In THz homodyne systems, optical delay lines are the key to time-resolved measurements but they come with a high cost and complexity. They also limit the application of the system in industrial environment, due to their sensitivity to vibrations. Another important point is the scanning speed, for which the mechanical delay line sets severe limitations. Frequency scanning-based systems need a change in THz frequency to recover phase information. Furthermore, there is a tradeoff between phase sensitivity and necessary tuning range. This tradeoff is based on the difference in the length of transmitter and receiver arm in the setup. With our approach, we can introduce a controllable phase shift at 280 GHz by frequency tuning of both lasers. For that purpose, chirped mirrors were designed and introduced into a standard continuous wave Terahertz homodyne system, in order to induce a variable phase shift. In our chirped mirror-based configuration, the phase shift between both optical modes depends on the center frequency of the lasers. Thus, moving the delay stage can be replaced by variation of the center frequency in order to record a THz trace. This means that the measurements are no longer limited by the speed of the delay line. This phase shift is independent of the path length difference in the setup and does not need phase modulators. Simulations show, that these mirrors may achieve a phase shift up to π inside the C-Band for a difference frequency of 280 GHz. To confirm the calculated behavior of the chirped mirrors, initial characterization measurements were performed. We modified an existing delay stage-based THz system to include the chirped mirrors in front of the receiver. This enables the direct comparison while keeping all other parameters constant.
Optical frequency combs (OFCs) have been revolutionizing numerous fields in metrology and spectroscopy. So far, self-referenced OFCs have been mainly based on modelocked fiber or solid-state lasers at wavelengths imposed by the respective gain materials. VECSELs have a large flexibility in their emission wavelength offered by bandgap engineering, making them ideally suited for applications in spectroscopy and sensing. In addition, VECSELs can easily operate at GHz repetition rates, thus enabling a high power per comb mode and the ease of access to individual optical lines. Multi-GHz OFCs are also advantageous for low-noise RF generation by optical-to-microwave frequency division, because it enables operation at lower noise levels in the photo-detection of the comb pulse train.
In this presentation, we will first review the initial work on the detection, noise characterization and stabilization of the carrier envelope offset (CEO) frequency of VECSELs. Then we discuss future application areas, such as their use in low noise microwave generation. In the traditional approach of locking the OFC to an ultrastable reference laser, the achieved phase noise of the generated microwave directly depends on the properties of the optical lock. This is a challenge for VECSEL combs, which currently exhibit higher noise than state-of-the-art ultrafast fiber or bulk lasers. A new RF-generation scheme appears promising for this task, in which a free-running or RF-locked OFC acts as a transfer oscillator. The method does not require any optical lock of the OFC and circumvents the need for high bandwidth actuators.
KEYWORDS: Semiconductor lasers, Semiconductors, Modulation, Optical amplifiers, Diodes, Signal to noise ratio, Fiber amplifiers, Disk lasers, Amplitude modulation, Frequency combs
Semiconductor lasers are a promising technology to make optical comb systems more accessible and cost-efficient. We stabilized the carrier-envelope offset (CEO) frequency of a semiconductor disk laser. The laser was modelocked by a SESAM and generates pulses at a wavelength of 1034 nm. It operates at a repetition frequency of 1.8 GHz. The 270-fs pulses are amplified to 3 W and compressed to 120 fs. A coherent octave-spanning supercontinuum spectrum is generated in a highly nonlinear fiber. Using a standard f-to-2f interferometer, we detect the CEO beat with a signal-to-noise ratio of ~30 dB. By applying a feedback signal to the pump current, the CEO frequency is phase-locked to an external reference.
We present the carrier envelope offset (CEO) frequency stabilization of a fiber laser using two novel methods. The first one is based on cross gain modulation (XGM) implemented with a low-power auxiliary light sharing the laser gain medium. The second method is based on opto-optical-modulation (OOM) of a semiconductor absorber mirror in the laser cavity, used for the first time in a fiber laser. We achieved CEO stabilization with feedback bandwidths of up to 600 kHz using these two easy-to-integrate methods. A tight CEO lock was achieved with a residual integrated phase noise of less than 400 mrad.
We present the self-referenced stabilization of the carrier-envelope offset (CEO) frequency of a semiconductor disk laser. The laser is a SESAM-modelocked VECSEL emitting at a wavelength of 1034 nm with a repetition frequency of 1.8 GHz. The 270-fs pulses are amplified to 3 W and compressed to 120 fs for the generation of a coherent octavespanning supercontinuum spectrum. A quasi-common-path f-to-2f interferometer enables the detection of the CEO beat with a signal-to-noise ratio of ~30 dB sufficient for its frequency stabilization. The CEO frequency is phase-locked to an external reference with a feedback signal applied to the pump current.
