Search

Quantum optics experiments frequently require the separation of single-photon-level signals from strong classical fields. In circumstances in which the signals are spectrally close, and one cannot make use of relatively simple separation methods based around differences in polarization state, optical mode, or beam direction, it is necessary to exploit the frequency difference itself as the means for signal separation. We have constructed and characterized an efficient and robust fiber-based filter, consisting of an all-fiber Fabry-Perot resonator, to achieve this goal. Our filter shows 31dB of suppression of unwanted signals and 76% transmission of the desired signal. The transmission of the filter was stabilized to within 2% of its maximum for over 35hours through simple temperature stabilization.

Fiber lasers are a great source for tunable lasers due to the wide and relatively flat gain spectra of rare earth transitions in a glassy host (as compared to crystals). Thulium (Tm)-doped fibers, in particular, offer an extremely wide tunability of up to 330 nm in the 2μm wavelength region in a dual gain module configuration1. More recently, new concepts have emerged, which allow the synchronized emission of two or even more wavelengths2. These sources are particularly useful for nonlinear frequency conversion via four-wave mixing (FWM) or difference frequency generation (DFG). We will present a very versatile fiber-integrated approach based on Fiber-Bragg-Grating (FBG) arrays implemented in a theta-shaped cavity. The Tm-doped fiber source emits typical average powers of 0.5W and is tunable from 1931nm to 2040nm. The emission linewidth follows the spectral characteristic of the FBG and is typically 30GHz in our case. This concept allows a constant wavelength-independent repetition rate as well as a synchronous emission of two or even three independently tunable wavelengths. The tuning is performed purely electronically by optical gating, and in addition the pulse duration can be tuned between 4ns and 25ns. The switching speed is very fast and was measured to be less than 10μs. These experiments will be contrasted with a different approach based on a VLMA fiber associated to a set of two volume Bragg gratings (VBG), one of them being angle-tunable. This concept allows pulsed (Q-switched) as well as CW operation and features a continuous and wider tunability of up to 144nm especially and also the dual wavelength mode. The output power was > 4.5W in CW mode and pulse peak power of 12kW have been obtained in the Q-switched mode with pulse durations of 25ns.

International audience; We demonstrate a widely tunable Q-switched dualwavelength fiber laser emitting synchronized pulses in the 2 µm spectral range. Owing to the use of a thulium doped rod-type Fully-Aperiodic Large Pitch Fiber (FA-LPF), together with an acousto-optic modulator (AOM), and two Volume Bragg Gratings (VBGs), the wavelength separation was shown to be continuously tunable from 1 nm to 120 nm (~0.1-10 THz). A peak power of more than 8 kW was demonstrated over the whole tuning range for a repetition rate of 1 KHz and a 26 ns pulse duration. The repetition rate was modulated from 1 kHz to 30 kHz, and the laser pulse duration measured between 23 ns and 130 ns, depending of the repetition rate and the wavelength separation.

Based on favorable properties with respect to beam quality, efficiency and applicability, fiber-integrated lasers replace costly bulk lasers in various application fields, such as in academia, industry, medical technology and life sciences. Additionally, rare-earth doped fibers exhibit broad gain regions, providing huge potential to develop broadly tunable fiber lasers for spectroscopy. Recently a fiber-integrated tuning concept based on an FBG array as discrete spectral filter was demonstrated, exhibiting a unique flexibility to tailor spectral and temporal emission properties. In this work, we present the prospects of this concept to address Differential Absorption LIDAR (DIAL) for environmental sensing. With tunable multi-wavelength operation for enhanced measurement speeds, and customized spectral emission lines to probe specific molecules, it may provide a fast and cost-efficient solution with excellent usability.

In this paper, we demonstrate a widely tunable dual-wavelength fiber laser, which is based on a thulium-doped fiber and volume Bragg gratings triggering the emission wavelengths in the 2-μm band. A continuously tunable wavelength separation range (Δλ), from 1 to 144 nm (0.08–11.47 THz) is demonstrated, with > 4.5 W of output power over the tuning range. The laser emission exhibits a linewidth (FWHM) lower than 0.1 nm independent from wavelengths and Δλ, with more than 45 dB of a signal-to-noise contrast. A maximum output power of 7 W is obtained for shorter Δλ (1–14 nm), corresponding to a slope efficiency of 33% with respect to the incident pump power at 790 nm.

