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We present a new design of a robust cavity-enhanced frequency comb-based spectrometer operating under the continuous-filtering Vernier principle. The spectrometer is based on a compact femtosecond Er-doped fiber laser, a medium finesse cavity, a diffraction grating, a custom-made moving aperture, and two photodetectors. The new design removes the requirement for high-bandwidth active stabilization present in the previous implementations of the technique, and allows scan rates up to 100 Hz. We demonstrate the spectrometer performance over a wide spectral range by detecting CO2 around 1575 nm (1.7 THz bandwidth and 6 GHz resolution) and CH4 around 1650 nm (2.7 THz bandwidth and 13 GHz resolution). We achieve absorption sensitivity of 5 × 10−9 cm-1 Hz-1/2 at 1575 nm, and 1 × 10−7 cm-1 Hz-1/2 cm-1 at 1650 nm. We discuss the influence of the scanning speed above the adiabatic limit on the amplitude of the absorption signal.

BACKGROUND Noninvasive assessment of metabolic processes that sustain regeneration of human retinal visual pigments (visual cycle) is essential to improve ophthalmic diagnostics and to accelerate development of new treatments to counter retinal diseases. Fluorescent vitamin A derivatives, which are the chemical intermediates of these processes, are highly sensitive to UV light; thus, safe analyses of these processes in humans are currently beyond the reach of even the most modern ocular imaging modalities. METHODS We present a compact fluorescence scanning laser ophthalmoscope (TPEF-SLO) and spectrally resolved images of the human retina based on two-photon excitation (TPE) with near-infrared (IR) light. A custom Er:fiber laser with integrated pulse selection, along with intelligent post-processing of data, enables excitation with low laser power and precise measurement of weak signals. RESULTS We demonstrate spectrally resolved TPE fundus images of human subjects. Comparison of TPE data between human and mouse models of retinal diseases revealed similarity with mouse models that rapidly accumulate bisretinoid condensation products. Thus, visual cycle intermediates and toxic byproducts of this metabolic pathway can be measured and quantified by TPE imaging. CONCLUSION Our work establishes a TPE instrument and measurement method for noninvasive metabolic assessment of the human retina. This approach opens the possibility for monitoring eye diseases in the earliest stages before structural damage to the retina occurs. FUNDING NIH, Research to Prevent Blindness, Foundation for Polish Science, European Regional Development Fund, Polish National Agency for Academic Exchange and Polish Ministry of Science and Higher Education.

We report a study on pulse dynamics in figure-nine Tm-doped all-polarization maintaining fiber laser. We analyzed laser operation from self-starting with multi-pulse dynamic to single-pulse operation by decreasing the pump power from the mode-locking threshold. By choosing a reliable setting of waveplates, our laser was generating pulses at the central wavelength of 1985 nm with a half-width of the spectrum, pulse duration, and pulse energy equal 6.4 nm, 650 fs, 177 pJ for the output port and 19.2 nm, 1279 fs, 57 pJ for the reject port in the single-pulse state. In the multi-pulse state, we recorded optical spectra, temporal waveforms, and average power at both exit ports. By analyzing temporal traces and output to reject port ratio power, we can distinguish between eight states of operation which follow an exact pattern. In the case of the single-pulse regime, we performed a further laser characterization, including relative intensity noise.

We use a 3.3 µm high-power continuous wave pump and a 1.67 µm comb probe to detect transitions in the 3ν3 ← ν3 range of methane with sub-Doppler resolution over 6 THz of bandwidth. We achieve high absorption sensitivity for the comb probe using an enhancement cavity and a Fourier transform spectrometer with auto-balanced detection.