Nowadays, space-borne differential absorption lidar (DIAL) instruments are under investigation by space agencies to monitor the integrated column density or the atmospheric density profile of gaseous species from space to ground.
Manufacturing processes from the private and academic sectors were used to deposit anti-reflective and high-reflective coatings composed of Ta2O5 - SiO2 multilayers. Used deposition techniques included three Ion Assisted Deposition (IAD) systems and an Ion Beam Sputtering (IBS) system. Coatings were performed on fused silica (Corning 7980) substrates polished by two different suppliers. LIDT Measurements were performed using a Q-Switched Nd:YAG laser operating at 1064nm. The paper presents a comparison of the coatings in terms of laser damage threshold values, optical properties and surface quality.
Tremendous progress has been achieved in the last years in the field of ultrafast high-power sources. Among the
different laser technologies driving this progress, thin-disk lasers (TDLs) have gained significant ground, both from
amplifiers and modelocked oscillators. Modelocked TDLs are particularly attractive, as they allow for unprecedented
high energy and average powers directly from an oscillator. The exponential progress in the performance of these
sources drives growing needs for efficient means of beam delivery and pulse compression at high average power (<
100 W) and high peak power (> 10 MW). This remains a challenging regime for standard fiber solutions:
microstructured large-mode-area silica photonic-crystal fibers (PCFs) are good candidates, but peak powers are limited
to ≈4-6 MW by self-focusing. Hollow-core (HC) capillaries are adapted for higher peak powers, but exhibit high losses
and are not suitable for compact beam delivery. In parallel to the progress achieved in the performance of ultrafast laser
systems, recent progress in novel hollow-core PCF designs are currently emerging as an excellent solution for these
challenges. In particular, Inhibited-coupling Kagome-type HC-PCFs are particularly promising: their intrinsic guiding
properties allow for extremely high damage thresholds, low losses over wide transmission windows and ultra-low
dispersion.
In our most recent results, we achieve pulse compression in the hundred-watt average power regime using
Kagome-type HC-PCFs. We launch 127-W, 18-μJ, 740-fs pulses from our modelocked TDL into an Ar-filled fiber (13
bar), reaching 93% transmission. The resulting spectral broadening allows us to compress the pulses to 88 fs at 112 W of
average power, reaching 105 MW of peak power, at 88% compression efficiency. These results demonstrate the
outstanding suitability of Kagome HC-PCFs for compression and beam delivery of state-of-the-art kilowatt-class
ultrafast systems.
The invention of the semiconductor saturable absorber mirror (SESAM) nearly 20 years ago was a major advancement
for the development of ultrafast laser systems. Today, SESAMs have become key devices for modelocking of numerous
laser types, including DPSSLs, fiber lasers, and semiconductor lasers. Semiconductors are ideally suited as saturable
absorbers because they can cover a broad wavelength range and yield short recovery times, supporting the generation of picosecond to femtosecond pulse durations. The macroscopic nonlinear optical parameters for modelocking can be optimized over a wide range by the design of the mirror structure and the choice of the semiconductor absorber. Furthermore, their damage threshold can be controlled making them ideally suited for high-power levels. In this presentation, we will focus on recent advances in SESAMs for cutting-edge ultrafast lasers. In particular, we will focus on recent damage and lifetime investigations of SESAMs designed for high-power oscillators. We will present guidelines for robust SESAMs in a large range of saturation parameters, and give an outlook towards novel SESAM designs that will enable future kW-level ultrafast oscillators.
Our femtosecond VECSELs have generated 1.05 W average output power. Numerical simulations have been successfully used to gain a better understanding, but initially have not predicted the average output power correctly. Only after we directly determined the correct gain parameters we got very good agreement. Numerical simulations show that weak gain saturation is beneficial for high-power operation. With a high-precision reflectivity measurement setup we measured the nonlinear change in reflectivity of the optically-pumped (OP) VECSEL gain chip as function of the incident pulse fluence, pump intensity, and heat-sink temperature. We also determined the small signal gain and the gain bandwidth.
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 [1]. 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 [1].
One of the main advantages of using VECSEL lasers for mode-locked operation is their power scalability. Best
performance data available for mode-locked semiconductor lasers have been achieved with optically pumped
VECSELs, reaching pulses in the femtosecond regime and average powers in the watt regime.1 This advantage
is challenging for electrically pumped VECSELs, where a homogeneous carrier injection into the center must be
provided in order to maintain a single-mode operation for large diameter devices.
