Low phase noise self-injection-locked diode laser with a high-Q fiber resonator: model and experiment: publisher's note.

This publisher's note contains a correction to Opt. Lett.49, 1933 (2024)10.1364/OL.514778.

Low phase noise self-injection-locked diode laser with a high-Q fiber resonator: model and experiment.

Low phase noise and narrow linewidth lasers are achieved by implementing self-injection locking of a DFB laser on two distinct fiber Fabry-Perot resonators. More than 45 dB improvement of the laser phase or frequency noise is observed when the laser is locked. In both cases, a frequency noise floor below 1 Hz2/Hz is measured. The integrated linewidth of the best of the two lasers is computed to be in the range of 400 Hz and appears to be dominated by vibration noise close to the carrier. The results are then compared with a model based on the retro-injected power and the Q factors ratio between the DFB laser and the resonator. This straightforward model facilitates the extraction of the theoretical performance of these sources close to the carrier, a characteristic still hidden by vibration noise.

Optimization of a fiber Fabry-Perot resonator for low-threshold modulation instability Kerr frequency combs.

We report a theoretical and experimental investigation of fiber Fabry-Perot cavities aimed at enhancing Kerr frequency comb generation. The modulation instability (MI) power threshold is derived from the linear stability analysis of a generalized Lugiato-Lefever equation. By combining this analysis with the concepts of power enhancement factor (PEF) and optimal coupling, we predict the ideal manufacturing parameters of fiber Fabry-Perot (FFP) cavities for the MI Kerr frequency comb generation. Our findings reveal a distinction between the optimal coupling for modulation instability and that of the cold cavity. Consequently, mirror reflectivity must be adjusted to suit the specific application. We verified the predictions of our theory by measuring the MI power threshold as a function of detuning for three different cavities.

Strategies for in-situ thin film filter monitoring with a broadband spectrometer.

Optical monitoring of thin film interference filters is of primary importance for two main reasons: possible error compensation and greater thickness accuracy of the deposited layers compared to non-optical methods. For many designs, the latter reason is the most crucial, because for complex designs with a large number of layers, several witness glasses are used for monitoring and error compensation with a classical monitoring approach is no longer possible for the whole filter. One optical monitoring technique that seems to maintain some form of error compensation, even when changing witness glass, is broadband optical monitoring, as it is possible to record the determined thicknesses as the layers are deposited and re-refine the target curves for remaining layers or recalculate the thicknesses of remaining layers. In addition, this method, if used properly, can, in some cases, provide greater accuracy for the thickness of deposited layers than monochromatic monitoring. In this paper, we discuss the process of determining a strategy for broadband monitoring with the goal of minimizing thickness errors for each layer of a given thin film design.

Fast interrogation wavelength tuning for all-optical photoacoustic imaging.

Optical detection of ultrasound for photoacoustic imaging provides a large bandwidth and high sensitivity at high acoustic frequencies. Therefore, higher spatial resolutions can be achieved using Fabry-Pérot cavity sensors than conventional piezoelectric detection. However, fabrication constraints during the deposition of the sensing polymer layer require precise control of the interrogation beam wavelength to provide optimal sensitivity. This is commonly achieved by employing slowly tunable narrowband lasers as interrogation sources, hence limiting the acquisition speed. We propose instead to use a broadband source and a fast-tunable acousto-optic filter to adjust the interrogation wavelength at each pixel within a few microseconds. We demonstrate the validity of this approach by performing photoacoustic imaging with a highly inhomogeneous Fabry-Pérot sensor.

Pulse duration dependent optical nonlinearities of Bi2Se3 thin films.