Mid-infrared comb-based Fourier transform spectroscopy allows high-resolution measurements of entire molecular absorption bands. However, most previous implementations were limited to wavelengths

Management of the intra-cavity net dispersion can be used as an optimization method of pulse energy and duration during the design phase of a sub-picosecond laser source. The dispersive regime is one of the key-factors affecting such aspects of laser operation as a pulse formation mechanism, a pulse shape, and a pulse train stability [1] , [2] . Another critical element in the construction of a femtosecond laser is the proper choice of a saturable absorber. The usage of a nonlinear amplifying loop mirror (NALM) equipped with a non-reciprocal phase shifter (NRPS) allows modeling of saturable absorber transmission characteristics in a broad range of the parameter space [3] . The integration of the two components described above allows for the development of a platform capable of a continuous and flexible change of the operating conditions of an ultrafast laser system.

International audience; Optical-optical double resonance (OODR) spectroscopy is a powerful tool for the experimental assignment of highly-excited molecular states, which in turn is needed for verification of the accuracy of theoretical predictions of high-temperature spectra observed in exoplanets and in combustion environments. Previous implementations of OODR used either continuous wave (cw) lasers, which limit the number of transitions that can be detected, or pulsed lasers, which limit the spectral resolution. Recently, we demonstrated OODR with a cw pump and a frequency comb probe and applied it to the detection and assignment of methane transitions in the 3ν 3 ← ν 3 range with sub-Doppler resolution over 200 cm -1 of bandwidth [1]. The pump [see Fig. 1(a) ] was a 1 W 3.3m idler of a cw optical parametric oscillator (cw-OPO), stabilized to the Lamb dip in a selected CH 4 transition in the ν 3 band using a signal from a reference cell. The probe was an amplified fully-stabilized Er:fiber comb ( f rep = 250 MHz), whose center wavelength was shifted to 1.67m using a soliton self-frequency shift fiber (SSSF). The sample of pure CH 4 was contained in an 80-cm-long single-pass cell cooled by liquid nitrogen. The probe spectra were detected using a Fourier transform spectrometer (FTS) with comb-mode-limited resolution [2] , and the final interleaved spectra had 2 MHz sampling point spacing. Figure 1(b) shows the 3ν 3 ← ν 3 R(1) transition at 6046.36008(5) cm -1 , detected with the pump on the ν 3 R(0) line. We measured, fit and assigned 36 probe transitions with the pump tuned to 9 different transitions. Figure 1(d) shows a comparison of the probe transition wavenumbers to predictions from the TheoReTS database [3] , demonstrating agreement within 1 cm -1. © 2021 IEEE.

Cavity ring-down spectroscopy (CRDS) based on broadband light sources offer the high sensitivity of CRDS with the advantages of a multiplex acquisition. Previous works have explored the possibility of analysing the broadband cavity decay using time-resolved FTIR and a pulsed dye laser [1] , and using a virtual image phased array combined with a frequency comb source [2] . Here, we exploit the ability to lock the optical frequency comb to the ring-down cavity to ensure a high throughput for the time-resolved Fourier transform spectrometer.

Since the first demonstration of high-precision distance measurements based on two optical frequency combs [1] , simplifying the measurement scheme has been an ongoing effort. Compared to established time-off-light techniques, the precision of dual-comb-ranging (DC-R) is increased by a scaling factor f rep,2 /Δ f rep ; where f rep,2 is the repetition rate of the comb used for the distance measurement, and Δ f rep is the difference in repetition rates between the two combs, i.e. | f rep,1 - f rep,2 |. However, for state-of-the-art DC-R, two phase-stable combs are needed and the measurement becomes ambiguous if the distance L is longer than c/ f rep , which is typically a few meters only [1] . It has been demonstrated that by using a single-cavity dual-comb, the complexity of the measurement setup can be decreased [2] , [3] . However, the short non-ambiguity range is still a problem limiting the applications of DC-R. Different methods have been presented to extend the non-ambiguity range [2] , [3] , e.g. by using the Vernier effect or by introducing an additional reference arm, which however increases the complexity of the measurement.

