Laser Configuration Research Articles

High-repetition-rate pulse generation via self-sustaining cross-gain modulation in a semiconductor-based laser.

We present a simple and efficient method for generating regular pulse trains with GHz pulse repetition rates in lasers based on semiconductor optical amplifiers (SOAs). This method enables pulse formation without active modulation or saturable absorption of the generated radiation. The method relies upon the self-sustaining cross-gain modulation which is achieved by adding the negative optical feedback (NOF) to a ring laser configuration. The resulting modulation of laser gain is shown to be restricted to the frequencies which match both the spacing of longitudinal laser modes and the highest peaks in the NOF-induced instability gain spectrum. This enables the reproducible stationary pulse generation at the strictly defined repetition rates. The feasibility of the method was confirmed by the stable generation of sub-nanosecond pulses at repetition rates up to 1.79 GHz in a SOA-based laser with a simple fiber cavity.

L-Band Erbium Fiber Laser with Tunable and Narrow Sub-kHz Linewidth Output

In this paper, a wavelength-selectable erbium-doped fiber (EDF) laser in L-band bandwidth with a simple structure and low cost is described. To obtain stable single-longitudinal-mode (SLM) and narrow laser linewidth, the 2 m unpumped EDF-based saturable absorber (SA) and 100 m single-mode fiber (SMF)-induced Rayleigh backscattering (RB) injection effect are cascaded in the ring cavity of the fiber laser. The EDF SA and feedback RB injection can cause the auto-tracking ultra-narrow bandwidth filter effect to suppress the multiple longitudinal mode oscillations and narrow the laser linewidth. Thus, the tunable SLM wavelength output of the EDF laser with sub-kHz linewidth can be achieved. In the measurement, two EDF ring lasers can be executed when the placement of EDF-based SA and SMF-induced RB are switched to experiment with different output characteristics. The experimental results show that the wavelength bandwidths of the two lasers are between 1558.0 and 1618.0 nm. Additionally, the optimal optical signal-to-noise ratio (OSNR), wavelength linewidth, output power, and stability of the presented EDF laser configuration are also achieved and discussed.

Investigation of the wake region behind a hemispherical turret via laser Rayleigh scattering.

Planar and volumetric density measurements in the wake region behind a mounted hemispherical turret are obtained using laser Rayleigh scattering. The measurements are conducted in a Mach 2 wind tunnel facility at the Kirtland Air Force Base. Quantitative measurements of density and contour plots with lines of constant density are computed, thus enabling visualization of the turret wake's fluid dynamics. A new, to the best of our knowledge, laser diagnostic methodology and configuration for capturing laser images is also presented. This methodology enables further suppression of background light scattering. Multi-dimensional single-shot and time-average measurements are recorded at multiple axial locations behind the turret. The images acquired reveal turbulent regions of the wake flow, and a discussion of the observed phenomena is presented.

Multi-wavelength fiber laser via evanescent field confinement with Al2O3 nanolaminate-coated thinned fiber

We present the fabrication and analysis of an Erbium-doped optical fiber laser emitting at multiple wavelengths. This is accomplished via a filter constructed from a fiber taper coated with alumina (Al2O3). The taper is created using the flame brushing induction technique with a combination of Butane (25 %) and Oxygen (75 %). The diameter is reduced to a 20 µm waist and 2 mm burner displacement. The thin film of Al2O3 was deposited via atomic layer deposition (ALD), resulting in a 15 nm layer. The coated fiber taper was then inserted into a ring fiber laser cavity functioning as a selector filter. This fiber laser produced an output comprising six simultaneous wavelengths: 1531.14, 1531.79, 1532.45, 1541.85, 1544.45, and 1562.22 nm. Laser signal stability tests were performed for one hour at 25 °C at the six emission wavelengths, resulting in a maximum optical power fluctuation of 0.15 to 0.33 dB, a wavelength shift variation of 0.02 nm, and a signal-to-noise ratio of 31.79 dB in average. The laser lines emitted have an average spectral width of 0.06 nm. Our results show that the typical performance of the fiber laser configuration based on an optical fiber taper was improved by the addition of the Al2O3 layer and can be further improved. Our findings demonstrate that the standard operation of the fiber laser setup, which utilizes an optical fiber taper, was enhanced through the incorporation of an Al2O3 layer and has the potential for further development.