Topological insulators, such as the Bi2Se3 material, exhibit significant optical nonlinearities. This work investigates the impact of the pulse duration on the nonlinear optical responses of Bi2Se3 layers. Scanning electron microscopy studies have been performed to reveal the crystalline structure of the samples. The nonlinear optical performance has been investigated for a wide range of pulse durations, from 400 fs to 10 ps, using 1030 nm laser excitation. The nonlinear absorption coefficients recorded in this study range from -1.45 x10-7 m/W to -4.86 x10-7 m/W. The influence of two different mechanisms on optical nonlinearities was observed and discussed. Identical experimental conditions have been employed throughout the studies allowing a direct comparison of the results.

Impact of pump pulse duration on modulation instability Kerr frequency combs in fiber Fabry-Pérot resonators.

We report an experimental investigation on the impact of the pump pulse duration on the modulation instability process in fiber Fabry-Pérot resonators. We demonstrate that cross-phase modulation between the forward and the backward waves alters significantly the modulation instability process. By varying the pump pulse duration, we show the modification of the modulation instability threshold and frequency. These experimental observations are in excellent agreement with theoretical predictions.

Observation of modulation instability Kerr frequency combs in a fiber Fabry-Pérot resonator.

We report the experimental observation of a modulation instability induced Kerr frequency comb in an all fiber Fabry-Pérot resonator. We fully characterized, in intensity and phase, the frequency comb using a commercial 10 MHz resolution heterodyne detection system to reveal more than 125 comb teeth within each of the modulation instability sidelobes. Moreover, we were able to reveal the fine temporal structure in phase and intensity of the output Turing patterns. The experimental results are generally in good agreement with numerical simulations.

Design and fabrication of As2S3-based non-quarterwave multi-cavity Fabry-Perot filters with the laser-adjustable central wavelength.

We present a thorough study of the use of As2S3 thin films for the fabrication of high-performance multi-cavity bandpass filters. We show that such layers can be used inside a non-quarterwave multi-cavity Fabry-Perot structure to produce local changes of the central wavelength of the filter using photosensitive properties of this material. In particular, we study the impact of these index changes on the spectral performances of the filters and show how to adapt the design of the Fabry-Perot structures to produce a spectral shift without degrading the bandpass profile. Double- and three-cavity Fabry-Perot filters are theoretically and experimentally studied.

Multipass lock-in thermography for the study of optical coating absorption.

The development of high-power lasers requires optics with very low absorption to avoid detrimental thermal effects. In this work, we discuss our recent developments on the use of lock-in thermography to measure absorption. We apply this technique in a multipass configuration to increase the effective power on the tested samples. We present a system based on a kW-class ytterbium fiber laser operating at 1.07 µm wavelength, which enables exposing samples to 5 kW effective power and measuring absorption in the ppm range. The implementation, calibration procedure, and obtained performance are discussed with some applications to single-layer coatings of HfO2,Ta2O5,TiO2,Nb2O5, and SiO2 deposited by plasma-assisted electron beam deposition.

Automated optical monitoring wavelength selection for thin-film filters.

In this paper we study the wavelength selection process for optical monitoring of thin film filters. We first discuss the technical limitations of monitoring systems as well as the criteria defining the sensitivity of different wavelengths to thickness errors. We then present an approach that considers the best monitoring wavelength for each individual layer with a monitoring strategy selection process that can be fully automated. We finally validate experimentally the proposed approach on several optical filters of increasing complexity. Optical interference filters with close to theoretical performances are demonstrated.

Beam-size effects on the measurement of sub-picosecond intrinsic laser induced damage threshold of dielectric oxide coatings.

Laser-induced damage experiments on HfO2 and Nb2O5 thin films were performed with 500 fs pulse duration at 1030 nm wavelength. Threshold fluences as a function of beam size have been determined for effective beam diameters ranging from 40 to 220 µm, in a single shot regime. The results suggest no beam-size effect related to material properties in the investigated range, but size effects related to the metrology. The results indicate the importance of appropriate focusing conditions and beam measurement to qualify the optics for use in lasers with large beam sizes.

In-situ interferometric monitoring of optical coatings.