We report the generation of coherent octave-spanning supercontinuum in an all-fiber system, without any free-space optical components. The setup uses the femtosecond fiber laser as a pump and an all-normal dispersion microstructured fiber as a medium for supercontinuum generation. The generated spectrum is characterized both experimentally and numerically and shows a broad bandwidth (1.1−2.2 μm), a high signal to noise ratio reaching 100 at maximum, a high coherence (closing to 1), linear polarization and average output power up to 57 mW. The source is characterized by exceptional simplicity and does not require any alignment (the nonlinear fiber is spliced to the pump) which finally opens the path to outside-lab applications of supercontinuum radiation.

We implement a cavity to enhance the absorption of a frequency comb probe in a double-resonance measurement of sub-Doppler 3ν3 ← ν3 methane transitions. This yields two orders of magnitude better sensitivity in 15 times shorter acquisition time compared to previous work using a single-pass cell. © 2021 The Authors OCIS codes: 300.1030, 300.6190, 300.6320, 300.6390.

We report the development of a widely-tunable femtosecond fiber laser system and its application for two-photon vision studies. The source is based on an Er-doped fiber laser with spectral shift up to 2150 nm, followed by a second harmonic generation module to generate a frequency-doubled beam tunable from 872 to 1075 nm. The source delivers sub-230 fs pulses with nearly-constant duration over the entire tuning range, with output powers between 0.68–1.24 mW, which corresponds to a pulse energy of 13.2–24.1 pJ. Such pulse energy is sufficient for employing a system for measurements of two-photon scotopic spectral sensitivity of two-photon vision in humans. The laser parameters allow for very efficient and safe two-photon stimulation of the human visual system, as proved by a good separation between one- and two-photon thresholds for wavelengths below 950 nm, which we have confirmed for 3 healthy subjects.

Optical-optical double-resonance spectroscopy with a continuous wave pump and frequency comb probe allows measurement of sub-Doppler transitions to highly excited molecular states over a wide spectral range with high frequency accuracy. We report on assessment and characterization of sub-Doppler double-resonance transitions in methane measured using a 3.3-$\ensuremath{\mu}\mathrm{m}$ continuous wave optical parametric oscillator as a pump and a 1.67-$\ensuremath{\mu}\mathrm{m}$ frequency comb as a probe. The comb spectra were recorded using a Fourier transform spectrometer with comb-mode-limited resolution. With the pump tuned to nine different transitions in the ${\ensuremath{\nu}}_{3}$ fundamental band, we detected 36 ladder-type transitions to the $3{\ensuremath{\nu}}_{3}$ overtone band region, and 18 V-type transitions to the $2{\ensuremath{\nu}}_{3}$ overtone band. We describe in detail the experimental approach and the pump stabilization scheme, which currently limits the frequency accuracy of the measurement. We present the data analysis procedure used to extract the frequencies and intensities of the probe transitions for parallel and perpendicular relative pump-probe polarization. We compare the center frequencies and relative intensities of the ladder-type transitions to theoretical predictions from the TheoReTS and ExoMol line lists, demonstrating good agreement with TheoReTS.

Using a compact fiber-based difference frequency generation comb and a Fourier transform spectrometer we record spectra of the N2O ν1 band at 1285 cm-1 in the Doppler limit. Fitting Gaussian line shapes to the individual absorption lines yields center frequencies with

We use a Fourier transform spectrometer based on a compact mid-infrared difference frequency generation comb source to perform broadband high-resolution measurements of nitrous oxide, $^{14}$N$_2$$^{16}$O, and retrieve line center frequencies of the $\nu$$_1$ fundamental band and the $\nu$$_1$ + $\nu$$_2$ - $\nu$$_2$ hot band. The spectrum spans 90 cm$^{-1}$ around 1285 cm$^{-1}$ with a sample point spacing of 3 ${\times}$ 10$^{-4}$ cm$^{-1}$ (9 MHz). We report line positions of 72 lines in the $\nu$$_1$ fundamental band between P(37) and R(38), and 112 lines in the $\nu$$_1$ + $\nu$$_2$ - $\nu$$_2$ hot band (split into two components with e/f rotationless parity) between P(34) and R(33), with uncertainties in the range of 90-600 kHz. We derive upper state constants of both bands from a fit of the effective ro-vibrational Hamiltonian to the line center positions. For the fundamental band, we observe excellent agreement in the retrieved line positions and upper state constants with those reported in a recent study by AlSaif et al. using a comb-referenced quantum cascade laser [J Quant Spectrosc Radiat Transf, 2018;211:172-178]. We determine the origin of the hot band with precision one order of magnitude better than previous work based on FTIR measurements by Toth [http://mark4sun.jpl.nasa.gov/n2o.html], which is the source of the HITRAN2016 data for these bands.

