Chirped Mirror Research Articles

Broadband chirped mirrors with a porous top layer for smooth group delay dispersion.

The effectiveness of ultrashort pulse compression depends on the coatings group delay dispersion (GDD) characteristics in chirped mirror (CM)-based optical systems. Current porous-layer-based CMs with low GDD oscillations are limited to less than half optical octave spectral bandwidth and the stability of their spectral parameters is still unknown. In order to address these limitations, a broadband CM (covering 400 nm bandwidth) with a porous top layer was deposited and spectral parameters were measured at different environmental conditions. The modeled refractive index of the porous layer varied by approximately 1% due to the changes in humidity. Deposited coatings exhibited low sensitivity of GDD oscillations to deposition errors of porous layer and humidity in the surrounding environment.

Spectral broadening and pulse compression of a 400 μJ, 20 W Yb:KGW laser using a multi-plate medium

We investigate the potential of a multi-plate medium consisting of thin fused silica plates to generate few-cycle pulses from a moderately high energy (400 μJ) and average power (20 W) Yb:KGW laser centered at 1025 nm. By optimizing the thicknesses and positions of the plates, we mitigate the losses associated with spatial and spectral distortions that can accompany self-focusing in bulk solids. Pulses with an initial duration of ∼280 fs were compressed using chirped mirrors, after spectral broadening in a multi-plate medium consisting of 6 mm of fused silica in total, by a factor of >5 to 50 fs. Further spectral broadening in a second stage also consisting of 6 mm of fused silica in total enabled compression to 18 fs with 40 μJ pulse energy, with the energy efficiency limited primarily by the geometry of the chirped mirror compressors.

Designing chirped aperiodically poled structures for high-energy single-cycle terahertz generation

We introduce a collinear scheme for the highly efficient generation of broadband single or few-cycle high-power terahertz (THz) pulses using optical rectification. For this purpose, two concepts are introduced and thoroughly analyzed. The first concept is the generation of chirped broadband terahertz pulses using chirped aperiodically poled electro-optic crystals. The second concept involves the compression of the terahertz pulses extracted from the aperiodically poled structure using a chirped mirror. An illustrative design employing cryogenically cooled aperiodically poled lithium niobate crystals and the appropriate chirped mirrors is presented. It is shown that the presented design allows the optical pulse to be re-used in subsequent generation stages, resulting in optical-to-THz conversion efficiencies in excess of 5% for terahertz radiation centered around ∼0.3 THz. Analytic solutions and numerical calculations are presented. In order to design a crystal with the optimum conversion efficiency, we take advantage of binary optimization techniques. This work paves the way for the generation of broadband terahertz radiation with several mJ of pulse energy using large-area aperiodically poled structures.

Flat focusing in reflection from a chirped dielectric mirror with a defect layer

Recently, the principle of flat focusing based on one-dimensionally chirped dielectric mirrors has been proposed and experimentally demonstrated. The flat chirped mirror causes anomalous diffraction of the beam during reflection, opposite to the normal diffraction in free space propagation. The anomalous diffraction compensates the normal diffraction of the beam resulting in focusing after reflection. For a better focusing performance and for a larger near-field focal distance, a stronger anomalous diffraction is required. We show that the anomalous diffraction can be enhanced by introducing a defect layer in the chirped mirror, as the structure becomes similar to a Gires–Tournois interferometer. The focal distance can be substantially increased due to the defect layer. In our specific structure, the focal distance shows an increase from 19 to 39 μm, numerically.

Two-octave-spanning dispersion-controlled precision optics for sub-optical-cycle waveform synthesizers

Two-octave-spanning precision dispersive mirror systems are demonstrated, providing a new enabling technology for pulse-energy and bandwidth scaling of sub-cycle optical waveform synthesizers. We propose and characterize new dispersion management schemes with advanced dielectric coating designs. Based on an analytic dual adiabatic matching structure, we implement a chirped dichroic mirror, to efficiently optimize the beam combining from different spectra, and a double-chirped mirror pair, to avoid unwanted nonlinearity during beam propagation, with custom-tailored dispersion and reflectivity over more than two octaves of bandwidth ranging from 0.49 to 2.3 μm, supporting 1.9-fs-short sub-optical-cycle pulses. The multilayer coating structures can also be applied to the design of chirped-fiber Bragg gratings and general optical filters. The proposed designs and schemes will benefit ultrabroadband applications requiring precise dispersion management, especially enabling the generation of intense sub-optical-cycle light transients.

Thermal-dynamical analysis of blister formation in chirped mirror irradiated by single femtosecond lasers.

