Optical frequency combs based on broadband-gain bulk lasers, due to the low intrinsic linewidth and sub-GHz repetition rates, have gained tremendous interest for applications such as high-resolution spectroscopy, dual-comb spectroscopy or LIDAR. However, susceptibility to mechanical and acoustic perturbations, the complexity of optical pumping and the larger physical size of these lasers has motivated research toward chip-based integrated extended cavity diode lasers with low-loss Si3N4 waveguide feedback circuits for low repetition rates. In diode lasers, mode-locking via saturable absorbers is generally used for generating frequency combs, however, the short upper-state carrier lifetime results in repetition rates of at least a few GHz. Here, we demonstrate absorber-free, passive mode-locking as well as hybrid mode-locking at sub-GHz repetition rates using a long Si3N4 feedback circuit with three highly frequency-selective microring resonators for extending the cavity roundtrip length to more than 0.6 m. This enables frequency-domain mode-locking in the form of a continuous wave, with a line spacing of around 500 MHz. Hybrid mode-locking, in addition to passive mode-locking, is demonstrated by adding a weak AC drive current with a frequency close to 500 MHz. This stabilizes the repetition rate and reduces the Gaussian component of the laser’s RF linewidth attaining a negligible Lorentzian component. Our numerical simulations predict that further lowering of the repetition rate and line spacings might be achievable with further cavity length extension.
We demonstrate the first on-chip laser frequency comb based on hybrid integration with low-loss Si3N4 waveguide circuits. The laser comprises an InP diode amplifier of which a small fraction is reverse biased for passive locking, while a Si3N4 feedback waveguide extends the optical cavity to a roundtrip length of 15 cm. The generated comb densely covers a 25 nm broad spectrum, at a 3 dB level, with more than 1600 comb-lines at 2 GHz spacing. With such properties, hybrid integrated diode lasers show great promise for widespread use in applications such as integrated microwave photonics or metrology.
R. van der Meer, B. Krishnan, I. Kozhevnikov, M. De Boer, B. Vratzov, H. M. Bastiaens, J. Huskens, W. van der Wiel, P. Hegeman, G. C. Brons, K.-J. Boller, F. Bijkerk
Lamellar Multilayer Gratings (LMG) offer improved resolution for soft-x-ray (SXR) monochromatization, while
maintaining a high reflection efficiency in comparison to conventional multilayer mirrors (MM). We previously used a
Coupled-Waves Approach (CWA) to calculate SXR diffraction by LMGs and identified a single-order regime in which
the incident wave only excites a single diffraction order. We showed that in this regime the angular width of the zeroth-order
diffraction peak simply scales linearly with Γ (lamel-to-period ratio) without loss of peak reflectivity. However,
the number of bi-layers must then be increased by a factor of 1/Γ. Optimal LMG resolution and reflectivity is obtained in
this single-order regime, requiring grating periods of only a few hundred nm, lamel widths < 100nm and lamel
heights > 1μm [1]. For the fabrication of LMGs with these dimensions, we use a novel process based on UV-NanoImprint
Lithography (UV-NIL) and Bosch-type Deep Reactive Ion Etching (DRIE). Successful fabrication of
LMGs with periods down to 200nm, line widths of 60nm and multilayer stack heights of 1μm is demonstrated. SXR
reflectivity measurements were performed on these LMGs at the PTB beamline at the BESSYII synchrotron facility. The
measurements demonstrate an improvement in resolution by a factor 3,5 compared to conventional MMs. Further
analysis of the SXR reflectivity measurements is currently being performed.
The net small-signal gain was measured in a discharge-pumped KrCl laser (222 nm) operated at high gas
pressures of around 3.3 bar. The pump power-density was varied between 200-650 kW cm-3. The experiments
were carried out on a three-electrode laser system. The net small-signal gain was measured in an oscillator-amplifier configuration and reached up to 1.4 % cm-1. Values for the small-signal gain g0 of 1.50 % cm-1, the
nonsaturable absorption &agr; of 0.07 % cm-1 and the saturation intensity Isat of 0.8 MW cm-3 were calculated for
a specific power-density of 320 kW cm-3.
The influence of electrode materials and surface roughness on the discharge homogeneity of F2 based excimer
laser gas mixtures is investigated in a small x-ray preionised discharge chamber. The temporal and spatial
evolution of the discharge is monitored by taking photographs of the discharge luminosity with an ICCD camera.