We use offset-frequency-free difference frequency generation comb sources and a Fourier transform spectrometer with comb-mode-width limited resolution to measure and analyze spectra of molecular species of atmospheric relevance: CH3I and CH2Br2 around 3000 cm-1, and 14N 162O around 1280 cm-1.

We report an investigation of dispersion management of an all-polarization-maintaining Er-fiber oscillator mode-locked via a nonlinear amplification loop mirror in a figure-nine cavity configuration with two output ports. The performance of the laser was investigated within the net cavity dispersion ranging from −0.034 ps2 to +0.006 ps2. We show that the spectral and temporal phase of the pulses at both figure-nine outputs have clearly different characteristics. One of the laser outputs provides pulses with significantly better quality; nonetheless, the rejection output also offers ultrashort pulses with broad spectra. Pulses as short as 79 fs with an energy of 83 pJ were generated directly from the laser in the near-zero dispersion regime.

We report the design and characteristics of a simple and compact mode-locked Er-doped fiber laser and its application to broadband cavity-enhanced spectroscopy. The graphene mode-locked polarization maintaining oscillator consumes less than 5 W of power. It is thermally stabilized, enclosed in a 3D printed box, and equipped with three actuators that control the repetition rate: fast and slow fiber stretchers, and metal-coated fiber section. This allows wide tuning of the repetition rate and its stabilization to an external reference source. The applicability of the laser to molecular spectroscopy is demonstrated by detecting CO2 in air using continuous-filtering Vernier spectroscopy with absorption sensitivity of 5.5 × 10−8 cm−1 in 50 ms.

A femtosecond frequency-doubled erbium-doped fiber laser with an adjustable pulse repetition rate is developed and applied in two-photon excited fluorescence microscopy. The all-fiber laser system provides the fundamental pulse at 1560 nm wavelength with 22 fs duration for the second harmonic generation, resulting in 1.35 nJ, 60 fs pulses at 780 nm. The repetition rate is adjusted by a pulse picker unit built-in within the amplifier chain, directly providing transform-limited pulses for any chosen repetition rate between 1 and 12 MHz. We employed the laser source to drive a scanning two-photon excited fluorescence microscope for ex vivo rat skin and other samples’ imaging at various pulse repetition rates. Due to compactness, ease of operation, and suitable pulse characteristics, the laser source can be considered as an attractive alternative for Ti:Sapphire laser in biomedical imaging.

Laboratory astrophysics aims at studying species and processes occurring in the interstellar medium and planetaiy atmospheres. Cavity ring-down spectroscopy combined with optical frequency comb Fourier transform spectroscopy techniques will be introduced.

We report a compact, actively stabilized fiber-based mid-infrared frequency comb source covering the 6.5 – 9 μm wavelength range with up to 5 mW of average output power at 125 MHz repetition frequency.

We present a robust cavity-enhanced comb-based spectrometer with a 6.6 GHz resolution and 60 Hz acquisition rate, based on the continuous-filtering Vernier principle, a fixed diffraction grating, a custom-made chopper wheel, and a low-bandwidth comb-cavity stabilization scheme. We measure a CO2 spectrum with 5 x 10-8 cm-1 sensitivity in 17 ms.