An improved measurement of frequency-dependent shear properties of highly compliant materials using a dynamic Kibble’s method

While methods exist to characterize the Young’s modulus and loss factor of viscoelastic materials, there are fewer methods to characterize the shear modulus and loss factor in shear. We previously presented an experimental apparatus that applied dynamic torque on rod-like viscoelastic materials with diameters of 0.5 cm to 2 cm and lengths of 10 cm to 30 cm. The torque was applied using Lorentz forces and measured using a dynamic version of Kibble’s method (similarly, the Watt method). Here, we present a modified version of this apparatus that has several notable improvements over the previously reported version. Improvements include a stronger, more uniform magnetic field and a direct measurement of the angular response of the specimen using a laser and photo-diode configuration. Moreover, the direct measurement of the angular response eliminates the need to know other geometric quantities (such as the radius of the electromagnetic coil), which further reduces the uncertainty of the measured torque. The measurement of the angular response and applied torque are fit to models in order to determine the shear moduli and loss factor. We demonstrate this improved technique on rod-like specimens of different highly-compliant viscoelastic materials.

Selectable and stable single-frequency fiber laser with flattened power range and sub-kHz linewidth output

To achieve stable and tunable single-longitudinal-mode (SLM) wavelength output, an erbium-doped fiber (EDF) laser with two fiber ring architecture and unpumped EDF based saturable absorber (SA) is demonstrated experimentally. The acquired optical signal to noise ratio (OSNR), wavelength linewidth, output stability and power of the fiber laser over an available tuning bandwidth of 1523.0 to 1569.0 nm are also implemented. Furthermore, this EDF laser configuration also can distribute the output power uniformly in the span of 1527.0 to 1569.0 nm within 0.53 dB power difference.

Phase-matching in terahertz quantum cascade laser sources based on Cherenkov difference-frequency mixing

Terahertz quantum cascade laser sources based on intra-cavity Cherenkov difference-frequency generation in dual-wavelength mid-infrared quantum cascade lasers are currently the only monolithic semiconductor laser technology that can deliver continuous-wave coherent terahertz output at room temperature. Because the Cherenkov difference-frequency generation process enables terahertz radiation generation and extraction across a wide range of frequencies, it is often assumed that phase-matching conditions for this process are automatically fulfilled. We theoretically analyze and experimentally demonstrate that phase-matching plays an important role in these devices, and significant improvements in terahertz power output can be achieved by adjusting the waveguide configuration of the quantum cascade lasers to provide better phase-matching.

Study on Dual-Wavelength Wide-Spectrum Erbium-Doped Fiber Ring Laser for Optical Fiber Interferometric Sensing System

In this paper, a novel dual-wavelength wide-spectrum erbium-doped fiber ring laser (EDFRL) for optical fiber sensing system is proposed and experimentally demonstrated. The output of EDFRL is theoretically simulated and classified depending on the theoretical analysis of laser threshold condition into “excited state” and “nonexcited state”. The influence of pump wavelength, fiber laser configuration, EDF length and output coupling ratio of EDFRL is discussed. The EDFRL pumped at 1480 nm wavelength and backward-pumped mode could achieve the stable maximum wide-spectrum output power. The output power fluctuation is lower than 0.06 dB and the center wavelengths shift is less than 0.01 nm. The practicability of EDFRL in a hybrid M-Z and Sagnac interferometer optical fiber sensing system is demonstrated through the comparison of SLD and EDFRL. Consequently, the EDFRL proposed in this study is expected to replace the traditional SLD in the optical fiber interferometric sensing system at a lower cost.

Molybdenum gallium carbide as saturable absorbers in 1.5-μm passive mode-locked fibre laser

A saturable absorber (SA) is one of the important components in a passive mode-locked fibre laser configuration to attain picosecond- to femtosecond-competent ultrashort pulses for fulfilling the imperatives of industries, particularly engineering and life sciences. The substantiation of a newfound SA in the generation of mode-locked pulses utilising molybdenum gallium carbide (Mo2Ga2C)—a MAX phase nanomaterial—was authenticated in this work. A Mo2Ga2C SA thin film was fabricated via a simple solution with polyvinyl alcohol (PVA) as the hosting medium. Linear absorption, modulation depth, non-saturable absorption, and saturation intensity of the 17.141-μm Mo2Ga2C SA were found to be 8.29%, 19.82%, 9.21% and 2.85 kW/cm2, respectively. The fabricated Mo2Ga2C SA was then implemented into an erbium-doped fibre laser (EDFL) system to realise the eminently stable mode-locked fibre laser at a centre wavelength of 1559.2 nm and rudimentary signal-to-noise ratio (SNR) of 54.8 dB. The pulse repetition rate and pulse width were measured to be 968.72 kHz and 2.028 ps successively. The maximum pulse energy and peak power are 10.219 nJ and 24.12 mW. In corroboration with previous reports, the implementation of Mo2Ga2C as a mode-locker is propitious in validating the impressively stable and ultrafast nonlinear fibre laser system for further multidisciplinary applications, for instance, medical instrumentation, sensing and range finding, communication, and military due to its excellent optical, physical, thermal, and mechanical properties.