In this paper we investigate the current injection from the bottom contact of a VECSEL design, and estimate
the leakage of the hole current. Then we introduce two designs that can reduce the leakage current and enhance
the injection into the center of the device, thereby increasing the simulated output power by more than 20% in
CW-mode while maintaining an optimal gain profile suitable for single-mode operation.
We report on power scaling of a modelocked thin disk laser (TDL) based on the broadband mixed sesquioxide material
Yb:LuScO3 (22 nm full width half maximum (FWHM) emission bandwidth). In a first experiment, we could demonstrate
pulse durations as short as 195 fs at a moderate average power of 9.5 W. Furthermore, we were able to power scale our
TDL while keeping the pulses short reaching 23 W at a pulse duration of 235 fs. A key element to achieve this result was
the design of new SESAM structures with multiple quantum wells (QW) and a suitable dielectric topcoating, resulting in
SESAMs with appropriate parameters for short pulse geneartion, low two-photon absorption (TPA) and high damage
thresholds. We will present SESAM optimization guidelines for short pulse generation from high-power modelocked
oscillators.
VECSELs are excellent high power semiconductor lasers with diffraction-limited circular output beam and outstanding
modelocking performance. The output power can be scaled up by simply increasing the mode area on the gain region.
Electrical pumping requires doped layers and also requires changes in the epitaxial design. Crucial for high power
operation is a low electrical resistance, because electrical power heats the device. The p-doped mirror gives the largest
contribution to the electrical resistance. There are certain possibilities to reduce the resistance while keeping the optical
losses as low as possible. Among these techniques are graded interfaces and improved doping schemes.
We present gain characterization measurements for different VECSEL structures with quantum well (QW) and quantum
dot (QD) layers in the active region. We use a high-precision reflectivity measurement setup to determine the change in
reflectivity of the pumped gain chip with varying pulse fluences. In this way the gain saturation behavior and the smallsignal
gain for several structures on different heat-spreaders were measured. The characterization was performed with
femtosecond and picosecond pulses for varying pump powers and heat-sink temperatures. We measured a small-signal
gain of up to 5% and saturation fluences in the range of 30-80 μJ/cm2 for both QW and QD VECSELs. With an
additional measurement setup we determined the gain spectra of two gain chips using a tunable cw probe beam. We
measured gain bandwidths FWHM of 26 nm (QD-structure) and 30 nm (QW-structure).
We present high average power femtosecond VECSELs based on both quantum dot (QD) and quantum well (QW) gain
with extremely low dispersion. 1.05 W in 784-fs pulses could be achieved from a QD-VECSEL modelocked by a QDSESAM
with fast recovery dynamics. A similar QW-gain structure modelocked by the same SESAM enabled stable
480-fs with an average output power of 300 mW at a repetition rate of 7 GHz. Furthermore, we investigated repetition
rate scaling by changing the cavity length. We demonstrated fundamentally modelocked pulses over a tuning range from
6.5 GHz to 11.3 GHz. Without any realignment of the cavity over the whole tuning range, the pulse duration remained
nearly constant around 625 fs (±3.5%) while the output power was 169 mW (±6%). The center wavelength changed
only about ±0.2 nm around 963.8 nm. A tunable repetition rate can be of interest for various metrology application such
as optical sampling by laser cavity tuning.
We present timing jitter measurements of an actively stabilized SESAM modelocked VECSEL generating 4-ps pulses
with 2-GHz repetition rate and 20-mW average output power. The repetition rate was phase-locked to a reference source
using a piezo actuator. The timing phase noise power spectral density of the laser output was measured with an Agilent
E5052B Signal Source Analyzer. The resulting rms timing jitter integrated over an offset frequency range from 1 Hz to
1 MHz gives a timing jitter of below 80 fs, several times lower than previous modelocked VECSELs and comparable to
the noise performance of ion-doped solid-state-lasers.
KEYWORDS: Semiconductor lasers, Green fluorescent protein, Second-harmonic generation, Semiconductors, In vivo imaging, Laser systems engineering, Microscopy, Disk lasers, Laser applications, Laser tissue interaction
Long term in vivo observations at large penetration depths and minimum sample disturbance are some of the key factors that
have enabled the study of different cellular and tissue mechanisms. The continuous optimization of these aspects is the main
driving force for the development of advanced microscopy techniques such as those based on nonlinear effects. Its wide
implementation for general biomedical applications is however, limited as the currently used nonlinear microscopes are
based on bulky, maintenance-intensive and expensive excitation sources such as Ti:sapphire ultrafast lasers.