We present a new robust approach to cavity-enhanced comb spectroscopy based on Vernier filtering, a fixed diffraction grating, custom-made chopper wheel, and a low bandwidth comb-cavity stabilization scheme. We measure a CO2 spectrum with a few GHz resolution and 5 x 10-8 cm-1 sensitivity in 9.4 ms.

We report the measurement of sub-Doppler double-resonance transitions in methane using a 3.3 $\mu$m continuous wave optical parametric oscillator to pump transitions in the fundamental $\nu_3$ band and a 1.67 $\mu$m frequency comb to probe ladder-type transitions in the 3$\nu_3$ band over 200 cm$^{-1}$ of bandwidth. We detected 36 ladder-type resonances for 9 pumped transitions with average center frequency accuracy of 1.7 MHz, limited by the pump frequency stability. The lines are assigned using the TheoReTS line list, and the intensity ratios for different relative pump/probe polarizations. This method provides accurate assignment of highly excited energy levels that cannot be done using high temperature spectra.

Dual-comb (DC) ranging is an established method for high-precision and high-accuracy distance measurements. It is, however, restricted by an inherent length ambiguity and the requirement for complex control loops for comb stabilization. Here, we present a simple approach for expanding the ambiguity-free measurement length of DC ranging by exploiting the intrinsic intensity modulation of a single-cavity dual-color DC for simultaneous time-of-flight and DC distance measurements. This measurement approach enables the measurement of distances up to several hundred kilometers with the precision and accuracy of a DC interferometric setup while providing a high data acquisition rate ( ≈ 2 k H z ) and requiring only the repetition rate of one of the combs to be stabilized.

A compact and robust all-fiber difference frequency generation-based source of broadband mid-infrared radiation is presented. The source emits tunable radiation in the range between 6.5 µm and 9 µm with an average output power up to 5 mW at 125 MHz repetition frequency. The all-in-fiber construction of the source along with active stabilization techniques results in long-term repetition rate stability of 3 Hz per 10 h and a standard deviation of the output power better than 0.8% per 1 h. The applicability of the presented source to laser spectroscopy is demonstrated by measuring the absorption spectrum of nitrous oxide (N2O) around 7.8 µm. The robustness and good long- and short-term stability of the source opens up for applications outside the laboratory.

In recent years, optical frequency combs are gaining attention in many fields of science, e.g. frequency metrology, optical clocks, and high-resolution spectroscopy. Most sophisticated comb sources can be repetition rate stabilized down to 10−16 uncertainty in 1 s averaging time [1] (bounded by the stability of the reference clock). Therefore compact, low-cost and energy efficient fiber-based sources are highly demanded on the market [2]. Accurate thermal stabilization of the fiber laser cavity is the first step to obtain stable repetition frequency, which is critical for optical frequency comb spectroscopy schemes [3]. Moreover, cavity-enhanced spectroscopic techniques like Vernier spectroscopy requires additional detuning of the comb repetition rate from the cavity mode-spacing [4].