Modern optical measurement technologies such as structured light microscopy or fringe-projection profilometry rely fundamentally on structured illumination of the specimen or probe. Miniaturizing the applied illumination concept enables the availability of these methodologies even in spatial domains that have remained inaccessible so far. Here we introduce a design methodology to realize complex illumination patterns with high diffraction efficiencies in a strongly miniaturized and functional integrated approach. This is achieved by combining the advantages of refractive freeform wavefront tailoring and diffractive beam shaping. This novel concept overcomes classical stray light issues known from conventional diffractive beam shaping and remains valid for micro-optical systems, i.e., beyond the geometric optical regime. Moreover, the design process is in particular optimized to reduce the aspect ratio of the obtained surface features. This strongly improves the manufacturability and as-built performance of the designed optical element, and the feasibility of the approach is demonstrated by the design and realization of monolithic beam shaping units on the tips of optical fibers via two-photon direct laser writing. This provides the means to realize complex illumination patterns in an integrated and mechanically flexible approach.

We present discretely tunable all-fiber lasers based on a theta ring cavity and an FBG array featuring single- and synchronized multi-wavelength emission. They cover extended tuning ranges of 50nm (Yb band) and 79nm (Tm band).

Fiber lasers provide the perfect basis to develop broadly tunable lasers with high efficiency, excellent beam quality and user-friendly operation as they are increasingly demanded by applications in biophotonics and spectroscopy. Recently, a novel tuning scheme has been presented using fiber Bragg grating (FBG) arrays as fiber-integrated spectral filters containing many standard FBGs with different feedback wavelengths. Based on the discrete spectral sampling, these reflective filters uniquely enable tailored tuning ranges and broad bandwidths to be implemented into fiber lasers. Even though the first implementation of FBG arrays in pulsed tunable lasers based on a sigma ring resonators works with good emission properties, the laser wavelength is tuned by a changing repetition rate, which causes problems with applications in synchronized environments. In this work, we present a modified resonator scheme to maintain a constant repetition rate over the tuning range and still benefit from the advantages of FBG arrays as filters. With a theta ring cavity and two counter propagating filter passes, the distributed feedback of the FBG array is compensated resulting in a constant pulse round trip time for each filter wavelength. Together with an adapted gating scheme controlling the emission wavelength with a modulator, the tuning principle has been realized based on a Ytterbium-doped fiber laser. We present first experimental results demonstrating a tuning range of 25nm, high signal contrast and pulse durations of about 10ns. With the prospect of tailored tuning ranges, this pulsed fiber-integrated laser may be the basis to tackle challenging applications in spectroscopy.

We present a fiber-integrated laser enabling independent tuning of two emission wavelengths with a synchronized pulsed emission. The discrete tuning concept comprises a theta cavity fiber laser (TCFL), a fiber Bragg grating (FBG) array as a versatile spectral filter, facilitating tailored tuning ranges, and optical gating to control the emission spectrum. A novel electrical driving scheme uniquely enables independently tunable multi-wavelength emission from a single laser oscillator. Tunable dual-wavelength emission is experimentally investigated with a ytterbium (Yb)-doped TCFL using an FBG array with 11 gratings. Over a tuning range of 25 nm, 55 wavelength pairs have been demonstrated with high signal contrast (��� 40 dB) and narrow linewidth (< 40GHz). Based on the demands of prospective applications, pulse synchronicity is studied with a fiber-based time-delay spectrometer (TDS) simultaneously measuring the joint temporal and spectral pulse properties down to a single-pulse analysis. Accordingly, tunable and fully synchronized dual-wavelength emissions have been verified by driving the TCFL with optimized electrical gating parameters. This unique operation mode achieved in a cost-efficient fiber-integrated laser design targets novel applications e.g. in nonlinear spectroscopy and biophotonics.

We present an independently tunable pulsed dual-wavelength MOPA at 2µm based on a fiber-integrated theta ring oscillator. With a tailored tuning range for THz generation of 50nm, an output power of 12W has been achieved.

We present a discrete tuning concept based on a theta ring cavity and an FBG array as spectral filter that enables an independently tunable dual-wavelength emission. Pulse synchronicity is analyzed based on a Time-Delay Spectrometer.

Over the past years, Thulium (Tm) -doped fiber lasers in the 2μm region have gained a lot of interest due to many potential applications in materials processing and biophotonics. Based on the broad gain regions spanning from 1800nm to 2100nm, they offer the perfect basis to implement broadly tunable and user-friendly light sources like they are increasingly demanded in spectroscopic applications. Recently, a novel tuning mechanism based on a fiber Bragg grating (FBG) array as versatile spectral filter has been reported. This concept combines unique spectral freedom for customized tuning ranges and ultrabroad bandwidths with a fiber-integrated setup in order to maintain the advantages of the waveguide geometry. In this work, we demonstrate such a dispersion tuned and pulsed fiber laser in the Tm domain around 1950nm using a modulator and a discrete FBG array to control the emission wavelength. In order to comply with the demands of potential applications in biophotonics, for the first time, this tuning concept is realized in a polarization maintaining (PM) configuration ensuring linearly polarized output. While a simple FBG array is employed containing five gratings inscribed in PM fiber, we also outline the prospect to implement FBGs fabricated in a standard single mode fiber. The emission characteristics of the system are investigated showing pulse durations down to 11ns and a good spectral signal contrast. In order to highlight the prospect for tunable high-power operation, we have also implemented an amplification stage scaling the average power to more than 25W.