We present the suitability of a portable (140x240x70 mm) ultrafast semiconductor disk laser (SDL) source, to be used in
nonlinear microscopy. The SDL is modelocked by a quantum-dot semiconductor saturable absorber mirror (SESAM). This
enables the source to deliver an average output power of 287 mW with 1.5 ps pulses at 500 MHz, corresponding to a peak
power of 0.4 kW. The laser center wavelength (965 nm) virtually matches the two-photon absorption cross-section of the
widely used Green Fluorescent Protein (GFP). This property greatly relaxes the required peak powers, thus maximizing
sample viability. This is demonstrated by presenting two-photon excited fluorescence images of GFP labeled neurons and
second-harmonic generation images of pharyngeal muscles in living C. elegans nematodes. Our results also demonstrate that
this compact laser is well suited for efficiently exciting different biological dyes. Importantly this non expensive, turn-key,
compact laser system could be used as a platform to develop portable nonlinear bio-imaging devices, facilitating its widespread
adoption in biomedical applications.
Using quantum well gain materials, ultrafast VECSELs have achieved higher output powers (2.1 W) and shorter pulses (60 fs) than any other semiconductor laser. Quantum dot (QD) gain materials offer a larger inhomogeneously broadened bandwidth, potentially supporting shorter pulse durations. We demonstrate the first femtosecond QD-based VECSEL using a QD-SESAM for modelocking, obtaining 63 mW at 3.2 GHz in 780-fs pulses at 960 nm. In continuous wave operation we obtained 5.2 W using an intra-cavity diamond heat spreader, which has been the highest output power from a QD-VECSEL so far. Further power scaling is thus expected also for modelocked operation.
We present timing jitter measurements of a free-running SESAM modelocked VECSEL generating 8-ps pulses with
1.88-GHz repetition rate and 80-mW average output power. We observed very good performance comparable with iondoped
solid-state-lasers which typically show excellent stability. We measured the two-sided noise power spectral
density at the 10th harmonic of the laser output with the von der Linde method. The rms timing jitter integrated over an
offset frequency range from 100 Hz to 100 kHz gives a free-running timing jitter of ≈400 fs which is an upper limit
because the measurement was already system noise limited above 10 kHz.
Semiconductor lasers have the potential to drastically reduce complexity and cost of high power ultrafast lasers.
Optically-pumped VECSELs achieved >20 W cw-power in fundamental transverse mode. Passive modelocking with a
SESAM enabled 2.1-W average power, sub-100 fs duration, and 50-GHz repetition rate. In 2007, the integration of both
elements was demonstrated, the MIXSEL (modelocked integrated external-cavity surface-emitting laser). Here we
present a novel MIXSEL design based on a low-saturation fluence quantum dot (QD) absorber layer in an antiresonant
structure. Improved thermal management with a CVD-diamond enabled a >30-fold power increase to 6.4 W, higher than
any other ultrafast semiconductor laser.
The combination of high output power and femtosecond pulses from VECSELs and MIXSELs would be very attractive
for many applications. To explore the limitations, a quantitative understanding of the pulse formation processes is
required. Our numerical simulations showed a good qualitative agreement with experimental results in the picosecond
regime. By minimizing intracavity group delay dispersion (GDD) and improving gain bandwidth and SESAM
parameters, our model predicts pulses as short as 250 fs. As a first step we minimized GDD with a top coating which
provides values between ±10 fs2 over a range of 30 nm around the design wavelength.
Fundamental mode operation along with high output power is a major challenge for an electrically pumped VECSEL
(EP-VECSEL) suitable for passive modelocking. We present an experimental study on the influence of the intermediate
DBR reflectivity on the beam quality and the output power of EP-VECSELs. For designs with reflectivities of 90%, 82%
and 71% the highest possible power for the best achievable beam quality was 15.1 mW (M2 = 1.1, 82% device). We can
demonstrate that a correctly chosen intermediate DBR reflectivity is necessary for both good beam quality and high
power, and that a trade-off in power has to be accepted.
Vertical External Cavity Surface Emitting Lasers (VECSELs) feature scaling to large active areas and combine
surface emission with high optical power output. In principle, they can be designed with electrical pumping
operating in continuous wave or passively mode-locked operation. In this paper, our design and modeling
activities towards a high power passively mode-locked VECSEL are described. In particular, design towards
single mode high-power CW operation is discussed as prerequisite for passive mode-locking.