Recent decade brought a tremendous progress laser-based gas detection techniques. This progress would not be possible without the relentless devotion of the scientific community in developing state-of-the-art sources capable of targeting strong absorption lines of gases localized in the mid-infrared (mid-IR) wavelength region. Although narrowband, semiconductor-based mid-IR lasers offer non-complex implementation their tuning range is usually limited to several cm−1. Therefore designing a sensor capable of simultaneous monitoring of several gas species, even with absorption profiles clustered nearby, requires incorporating individual expensive quantum cascade laser (QCL) or interband cascade laser (ICL) sources into a single platform. Recent development of reliable mid-IR comb sources, combined with clever methods of signal processing allowed for designing gas sensors capable of simultaneous monitoring of several gas analytes in fractions of a second [1]. Obtaining broadband pulsed radiation in the mid-IR region is usually accomplished by utilizing nonlinear conversion or in recently presented on-chip QCL combs [2]. Although QCL combs offer compactness and efficiency they do not allow straightforward control of the repetition frequency (f rep ) or the carrier-envelope offset frequency (f CEo ), which are critical in some particular cavity-enhanced comb-based sensing techniques, e.g. Vernier spectroscopy [3]. Here we present a fully-fiberized, f CEO -free mid-IR optical frequency comb source capable of covering 6–9 μm wavelength region with tunable f rep . The setup of the source is presented in Fig 1. The heart of the source is a ring-cavity graphene mode-locked laser generating sub-250 fs pulses at 1560 nm and f rep of 100 MHz. The pulses are split using a 50/50 coupler and directed to separate sections of the source. In the blue section the source the pulses are amplified to 80 mW in an all-fiber-based chirped pulse amplifier (CPA) and recompressed in a 8 m-long section of PM1550 fiber to ∼200 fs. In the red section a custom manufactured nonlinear fiber with tailored dispersion (PCF) is utilized to soliton-shift the 1.55 μm pulses into the 1.9–2 μm wavelength region. Subsequently the pulses are amplified in a CPA and recompressed to ∼200 fs. Combined 1.55 and 2 μm pulses are focused on a 3 mm-long orientation patterned gallium phosphide (OP-GaP) crystal to generate mid-IR radiation via differential frequency generation (DFG). Since both pulses originated from the same ML source, the generated mid-IR frequency comb is inherently fceo-free and can be used in laser-based gas sensors requiring stable carrier-envelope offset. The source is built entirely from PM single-mode fibers.

Supercontinuum (SC) sources are nowadays widely used as fundamental building blocks in systems enabling a multitude of technologies such as optical frequency metrology, optical coherence tomography and broadband spectroscopy to name a few [1]. Highly nonlinear fibers (HNLF) make up an excellent platform for SC generation as they not only offer high nonlinearity, but also dispersion-engineering and broadband single-mode operation. Broad SC are easily obtained by injecting ultrashort pulses into the anomalous dispersion regime of the HNLF, where the initial propagation is characterized by temporal compression and spectral broadening through solitonic effects. However, after compression each launched pulse tend to fission into several pulses, a process that is very sensitive to noise and can therefore lead to increased shot-to-shot fluctuations [2].

Antimony telluride (Sb2Te3) is an intermetallic compound crystallizing in a hexagonal lattice withR-3mspace group. It creates a c lose packed structure of anABCABCtype. As intrinsic semiconductor characterized by excellent electrical properties, Sb2Te3is widely used as a low-temperature thermoelectric material. At the same time, due to unusual properties (strictly connected with the structure), antimony telluride exhibits nonlinear optical properties, including saturable absorption. Nanostructurization, elemental doping and possibilities of synthesis Sb2Te3in various forms (polycrystalline, single crystal or thin film) are the most promising methods for improving thermoelectric properties of Sb2Te3. Applications of Sb2Te3in optical devices (e.g. nonlinear modulator, in particular saturable absorbers for ultrafast lasers) are also interesting. The antimony telluride in form of bulk polycrystals and layers for thermoelectric and optoelectronic applications respectively were used. For optical applications thin layers of the material were formed and studied. Synthesis and structural characterization of Sb2Te3were also presented here. The anisotropy (packed structure) and its influence on thermoelectric properties have been performed. Furthermore, preparation and characterization of Sb2Te3thin films for optical uses have been also made.

We present a direct comparison between two types of femtosecond 2 µm sources used for seeding of an ultrafast thulium-doped fiber amplifier based on all-normal dispersion supercontinuum and soliton self-frequency shift. Both nonlinear effects were generated in microstructured silica fibers, pumped with low-power femtosecond pulses at 1.56 µm originating from an erbium-doped fiber laser. We performed a full characterization of both nonlinear processes, including their shot-to-shot stability, phase coherence, and relative intensity noise. The results revealed that the solitons show comparable performance to supercontinuum in terms of relative intensity noise and shot-to-shot stability, despite the anomalous dispersion regime. Both sources can be successfully used as seeds for Tm-doped fiber amplifiers as an alternative to Tm-doped oscillators. The results show that the sign of chromatic dispersion of the fiber is not crucial for obtaining a stable, high-quality, and low-noise spectral conversion process when pumped with sub-50 fs laser pulses.