Noise-like pulse generation in 1.95 µm region using bulk α-alumina saturable absorber

This work presents a tapered fiber decorated with bulk α-alumina with 4.08% modulation depth and 21.67 MW/cm2 saturation intensity which was responsible to induce noise-like pulse (NLP) in an “eye-safe” wavelength region. The light-matter interaction within evanescent field of ultrathin tapered fiber established NLP in a ring configuration of thulium-doped fiber laser with a broad spectral bandwidth of 45 nm centered at 1948 nm wavelength. The NLP was emitted at 11.36 MHz repetition rate in a net anomalous laser cavity. The generated pulse had a maximum pulse energy of 2.46 nJ at 1587 mW pumping power. The robustness of the NLP performance was validated with 3 h stability test, which opens new possibilities for future research of this material in the wavelength of exceeding 1.9 μm.

Efficient use of all ports of a 3 × 3 coupler in a nonlinear amplifying loop mirror-based fiber laser.

We present an all-polarization-maintaining mode-locked fiber laser based on a nonlinear amplifying loop mirror utilizing a 3 × 3 coupler. A pump laser diode placed outside the cavity pumps both the oscillator and fiber amplifier. This laser configuration effectively utilizes all ports of the coupler, making the laser compact and low-cost, and has great potential as a seed laser for ultrashort pulse lasers.

CMOS Linear Laser Driver for Intermediate Frequency over Fiber (IFoF) Links

The main objective of the proposed linear laser driver (LLD) is to reduce signal distortion in an analog direct modulation laser configuration used for intermediate frequency over fiber links. This work draws on an open-loop configuration featuring two differential pair blocks in a cascade arrangement to achieve a bandwidth measurement of 415 MHz at the half-power point, a total harmonic distortion of 4.57% for a fundamental frequency of 100 MHz, and an amplitude of 100 mVpp. The LLD provides a gain of 12.3 dB for a differential output and an output impedance of 46 Ω. The design, layout, and integration correspond to the process design kit for TSMC 65-nm CMOS technology. Experimental results show the advantage over other previously reported laser drivers.

Paracetamol concentration-sensing scheme based on a linear cavity fiber laser configuration

A paracetamol concentration-sensing scheme based on a linear cavity fiber laser configuration is demonstrated experimentally. The laser cavity has a fiber sensor at one end, that allows refractive index measurements. The refractometer consists of a cleaved fiber tip combined with an FBG functioning as a reflecting mirror. The combination of a fiber loop mirror at the other end allows to reflect all the light from the FBG and refractometer, forming a linear cavity. By measuring the intensity variation of the Fresnel reflection at the fiber-to-liquid interface, the measured concentration is linear and have a concentration sensitivity of -8.74±0.34×10-5µW/(g/kg), over a range of 52.61 to 219.25 g/kg, and with a resolution of 2.77 g/kg.The results obtained present high stability and prove the potential of the fiber laser system to performed real-time measurements of concentration, in a non-invasive way.

Optimization of the Core Compound for Ytterbium Ultra-Short Cavity Fiber Lasers

Highly ytterbium-, aluminum- and phosphorus-co-doped silica fibers with low optical losses were fabricated by the MCVD method, utilizing an all-gas-phase deposition technique. Optical and laser properties of the active fibers with a phosphosilicate and aluminophosphosilicate glass cores doped with 1.85 mol% and 1.27 mol% Yb2O3 were thoroughly investigated. With the help of hydrogen loading, it was possible to induce highly reflective Bragg grating in both fiber samples using the standard phase-mask technique and 193 nm-UV laser irradiation. The ultra-short (less than 2 cm long) Fabry–Perot laser cavities were fabricated by inscribing two fiber Bragg gratings (highly and partially reflective FBGs) directly in the core of the fiber samples. The highest pump-to-signal conversion efficiency of 47% was demonstrated in such laser configuration using phosphosilicate fiber. The reasons for the low efficiency of aluminophosphosilicate fiber are discussed.

Photon-to-photon modulation in fiber laser system using emerging materials

Graphene was the first two dimension (2D) material demonstrated as temporal modulator in fiber laser system more than a decade ago. Following its success, other types of 2D materials with comparable and superior performance were discovered. In this review, we outlined the types of optical modulation into wavelength, amplitude, polarization, phase and temporal modulation. We also discussed each modulation working principles along with its fiber laser configuration. Additionally, we compiled each modulator’s performance based on a wide range of 2D materials that were published recently. Some novel and emerging materials such as quantum dot (QD), one dimension (1D), three dimension (3D), composite and organic materials were highlighted.