VECSELs modelocked with SESAMs are promising lasers for numerous applications. The MIXSEL concept integrates
both laser gain and saturable absorber regions in one epitaxially grown semiconductor structure. This enables a
particularly simple cavity design: only an external output coupler is needed. In the current MIXSEL realizations, the full
structure is grown in one single growth run. This rises a number of epitaxial growth challenges, which we discuss in this
paper.
We demonstrate monolithic distributed-Bragg-reflector tapered diode lasers having an output power up to 12 W, a small
spectral width of below ▵λ<10 pm and a beam quality close to the diffraction limit. This results in a brightness close to
1 GWcm-2sr-1. Due to these excellent electro-optical characteristics we achieved visible laser light up to P=1.8 W in a
single-path second harmonic generation experiment. This allowed us to develop compact Watt-class (P=1.1 W) visible
laser modules having an excellent beam quality (M²<3) with a narrow spectrum (▵λ<30 pm). The entire device is
integrated on a micro-optical bench with a volume below 20 cm³. In another application we demonstrate for the first time
a femtosecond gigahertz SESAM-modelocked Yb:KGW laser. Such a laser system benefits from the small spectral
emission and the focusability of the developed diode laser. A record peak power of 3.9 kW was achived. At the
repetition rate of 1 GHz, 281 fs pulses with an average output power of 1.1 W were generated. This Yb:KGW laser has a
high potential for stable frequency comb generation.
Ultrafast thin disk lasers achieve higher pulse energies and average power levels than any other modelocked oscillators.
The key components of SESAM modelocked thin disk lasers are used in reflection, which is an advantage for the
generation of ultrashort pulses with excellent temporal, spectral and spatial properties. We review the development and
report latest results. We report on successful scaling of a Yb:Lu2O3 thin disk laser to 141 W average power, setting a new record for mode-locked laser oscillators. Such performance is important for a growing number of applications such as
material processing or driving experiments in high field science.
Ultrashort pulsed laser systems (such as Ti:sapphire) have been used in nonlinear microscopy during the last years.
However, its implementation is not straight forward as they are maintenance-intensive, bulky and expensive. These
limitations have prevented their wide-spread use for nonlinear imaging, especially in "real-life" biomedical applications.
In this work we present the suitability of a compact ultrafast semiconductor disk laser source, with a footprint of
140x240x70 mm, to be used for nonlinear microscopy. The modelocking mechanism of the laser is based on a quantumdot
semiconductor saturable absorber mirror (SESAM). The laser delivers an average output power of 287 mW with 1.5
ps pulses at 500 MHz, corresponding to a peak power of 0.4 kW. Its center wavelength is 965 nm which is ideally suited
for two-photon excitation of the widely used Green Fluorescent Protein (GFP) marker as it virtually matches its twophoton
action cross section.
We reveal that it is possible to obtain two photon excited fluorescence images of GFP labeled neurons and secondharmonic
generation images of pharynx and body wall muscles in living C. elegans nematodes. Our results demonstrate
that this compact laser is well suited for long-term time-lapse imaging of living samples as very low powers provide a
bright signal. Importantly this non expensive, turn-key, compact laser system could be used as a platform to develop
portable nonlinear bio-imaging devices, facilitating its wide-spread adoption in "real-life" applications.
We demonstrate wafer-scale integration of a saturable absorber in a surface emitting semiconductor laser. Vertical
external cavity surface-emitting lasers (VECSELs) have high quality circular output beams, 2D-array scalability, and
high average power. To date, ultrafast VECSELs required a folded cavity with a separate saturable absorber device for
passive modelocking. In the result presented here, we integrate the saturable absorber into the same semiconductor
wafer, optimize its performance for integration with quantum dots and demonstrate stable passive modelocking in a
simple straight external cavity which allows for a fully monolithically wafer-integrated structure to reduce cost and
improve ease of mass production. We refer to this class of devices as the modelocked integrated external-cavity surface emitting laser (MIXSEL). Such devices would be ideally suited for many applications where the current ultrafast laser technology is considered to be too bulky and expensive.
We report on passively mode-locked think disk lasers with up to 60 W average power, nonlinear pulse compression to 33 fs with 18 W average power, and a fiber-feedback parametric oscillator generating 15 W in the 1.5-μm region.
We discuss the latest achievement on passively mode-locked high-power lasers, delivering tens of watts of average power in sub-picosecond pulses. The most promising concept is that of the passively mode-locked thin disk Yb:YAG laser which can so far generate up to 50 W of average power in sub-picosecond pulses.
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