Herein, we report on the possibility of ultrashort laser pulse generation in the broadband spectral range using a saturable absorber based on free-standing metallic carbon nanotube thin film. Erbium, thulium, and holmium-doped all-fiber lasers were mode-locked with a single saturable absorber containing a 300 nm thick material layer. Subpicosecond pulses were generated at 1559, 1938, and 2082 nm. Our work validates the broadband operation of metallic carbon nanotube-based saturable absorbers and highlights the suitable performance of nanomatematerial for ultrafast photonic applications.

A 125 MHz fiber-based frequency comb source in the mid-infrared wavelength region is presented. The source is based on difference frequency generation from a polarization-maintaining Er-doped fiber pump laser and covers a spectrum between 2900 cm−1 and 3400 cm−1 with a simultaneous bandwidth of 170 cm−1 and an average output power up to 70 mW. The source is equipped with actuators and active feedback loops, ensuring long-term stability of the repetition rate, output power, and spectral envelope. An absorption spectrum of ethane and methane was measured using a Fourier transform spectrometer to verify the applicability of the mid-infrared comb to multispecies detection. The robustness and good long- and short-term stability of the source make it suitable for optical frequency comb spectroscopy of hydrocarbons.

This paper presents the study on power scaling of an all-polarization maintaining (PM) fiber mode-locked laser using the multilayer chemical vapor deposition graphene/poly(methyl methacrylate) composite as a saturable absorber by changing the coupling ratio of the laser output coupler. Pulses with the duration of 148 fs were generated using 10% output coupler, which are the shortest pulses obtained from mode-locked erbium doped fiber laser working in anomalous dispersion regime, additionally the highest average output power of 15.66 mW was achieved by applying 70% output coupler. All-PM fiber resonator configuration provides self-start and stable operation. To our knowledge, this is the first comprehensive study of power scaling of erbium doped fiber laser with graphene saturable absorber. Conducted experimental work is in very good agreement with numerical simulations.

Effective, high energy ultrafast laser systems for practical applications requires laser sources with low and tunable repetition rate. Intrinsically, a mode-locked fiber laser operates at relatively high and fixed repetition rates (at the order of MHz). Such a high repetition rate limits the achievable pulse energy from an amplifier. To overcome this limitation, we demonstrate a fast, compact, and fully automatic custom made pulse picking system for operation with femtosecond lasers with repetition rates up to 50 MHz. It employs custom electronics with FPGA based signal processing unit and a fast 200 MHz acousto-optic modulator. Proposed solution is flexible and it can be used with various lasers, modulators and detectors. Therefore it is possible to operate e.g. at 1.55 Μm and 2 μm wavelengths. Moreover, the unit can automatically adopted to different repetition rates of seed lasers by adjustable time delay, to be in phase with optical signal. FPGA based signal processing unit can be reconfigured by a microcontroller to deliver several triggering options, digital delay line and frequency meter unit.

Actively mode-locked lasers offer varying degrees of flexibility for a wider range of applications than their passively modulated counterparts, due to their capability for electrically controlled ultrahigh repetition rate operation. Graphene based electrooptic modulators with unique advantages of broad operation bandwidth and ultrafast speed are suitable for light modulation in various optoelectronic applications. Here, an actively mode-locked laser with a graphene based electrooptic modulator is reported for the first time. The active mode-locking technique combined together with the intracavity nonlinear pulse shortening effect allows the generation of transform-limited 1.44 ps pulses with pulse energy of 844 pJ. The electrically controlled repetition rate of generated pulses, a key performance advantage of active mode-locking, is also demonstrated. These results provide a practical and effective approach for actively mode-locked lasers with broad operation bandwidth and compact footprint, which contributesa new way for applications of two-dimensional (2D) layered materials in ultrafast lasers.

