 620-fs pulses in 101 mW of average output power at 4.8 GHz pulse repetition rate. A novel single quantum well saturable absorber, whose parameters are discussed in detail, enabled the strong reduction in pulse duration by over an order of magnitude.
We present timing jitter measurements of a free-running and actively stabilized modelocked integrated external-cavity surface-emitting laser (MIXSEL) generating ps-pulses around 2-GHz repetition rate and over 600 mW of average output power with <0.15% rms amplitude modulation (AM) noise. The free running rms timing-jitter was 129 fs [100 Hz to 10 MHz] and 70 fs [300 Hz to 10 MHz], which is the lowest timing jitter of a free-running passively modelocked semiconductor laser to date. Actively stabilized to an electronic reference source using a piezo-actuator, an rms timing jitter of 31 fs was obtained, representing the lowest value ever measured from a passively modelocked semiconductor disk laser between 100 Hz and 100 MHz.
Optically pumped vertical external cavity surface emitting lasers (OP-VECSELs) evolved to high-power laser sources offering excellent beam-quality, wavelength flexibility and low-noise properties in a compact and simple cavity. Passively modelocked with a semiconductor saturable absorber mirror (SESAM), VECSELs demonstrated fs-pulses with multi-Watt average output powers at gigahertz repetition rates. Electrical pumping (EP) is an obvious step to make these semiconductor lasers even more compact and suitable for chip integration, potentially enabling access to applications such as data communication or optical clocking. With SESAMmodelocked EP-VECSELs, 57-ps pulses with an average output power of 40 mW and 9.5-ps pulses with 7.6 mW have been obtained. However, due to the intrinsic trade-off between electrical and optical properties in the design of EPVECSELs, short pulses at high average output power are difficult to achieve. This challenge was previously addressed in our theoretical guidelines for power scaling and modelocking optimization and later experimentally verified. Here, we report on the successful implementation of an improved design and fabrication scheme for EP-VECSELs, grown and fabricated at ETH Zurich. These lasers enabled a further decrease in pulse duration to 7.3 ps while increasing the average output power to 13.1 mW at 1.46-GHz repetition rate. The shortest pulse duration measured was 6.3 ps with an average power of 6.2 mW. In addition to the modelocking experiments, we present a thorough cw-characterization of our EP-VECSELs of different sizes and in different cavity configurations, pointing out the inevitable trade-off between high-power multi-mode and low-power single-mode operation limiting the modelocking performance.
In modelocked electrically pumped VECSELs (EP-VECSELs) the gain saturation strongly influences the pulse formation. Here we present a detailed gain characterization of EP-VECSELs as published the first time in . The spectral gain-distribution and the gain saturation behavior of two devices with different field-enhancement in the quantum-well gain layers are investigated. Comparing spectral bandwidth, small-signal gain and saturation fluence of the three devices, we chose the most suitable for modelocking experiments. Using a low-saturation fluence SESAM we have generated 9.5-ps-pulses with an average output-power of 7.6 mW at 1.4 GHz repetition-rate, which have been the
shortest pulses from an EP-VECSEL to date .