Capillary optical fibers with hole diameters of several micrometers are important for novel plasmonic applications and medical diagnosis. In order to ensure the optical functionality of these fibers, the diameter of the capillary hole needs to be realized with high accuracy. Here, we introduce a novel and noninvasive methodology to characterize optical fibers and discuss it for the assessment of capillaries. In this method, the fiber is side-illuminated by a coherent beam, and the resulting diffraction pattern is analyzed. This corresponds to an in-line holographic measurement in the presence of strong scattering. A numerical parameter retrieval allows us to characterize the capillary hole diameter with an accuracy of approximately 100 nm for radii between several hundreds of nanometers and several tens of micrometers.

An ytterbium-doped fiber laser mode-locked by the interaction of a fiber Bragg grating and longitudinal acoustic waves in a suspended core fiber is experimentally investigated. An optimized design of an acousto-optic modulator is also proposed. The results indicate output pulses with a width of less than 550 ps at a repetition rate of 10 MHz. The reduction of the power consumed by the transducer and the grating length points out to more efficient, compact and fast acousto-optic modulators for mode-locked all-fiber lasers.

We present two configurations of a discretely tunable Thulium-doped fiber laser using FBG arrays as versatile spectral filters for tailored tuning ranges. With pulses in the nanosecond regime, a tuning range of 84nm has been realized.

We demonstrate a novel tuning concept for pulsed fiber-integrated lasers with a fiber Bragg grating (FBG) array as a discrete and tailored spectral filter, as well as a modified laser design. Based on a theta cavity layout, the structural delay lines originating from the FBG array are balanced, enabling a constant repetition rate and stable pulse properties over the full tuning range. The emission wavelength is electrically tuned with respect to the filter properties based on an adapted temporal gating scheme using an acousto-optic modulator. This concept has been investigated with an Yb-doped fiber laser, demonstrating excellent emission properties with high signal contrast (35 dB) and narrow linewidth (150 pm) over a tuning range of 25 nm.

We present a method to tune the emission wavelength of pulsed fiber lasers using stepchirped fiber Bragg grating arrays as filters. First experimental results demonstrate excellent spectral characteristics and a tuning range of 18 nm.

The optical design and analysis of modern micro-optical elements with high index contrasts and large numerical apertures is still challenging, as fast and accurate wave-optical simulations beyond the thin-element-approximation are required. We introduce a modified formulation of the wave-propagation-method and assess its performance in comparison to different beam-propagation-methods with respect to accuracy, required sampling densities, and computational performance. For typical micro-optical components, the wave-propagation-method is found to be considerably faster and more accurate at even lower sampling densities compared to the different beam-propagation-methods. This enables realistic wave-optical simulations beyond the thin-element-approximation for micro-optical components. As an example, the modified wave-propagation-method is applied for in-line holographic measurements of strongly diffracting objects. From a direct comparison of experimental results and corre sponding simulations, the geometric parameters of a test object could be retrieved with high accuracy.

We have investigated the possibility to use a step-chirped fiber Bragg grating array for a time-delay-spectrometer. This concept allows a simultaneous spectral and temporal characterization of pulsed light in the nanosecond regime which can also be applied for the investigation of single pulses. A spectral resolution in the 100 picometer range has been achieved and used for the characterization of laser pulses in the nanosecond range.

