Femtosecond inscription of semi-aperiodic multi-notch fiber Bragg gratings using a phase mask.

We present an innovative concept of a semi-aperiodic phase mask design that enables the realization of multi-notch fiber Bragg gratings (FBG). This design utilizes the overlap and interference of near-infrared ultrashort laser pulses diffracted by short sequenced phase mask sections, which not only allows for a highly stable and reproducible inscription of a large number of wavelength filters but also paves the way towards full aperiodic phase masks. The semi-aperiodic FBG inscribed by this phase mask enables versatile notch filters showing multiple non-equidistant resonances. Those filters target applications, for instance in ground-based telescopes, where a large number of hydroxyl emission lines emitted in the upper atmosphere at near-infrared wavelengths restrict the observation of faint extraterrestrial objects.

Table-top high-energy 7 µm OPCPA and 260 mJ Ho:YLF pump laser.

We present a state-of-the-art compact high-energy mid-infrared (mid-IR) laser system for TW-level eight-cycle pulses at 7 µm. This system consists of an Er:Tm:Ho:fiber MOPA which serves as the seeder for a ZGP-based optical parametric chirped pulse amplification (OPCPA) chain, in addition to a Ho:YLF amplifier which is Tm:fiber pumped. Featuring all-optical synchronization, the system delivers 260 mJ pump energy at 2052 nm and 16 ps duration at 100 Hz with a stability of 0.8% rms over 20 min. We show that chirp inversion in the OPCPA chain leads to excellent energy extraction and aids in compression of the 7 µm pulses to eight optical cycles (188 fs) in bulk BaF2 with 93.5% efficiency. Using 21.7 mJ of the available pump energy, we generate 0.75 mJ energy pulses at 7 µm due to increased efficiency with a chirp inversion scheme. The pulse quality of the system's output is shown by generating high harmonics in ZnSe which span up to harmonic order 13 with excellent contrast. The combination of the passive carrier-envelope phase stable mid-IR seed pulses and the high-energy 2052 nm picosecond pulses makes this compact system a key enabling tool for the next generation of studies on extreme photonics, strong field physics, and table-top coherent X-ray science.

Watt-scale 50-MHz source of single-cycle waveform-stable pulses in the molecular fingerprint region.

We report a coherent mid-infrared (MIR) source with a combination of broad spectral coverage (6-18 µm), high repetition rate (50 MHz), and high average power (0.5 W). The waveform-stable pulses emerge via intrapulse difference-frequency generation (IPDFG) in a GaSe crystal, driven by a 30-W-average-power train of 32-fs pulses spectrally centered at 2 µm, delivered by a fiber-laser system. Electro-optic sampling (EOS) of the waveform-stable MIR waveforms reveals their single-cycle nature, confirming the excellent phase matching both of IPDFG and of EOS with 2-µm pulses in GaSe.

Mask aligner lithography using laser illumination for versatile pattern generation.

Mask aligner lithography is a well-established back-end fabrication process in microlithography. Within the last few years, resolution enhancement techniques have been transferred and adapted from projection lithography to further develop mask aligner lithography, especially concerning achievable resolution. Nonetheless, current technology using a mercury vapor lamp as a light source has reached its limits, e.g. for high-resolution pattering. Within this paper, we present the extension of the existing mask aligner illumination system by replacing the mercury vapor lamp with a solid-state laser. Full-field mask aligner lithography is guaranteed by a rotating diffuser expanding the laser beam and minimizing undesired speckle effects. An additional integrated galvanometer scanner allows a flexible choice of arbitrary angular spectrum distributions of the photomask illumination. We show versatile results like simple binary patterns of squares and triangles, as well as a more complex lateral shape like a blazed grating.

Resolution enhancement for advanced mask aligner lithography using phase-shifting photomasks.

The application of the phase-shift method allows a significant resolution enhancement for proximity lithography in mask aligners. Typically a resolution of 3 µm (half-pitch) at a proximity distance of 30 µm is achieved utilizing binary photomasks. By using an alternating aperture phase shift photomask (AAPSM), a resolution of 1.5 µm (half-pitch) for non-periodic lines and spaces pattern was demonstrated at 30 µm proximity gap. In a second attempt a diffractive photomask design for an elbow pattern having a half-pitch of 2 µm was developed with an iterative design algorithm. The photomask was fabricated by electron-beam lithography and consists of binary amplitude and phase levels.

Observation of the waveguide resonance in a periodically patterned high refractive index broadband antireflection coating.

Grating waveguide structures have been prepared by the deposition of a high refractive index broadband antireflection coating onto a patterned fused silica substrate. Aluminum oxide and hafnium oxide as well as mixtures thereof have been used as coating materials. Optical reflection measurements combined with atomic force microscopy have been used to characterize the structures. Upon illumination with a TE wave, the best structure shows a narrow reflection peak located at 633 nm at an incidence angle of about 17°. The peak reflectance of that sample accounts for more than 89%. Off-resonance interference structures appear strongly suppressed in the spectrum between 450 and 800 nm because of the characteristics of the designed antireflection layer. The structure thus possesses a notch filter spectral characteristic in a broad spectral range.

Multilevel blazed gratings in resonance domain: an alternative to the classical fabrication approach.

In this paper we present a novel technological approach for the fabrication of multilevel gratings in the resonance domain. A coded chromium mask is used to avoid alignment errors in electron beam lithography, which typically occur within the standard multistep binary micro-optics technology. The lateral features of all phase levels of the grating are encoded in a single chromium mask. The final profile of the structure is obtained by selective etching process for each level. This new technological method is applied for the fabrication of two different three-level gratings in resonance domain. The corresponding optical response as well as structural characterizations are presented and discussed. In particular, a first order diffraction efficiency of 90% is demonstrated for a grating period twice the wavelength at normal incidence.

Comparison of resonator-originated and external beam shaping.

The spatial shaping of laser beams is a subject of research in modern optics. Recently the introduction of diffractive elements in laser resonators has offered an alternative to external beam-shaping optics by mode shaping within the resonator. We describe the specification of the laser resonator mirrors to obtain by means of internal mode shaping a desired beam outside the resonator. Modal discrimination of the modified resonator and the mirror alignment sensitivity is discussed. Basic features of resonator-originated and external beam shaping are compared.

Polarization multiplexing of diffractive elements with metal-stripe grating pixels.

Diffractive elements with polarization multiplexing for the visible spectral region are demonstrated. The polarization-multiplexing property of the element is based on the polarization-dependent transmission characteristics of metal-stripe subwavelength period gratings. The proper dimensions of these gratings are estimated by rigorous calculations. The principle of polarization multiplexing by use of metal-stripe subwavelength period gratings is described for a diffractive element that has a binary amplitude transmission per polarization channel and is demonstrated by experimental results.