The laser-induced damage behaviors of chirped mirrors (CMs) are studied by single 800 nm, 38 fs lasers. The CMs provide group delay dispersion of around -60 fs² and average reflectivity of about 99.4% with bandwidth range of 200-300 nm at a central wavelength of 800 nm. Interestingly, a circular blister feature appears in the CMs at a wide range of laser fluence. An optical microscope, atomic force microscope, scanning electron microscope, and surface profiler are applied to describe the blister characteristics. An adiabatic expansion model of ideal gas is adopted to illustrate the formation dynamics of blisters. The evolution of blisters can be explained by partial evaporation of the film and a subsequent gas expansion, driving the bulging of the film stack up to the stress limit, where the blister fractures. According to this model, the energy absorption ratio of blisters increases monotonously with increasing laser fluence before the occurrence of the focal spot confinement effect.

Compression of long-cavity Ti:sapphire oscillator pulses with large-mode-area photonic crystal fibers

Motivated by the pulse compression challenge of novel long-cavity, high-pulse-energy Ti:sapphire laser oscillators, we report on ~280 nm supercontinuum generation and 4.5-times compression of close to transform limited, high-energy oscillator pulses using different large-mode-area photonic crystal fibers and standard chirped mirrors. As input, we used pulses of a long-cavity Ti:sapphire oscillator with 190 nJ pulse energy, 70 fs pulse length and 3.6 MHz repetition rate. Compressed pulses at the fiber/compressor output had a duration of 15–18 fs with up to 100 nJ pulse energy representing as much as 53 % throughput for the fiber/chirped mirror system. Using transform-limited input pulses, we could use short fiber pieces and thus a simple, low-dispersion chirped mirror compressor comprised of one pair of mirrors.

Simultaneous compression and characterization of ultrashort laser pulses using chirped mirrors and glass wedges

We present a simple and robust technique to retrieve the phase of ultrashort laser pulses, based on a chirped mirror and glass wedges compressor. It uses the compression system itself as a diagnostic tool, thereby making unnecessary the use of complementary diagnostic tools. We used this technique to compress and characterize 7.1 fs laser pulses from an ultrafast laser oscillator.

Group delay dispersion measurement of a dispersive mirror by spectral interferometry: comparison of different signal processing algorithms

We built a dispersive white-light spectral interferometer for precisely measuring the dispersion properties of a multilayer thin-film structure. A novel algorithm with improved robustness to measurement errors is presented by combining a windowed Fourier transformation with wavelet-based differentiation. Compared with previously published algorithms, this method shows substantial resistance to measurement errors. The group delay dispersion properties of bulk materials and a homemade chirped mirror are measured by our apparatus, and the measurement result manifests considerable accuracy and robustness. The technique shows reasonable potential for the characterization of ultrabroadband chirped mirrors.

Design and Fabrication of Chirped Mirror

Chirped mirrors (CMs) are designed and manufactured. The optimized CM provides a group delay dispersion (GDD) of around –60 fs2 and average reflectivity of 99.4% with bandwidth 200 nm at a central wavelength of 800 nm. The CM structure consists of 52 layers of alternating high refractive index Ta2O5 and low refractive index SiO2. Measurement results show that the control of CM manufacturing accuracy can meet our requirement through time control with ion beam sputtering. Because the GDD of CMs is highly sensitive to small discrepancies between the layer thickness of calculated design and those of the manufactured mirror, we analyze the error sources which result in thickness errors and refractive index inhomogeneities in film manufacture.

Chirp compression with single chirped mirrors and its assembly

Chirped mirrors are widely used in ultrafast optics as broadband devices for dispersion compensation. Time domain analysis in contrast to frequency domain one allows the direct investigation of pulse profile changing during chirp compression. We model the chirped pulse compression with single chirped mirror and assembly of similar mirrors. Our analysis shows there is similar effect of chirp compensation in both cases. However, injurious group delay oscillations in the single mirror strongly decrease pulse quality in assembly of mirrors and can even damage pulse waveform. Angle of incidence and polarization also play crucial role and should be taken into account.

Chirped-cavity dispersion-compensation filter design

A new basic structure of a dispersive-compensation filter, called a chirped-cavity dispersion-compensator (CCDC) filter, was designed to offer the advantages of small ripples in both reflectance and group-delay dispersion (GDD). This filter provides a high dispersion compensation, like the Gires-Tournois interferometer (GTI) filter, and a wide working bandwidth, like the chirped mirror (CM). The structure of the CCDC is a cavity-type Fabry-Perot filter with a spacer layer (2 mH or 2 mL) and a chirped high reflector. The CCDC filter can provide a negative GDD of -50 fs2 over a bandwidth of 56 THz with half the optical thickness of the CM or the GTI.