We present a new method for the in situ measurement of the amplitude and phase of the reflection coefficient of a plane substrate installed in a mechanical holder rotating at high speed (120 turns per minute) during the deposition of optical thin films. Our method is based on digital holography and uses a self-referenced scheme to cancel the effects of the severe constraints generated by the vibrational and thermal environment of the deposition machine.

Photosensitive chalcogenide metasurfaces supporting bound states in the continuum.

We study, both theoretically and experimentally, tunable metasurfaces supporting sharp Fano-resonances inspired by optical bound states in the continuum. We explore the use of arsenic trisulfide (a photosensitive chalcogenide glass) having optical properties which can be finely tuned by light absorption at the post-fabrication stage. We select the resonant wavelength of the metasurface corresponding to the energy below the arsenic trisulfide bandgap, and experimentally control the resonance spectral position via exposure to the light of energies above the bandgap.

Large aperture, highly uniform narrow bandpass Fabry-Perot filter using photosensitive As2S3 thin films.

We present a thorough study of the use of As2S3 thin films for the fabrication of high-performance bandpass filters. We show that these layers can be included inside a multilayer Fabry-Perot structure in order to produce large-aperture narrow bandpass filters. These layers can also exhibit a large local refractive index change up to 0.1 when they are exposed to actinic radiation at 470 nm. Kinetics of photosensitivity are presented. We show that these effects can then be implemented to locally modify the spectral performance of a Fabry-Perot filter after deposition. Highly uniform 65×60 mm2 filters with a 4 nm bandpass at 800 nm and ±0.1% fluctuations of the central wavelength are demonstrated using this technique.

Angularly tunable bandpass filter: design, fabrication, and characterization.

We present a thorough study of a bandpass filter for the near-infrared (IR) region, with bandwidth below 20 nm, high transmission, and a broad rejection band [350-1100] nm. This filter is angularly tunable between 0 and 50° and shows a shift of the central wavelength from 970 nm down to 880 nm, without major changes in its bandwidth and spectral shape for unpolarized light. We first provide a description of the design procedure and then carry out an experimental demonstration by using plasma-enhanced reactive magnetron sputtering. We finally present an accurate characterization of this class of filter by using a custom optical system.

High-performance thin-film optical filters with stress compensation.

We present a thorough description of high-performance thin-film optical filters with high flatness. These components can combine several tens or hundreds of layers and are manufactured using plasma-assisted reactive magnetron sputtering. Stress compensation is achieved using dual-side coatings with appropriate spectral function. Examples are described of highly reflecting mirrors at 515 nm with 15 nm flatness peak-to-valley up to and over 75 mm diameter aperture, narrow bandpass filters, and filters with broadband controlled transmission.

Trinary mappings: a tool for the determination of potential spectral paths for optical monitoring of optical interference filters.

An alternative new technique for the determination of efficient optical monitoring strategies of optical interference filters is presented. This technique relies on the analysis of optical monitoring signals for different optical monitoring wavelengths and the comparison of these signals with some pre-defined criteria in order to generate potential spectral paths compatible with a dedicated trigger point monitoring technique. Trinary mappings are then generated in order to determine possible optical monitoring strategies. This technique is finally implemented on various filter designs and experimentally validated.

Use of a broadband monitoring system for the determination of the optical constants of a dielectric bilayer.

This paper extends a method previously applied to the determination of the optical constants of a high-index thin film to a dielectric bilayer. This method is based on the time recording of the spectral transmittance of the stack during its deposition with the help of an in situ broadband monitoring system.

Coating stress analysis and compensation for iridium-based x-ray mirrors.

Iridium-based coatings for mirrors of x-ray telescopes are studied. In particular, stress-induced deformation is characterized and shown to be compressive and equal to -1786 MPa. Two methods for stress compensation are then studied. One relies on the deposition of silica on the back surface of the substrate and a second one relies on the deposition of a chromium sublayer. Advantages and drawbacks of each of these techniques are presented.