A Polarimetric Fiber Ring Laser Incorporating a Coupled Optoelectronic Oscillator and Its Application to Magnetic Field Sensing

A novel configuration for a polarimetric fiber ring laser incorporating a coupled optoelectronic oscillator (COEO) is proposed and experimentally demonstrated, and its application to magnetic field sensing is studied. The COEO-based polarimetric fiber ring laser has two mutually coupled loops: the fiber ring laser loop and the OEO loop. In the fiber ring laser loop, longitudinal modes break up into orthogonal polarization modes because of birefringence. The frequency of the polarization mode beat (PMB) signals is determined by the cavity birefringence. In the OEO loop, a microwave signal with its frequency equal to the PMB signal is generated. By feeding the oscillation mode to modulate the optical loop, mode-locking can be achieved, rendering the mode spacing of the laser equal to the frequency of the oscillating OEO mode. We can estimate the birefringence variation by measuring the oscillating frequency of the COEO. To validate the proposed sensing system, a circular birefringence change is introduced in a magneto-optic crystal via the Faraday rotation effect. Then, the magnetic field sensing is implemented. Such configuration can achieve single longitudinal oscillation and realize high-speed and high-precision measurements.

Modified Mach-Zehnder interferometer for spatial coherence measurement.

Spatial coherence of light sources is usually obtained by using the classical Young's interferometer. Although the original experiment was improved upon in successive works, some drawbacks still remain. For example, several pairs of points must be used to obtain the complex coherence degree (normalized first-order correlation function) of the source. In this work, a modified Mach-Zehnder interferometer which includes a pair of lenses and is able to measure the spatial coherence degree is presented. With this modified Mach-Zehnder interferometer, it is possible to measure the full 4D spatial coherence function by displacing the incoming beam laterally. To test it, we have measured only a 2D projection (zero shear) of the 4D spatial coherence, which is enough to characterize some types of sources. The setup has no movable parts, making it robust and portable. To test it, the two-dimensional spatial coherence of a high-speed laser with two cavities was measured for different pulse energy values. We observe from the experimental measurements that the complex degree of coherence changes with the selected output energy. Both laser cavities seem to have similar complex coherence degrees for the maximum energy, although it is not symmetrical. Thus, this analysis will allow us to determine the best configuration of the double-cavity laser for interferometric applications. Furthermore, the proposed approach can be applied to any other light sources.

Co-Existence of Watt-Level Dissipative Solitons and Synchronous Dual-Wavelength Mode-Locked Pulses in Yb Fiber Laser

We present a high-average-power laser configuration to simultaneously generate both dissipative solitons (DSs) and synchronous dual-wavelength mode-locked (ML) pulses using large-mode-area (LMA) fibers in combination with a birefringence-based chirped-pulse spectral filter. DSs with average power as high as 5.8 W and pulse width as short as 178 fs are obtained at the output port. Meanwhile, up to 1.5 W average power in synchronous dual-wavelength ML pulsed operation is also obtained at the rejected port with central wavelengths located at 1029.7 nm and 1046.1 nm respectively. To our knowledge, observation of such behavior with both DSs and synchronous dual-wavelength pulse generation simultaneously has not been yet realized in watt-level fiber lasers.

Sub-kHz linewidth 1.6-µm single-frequency fiber laser based on a heavily erbium-doped silica fiber.

We present a single-frequency erbium-doped fiber laser operated at 1608.8 nm using a homemade, heavily erbium-doped silica fiber as gain medium. The laser configuration is based on a ring cavity, which is combined with a fiber saturable absorber to achieve single-frequency operation. The measured laser linewidth is less than 447 Hz and the optical signal-to-noise ratio exceeds 70 dB. The laser exhibits an excellent stability, without any instance of mode-hopping during 1-hour observing. The fluctuations in both wavelength and power were measured to be 0.002 nm and less than 0.09 dB in a 45-minutes period. The laser produces over 14 mW of output power with a slope efficiency of 5.3%, which, to the best of our knowledge, is currently the highest power directly obtained from a single-frequency cavity based on an erbium-doped silica fiber above 1.6 µm.

Ultra-Flat Supercontinuum Generation in an All-Fiber Self-Q-Switched Er/Yb Laser

A supercontinuum source based on a self-Q-switched Er/Yb-codoped fiber laser has been experimentally demonstrated. At different pump powers, starting at a moderate value of 1.1 W, self-Q-switched pulses with a broad flat spectrum of ~ 560 nm width spanning over the range ~1600 – 2160 nm at −5 dB level (considering the Raman peak) are obtained. Such wide spectrum can be achieved directly at the output of the fiber laser resonator, making the structure compact and robust. With a maximum pump power of 4.9 W, the repetition rate, average output power, peak power and pulse energy are approximately 14.7 kHz, 465 mW, 66 kW and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$31~\mu \text{J}$ </tex-math></inline-formula> , respectively. This compact SC laser configuration has the merits of simple cavity design, low cost and good robustness, which provide a SC source that can be easily applied to numerous areas, such as spectroscopy, optical communications, metrology and biomedical applications.