In this work, we demonstrate a high-power, fiber-based chirped pulse amplification (CPA) setup utilizing Er- and Er/Yb-doped fibers, operating at 1555 nm central wavelength. The integrated all-fiber pulse-picker allows to reduce the repetition frequency down to the kHz-range, which enables generation of sub-picosecond pulses with energies above 2 μJ and pulse peak power exceeding 1 MW. The system utilizes an Er/Yb co-doped large mode area fiber in the final amplification stage. Thanks to the used mode-field adapters and fiber-based components, the setup is almost fully fiberized, except the bulk grating pulse compressor. In order to provide compactness and simplicity, the compressor was designed using dense 1100 lines per millimeter gratings, that allow to keep the small grating separation.

Ultra-high-pulse-repetition-rate lasers are essential for a number of applications, including, e.g., optical communication and ablation-cooled material processing. Despite several techniques to generate pulses with gigahertz-range repetition rate, incorporating mainly short-length resonators, more widespread applications are still limited by the lack of a robust, simple, and cost-effective solution. Here, we report for the first time, to the best of our knowledge, fully passive harmonic mode locking in an all-polarization-maintaining (PM) fiber laser. The design guarantees a fixed polarization state and stable operation, where the cavity harmonic number is controlled by the pump power only. Self-starting operation is provided by the antimony telluride (Sb2Te3) thin-film saturable absorber (SA), which facilitates multiple pulse operation. The SA acts by means of low modulation depth, low saturation fluence, and an inverse slope in the saturable absorption curve. The optimum features of the SA and limiting factors for high-repetition-rate pulse generation in this regime of operation are discussed. As a result, 2.2 ps pulses with 3 GHz repetition rate are generated at 1560 nm wavelength. The study reports a new approach towards an optimal SA for multi-gigahertz pulse generation in practical, all-PM fiber lasers.

The Laser & Fiber Electronics Group (LFEG) constitutes a team of young, skilled and ambitious researchers, doctoral candidates and students. The Group possesses great experience in applied optoelectronics and laser technology. Currently, it conducts research in several areas,mostly focusing on: ultrashort laser pulse generation using novel materials, development of pulsed fiber laser sources ranging from visible to mid-infrared, development of compact mid-infrared optical frequency combs, laser spectroscopy techniques, laser vibrometry, solid-state lasers, advanced analog and digital electronics, and investigations of light-matter interaction.

We present an all-fiber source generating laser pulses in the 6 - 9 μm spectral range. The developed setup is based on difference frequency generation (DFG) in an orientation-patterned gallium phosphide (OP-GaP) with six different quasi-phase matching (QPM) periods. The maximum 7.4 mW average output power of the idler centered at 7600 nm was obtained with the 7% photon conversion efficiency.

We report the first fully fiberized difference frequency generation (DFG) source, delivering a broadly tunable idler in the 6 to 9 μm spectral range, using an orientation-patterned gallium phosphide (OP-GaP) crystals with different quasi-phase matching periods (QPM). The mid-infrared radiation (MIR) is obtained via mixing of the output of a graphene-based Er-doped fiber laser at 1.55 μm with coherent frequency-shifted solitons at 1.9 μm generated in a highly nonlinear fiber using the same seed. The presented setup is the first truly all-fiber, all-polarization maintaining, alignment-free DFG source reported so far. Its application to laser spectroscopy was demonstrated by the absorption spectrum measurement of ν4 band of methane in 7.5 – 8.3 µm spectral range. The system simplicity and compactness paves the way for applications in field-deployable optical frequency comb spectroscopy systems for gas sensing.

We demonstrate an all-polarization maintaining (PM) fiber laser systems which allows for generation of ultrashort laser pulses in 2 μm spectral range. The developed laser setup is very compact and reliable. The average output power of the laser pulses centered at 1975 nm was 244 mW. While the pulse duration and energy were of 95 fs and 5.4 nJ, respectively.