Dispersion-Controlled Mirrors to Facilitate Fabrication of Ultra-Short Pulse Ti:Sapphire Oscillator

We designed a chirped mirror with layers having optical thicknesses greater than 100 nm and potential dispersion for a sub-6-fs Ti:sapphire oscillator. The novel design process facilitates fabrication. The dispersion characteristics can be reduced by utilizing three materials for the mirror. Our calculations indicate that fused silica, CaF2, and BK7 glass are candidate prism materials for intracavity dispersion compensation for such an oscillator combined with our designed chirped mirrors.

High-power sub-10-fs Ti:sapphire oscillators

The state of the art of mirror-dispersion-controlled (MDC) Ti:sapphire laser oscillators is reviewed. Owing to improvements in the cavity and mirror design, these systems can now routinely generate sub-10-fs pulses with peak powers exceeding the megawatt level. The unique compactness of MDC Ti:sapphire oscillators results in excellent noise characteristics, a nearly diffraction limited output, and a high reproducibility of performance. Employing a diode-pumped solid-state laser as a pump source allows the generation of sub-10-fs pulses from an all-solid-state laser for the first time. PACS: 42.55.Rz; 42.60.Mi; 42.65.Re Titanium-doped sapphire lasers [1] generating ultrashort pulses by Kerr-lens mode locking (KLM) [2–7] are now widely used for time-resolved studies in physics, chemistry, biology, and electronics, as well as for seeding high-power solid-state amplifier systems. After its first demonstration by Spence and co-workers in 1990 [2], the performance of KLM Ti:sapphire oscillators had been subject to a rapid progress [8], which temporarily culminated in the development of fused-silica-prism-controlled systems [9]. When detuned to ≈ 850 nm, these systems have been capable of generating pulses of around 10 fs or slightly shorter in duration [10, 11], which were limited by the fourth-order dispersion introduced by the prisms [12]. This performance came at the expense of a large (> 0.6) time–bandwidth product and a significant red shift of the spectrum from the gain peak of Ti:sapphire, impairing the suitability of this broadband output for seeding Ti:S amplifiers. With the advent of dispersion-engineered chirped multilayer dielectric mirrors [13] a new generation of mirrordispersion-controlled (MDC) Ti:sapphire lasers has been developed [14–17] which were able to overcome the limitations inherent in prism-controlled oscillators. In MDC oscillators the mirrors not only provide feedback but also introduce broadband negative dispersion indispensable for soliton-like pulse formation [18]. Hence they obviate the need for intracavity prisms, allowing the construction of femtosecond Ti:S oscillators containing no intracavity components other than the gain medium (and an optional aperture). This unprecedented simplicity and compactness come in combination with a reliable sub-10-fs performance in the 800-nm wavelength range. Owing to these unique features, sub-10-fs MDC Ti:sapphire oscillators are likely to become an important workhorse for a number of application fields, particularly where high time resolution, high peak power, solid-state ruggedness, and reliability are important. In this paper, we shall report on recent progress in MDC Ti:S oscillator technology, which has permitted the generation of femtosecond pulses with peak powers exceeding 1 MW for the first time directly from a laser oscillator. As this output is delivered in a nearly diffraction-limited beam, these sub-10-fs pulses are expected to be focusable to intensities well beyond the terawatt/cm2 level. This performance is unprecedented in a cw mode-locked laser. Yet higher peak powers can be achieved by cavity dumping [19, 20] or external amplification [21] at reduced repetition rates. The advanced sub-10-fs oscillator technology is the result of recent innovations in the design and optimization of KLM oscillators as well as of progress in chirped mirrors technology [22]. In what follows we shall focus on the laser design; details about the chirped mirror characteristics will be published elsewhere. After considering some general guidelines for optimizing KLM oscillators for maximum output pulse energy, the results of the optimization of MDC oscillators Kerr-lens mode-locked by using hard and soft apertures will be presented. The pulses delivered by the oscillators have been characterized temporally, spectrally, spatially (beam profile, M2), as well as in terms of their energy noise and timing jitter. Finally, the major characteristics of an allsolid-state sub-10-fs laser will be briefly summarized. 1 Design considerations for high-power KLM oscillators In femtosecond solid-state lasers a soliton-like interplay between self-phase modulation (SPM) and negative group delay dispersion (GDD) dominates the formation of an ultrashort pulse in the laser cavity. The separated action of SPM and GDD introduces a periodic perturbation to the soliton-like