Summary form only given. The development of pulsed light sources is crucially connected to proper characterization methods especially for the spectral and temporal emission behavior. In the nanosecond regime, the characterization in either domain is a straight forward task employing standard metrology like optical spectrum analyzers (OSA) and oscilloscopes. However, in contrast to the ultrashort-pulse scale, where simultaneous characterization in the time and frequency domain is already demonstrated, there is no proper technique to achieve the same for nanosecond pulses. In contrast to the common approach of a spectrometer, where the not directly accessible frequency domain is mapped onto a spatial scale, a Time-Delay Spectrometer (TDS) decodes the spectrum into a time dependent signal easily measureable with a fast single-point detector. This principle, realized in a setup employing optical fibers as time delay lines and step-chirped fiber Bragg grating (FBG) arrays as wavelength-selective mirrors, has already proven to be applicable for the spectral characterization of pulsed light sources. However, recent progress in the inscription of FBGs during the drawing process allows the gratings to achieve a much higher reflectivity of up to 40% enabling a much simpler structure of the FBG arrays employed in a TDS. Based on this modified structure, a TDS is realized not only characterizing in the frequency domain but also measuring the temporal behavior of the spectral components. In order to demonstrate the capabilities of the proposed method, a pulsed laser diode, usually employed as a seed source in a multiple stage fiber amplifier system, is investigated exemplarily. Based on the measurement with the TDS, spectrograms of the 10 ns pulses are generated plotting the information in the wavelength and time domain simultaneously. The emission behavior is characterized by a broad and weak emission at the pulse front which directly narrows down to a sharp and stable laser line. With a spectral and temporal resolution in the range of 100pm and 500ps respectively, the TDS shows very good agreement with referential measurements in both domains. As a unique feature, the TDS is also capable of performing the measurement for single pulses. This is of particular interest for an investigation of pulse-to-pulse variations which are disturbing in many applications of pulsed light sources. Especially in the frequency domain, this effect is difficult to measure. The TDS is a novel technique for accessing pulse-to-pulse variations in both domains and hence might offer important insights for the development of pulsed light sources.

Thulium (Tm)-doped fiber lasers offer a broad emission bandwidth in the 2-μm region, providing the perfect basis to develop broadly tunable laser sources, e.g., for spectroscopic applications. Recently, a tuning principle for pulsed fiber lasers has been reported, which is based on a fiber Bragg grating (FBG) array as a discrete spectral filter. This concept uniquely combines an unrivaled spectral freedom for tailored tuning ranges with a monolithic layout preserving the inherent advantages of fiber-integrated systems. In this study, we investigate this discrete tuning method using a Tm-doped fiber laser in the spectral domain around 1950 nm. While the laser emits linearly polarized light based on a polarization-maintaining (PM) resonator, we also examine the possibility of using standard FBG arrays inscribed in non-PM fiber. In order to highlight the prospect for tunable high-power operation, the tunable seed laser is implemented in a master oscillator power amplifier configuration scaling the average power to ∼28 W. With a tuning range of up to 76 nm, the emission characteristics of the system are investigated showing pulse durations down to 11 ns and a very good spectral signal contrast with narrow linewidth.

An approach for efficient coupling between different single-mode fibers is demonstrated. It is shown that short segments of graded-index fiber can provide broadband, extremely low-loss coupling even when the fibers have very different mode-field diameters.

We present a novel method to discretely tune the emission wavelength of pulsed fiber-integrated lasers. As spectral filter, a step-chirped fiber Bragg grating (FBG) array is employed combining a monolithic structure with an unrivaled design freedom enabling large tuning bandwidths as well as tailored spectral characteristics towards fingerprint tuning features. Together with an electrical control mechanism ensuring programmable operation, this tuning method promotes fiber-integrated lasers to access new fields of applications e.g. in biophotonics and distributed sensing. The potential of this tuning concept is investigated based on an Ytterbium-doped fiber laser. The system shows superb emission properties including excellent wavelength stability, high spectral signal contrast (up to 50dB) and narrow linewidth (15GHz) as well as adjustable pulse durations in the nanosecond range with peak powers up to 100W. Additionally, the unique spectral potential of this method is demonstrated by realizing filter designs enabling e.g. a record tuning range of 74nm for fiber-integrated lasers.

The interaction of a fiber Bragg grating and longitudinal acoustic waves in a three-air-holes suspended core fiber is experimentally investigated and employed to mode-lock an ytterbium-doped fiber laser. An optimized design of an acousto-optic modulator based on two piezoelectric transducers and 1 cm grating length is also proposed. For an electrical signal strength of 10 V applied to the modulator, the results indicate output pulses with a width of less than 550 ps at a repetition rate of 10 MHz. The reduction of the grating length and the power consumed by the transducer, when compared to previous studies, points out to more efficient, compact and fast acousto-optic modulators for mode-locked all-fiber lasers.

Tunable lasers allow the emission wavelength λ L to be adjusted within the gain bandwidth of the amplifying medium. Besides applications in telecommunications, tunable lasers are mostly employed in biophotonics for e.g. transmission and absorption measurements as well as in novel analysis methods such as RAMAN and CARS spectroscopy [1]. Based on broad spectral gain regions offering large tuning bandwidths, rare-earth doped fibers provide a promising basis to develop suitable systems. Besides the excellent beam quality and high efficiency, applications in spectroscopy would sustainably benefit from lasers in an all-fiber structure allowing robust, compact and easy-to-use systems to be realized.