It is found that for the same surface roughness, discharges with nickel coated aluminum electrodes are more
homogeneous than the discharges using chromium or gold coated aluminum, massive copper, aluminum, brass
and steel electrodes. Moreover, the surface roughness of copper electrodes does not have a large influence on the
homogeneity of discharges in F2 doped excimer laser gas discharges.
The influence of the buffer gases on the discharge homogeneity of F2doped excimer laser gas mixtures is investigated in a small x-ray preionised high-pressure discharge chamber. The spatial and temporal development of discharges in He/F2 and Ne/F2 gas mixtures is monitored via its fluorescence using an intensified CCD camera with a gating time of 300 ps. The formation and development of discharge filaments in He/F2 gas mixtures is completely different from that in Ne/F2. Under the same start up conditions, discharges in Ne/F2 are dominated by cathode hotspots where as no hot spots are visible in discharges in He/F2 gas mixtures. However, the discharges in Ne/F2 gas mixtures appear to be more uniform even though the discharge width decreases in time and hotspots are persistent on the cathode surface.
Discharge instabilities in F2 based excimer gas lasers are investigated using a small-scale discharge system. After preionizing the gas volume, a fast rising voltage pulse initiates the discharge. The temporal development of the discharge is monitored via its fluorescence by an intensified CCD camera with a gating time of 10 ns. Homogeneous discharges are produced in gas mixtures of He/1mbar F2 and He/1mbar F2/30mbar Xe at a total pressure of 2 bar for pump pulse duratins up to 70 ns (FWHM). The addition of Xe to He/F2 mixture does not lead to discharge instabilities while the introduction of more F2 results in hotspot and filament formation.
High-pressure gas discharge experiments are carried out in a novel three-electrode prepulse-mainpulse configuration with two discharge volumes. The design is capable to break down both x-ray preionized volumes at the same time. First experiments in Xe/HCl/Ne mixtures reveal spatially very homogeneous discharges for up to 350 ns with a power deposition of 260 kW/cm3. For discharges in Kr/HCl/Ne mixtures we observe also very homogeneous discharges for a similar long pulse duration when a low krypton concentration of 10 mbar is used. For discharges in mixtures with a higher krypton concentration of 100 mbar we still observe very homogeneous discharges for 200 ns. Furthermore the total discharge current is established 8-10 times faster than in other systems.
For some specific application areas like (V)UV lithography or special processing of certain materials with high average power (V)UV lasers excimer lasers have to be developed further. In this contribution we will summarise the recent progress of our VUV excimer laser programme on the ArF and F2 laser. Key point in our research programme is the production of long laser pulses in the order of 100 ns (FWHM). An existing laser chamber was modified and optimised for the ArF laser research programme. Different excitation circuits have been tested. For long pulse operation the laser is operated in the ferrite switched resonant overshoot mode using 18 cm2 of ferrite in the switch for a discharge volume of approximately 60 x 0,7 x 1,2 cm3. Long optical pulses of up to 116 ns have been obtained with a lean gas mixture and a very low self-inductance of the electrical circuit of the packing capacitors. For the F2 laser a new discharge chamber and a new X-ray preionisation source was designed. With this set-up it was possible to produce a F2 laser with a large optical cross-section of 1,5 x 2,4 cm2 operating at an intrinsic efficiency of 0,1%. With a different electrical circuit the system produced long optical output pulses of 70 ns (FWHM) in a gas mixture of helium and 3 mbar F2 at a total gas pressure of 2 bar.
High energy extraction from UV pre-ionized molecular fluorine lasers is hampered by the short period of discharge stability and the spatial inhomogeneity of the discharge. So far stable discharge operation is reported for small discharge cross sections with an area less than 0.8 cm2 and efficiencies in the order of 0.15%. To increase the energy extraction of the molecular fluorine laser the discharge cross section should be enlarged. Here we report on the successful operation of a molecular fluorine laser with a large discharge cross section of 1.5 multiplied by 2.4 cm2 (electrode spacing times discharge width) which operates at a high intrinsic efficiency of 0.45%. Crucial for obtaining this result is the development of a short pulse, high intensity x-ray preionization source.
KEYWORDS: Pulsed laser operation, Gas lasers, Molecular lasers, Neon, Vacuum ultraviolet, High power lasers, Electrons, Electron beams, Diodes, Resonators
A long pulse molecular F2 laser ((lambda) equals 157 nm) with an optical pulse width of 160 ns and an output energy of 1.7 J (4.6 MW/cm2) pumped by an electron beam has been realized. The only restriction for the optical pulse width of the laser seems to be the duration of the excitation pulse. No signs of self terminating laser pulses due to bottle-necking in the lower laser level have been observed.
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