Narrow-linewidth Laser Research Articles

Dynamic detection of acoustic wave generated by polarization maintaining Brillouin random fiber laser

The intrinsic spectral width and intensity dynamics of the acoustic wave generated by the Brillouin random fiber laser were characterized experimentally for the first time. These are important to the understanding of the dynamic noise properties of random fiber lasers based on stimulated Brillouin scattering. We demonstrate that the spectra of the acoustic wave in the gain medium are determined by the spectral convolution of pump light and its Stokes light, which is further an influential factor on the narrow linewidth of the random fiber laser resulting from the distributed feedback of coherent Rayleigh scattering. The power of probe light is weak to ensure the minimum disturbance to the random fiber laser. In the time domain, the intensity of the reflected probe light exhibits a similar output property to that the Brillouin random fiber laser with Gaussian probability distribution when both narrow linewidth lasers (on the order of several kHz) are used as pump light and probe light. In contrast, stochastic noise features with an exponential probability distribution are introduced to the intensity dynamics of the reflected probe light when the linewidth of pump light or probe light is several MHz. The phase noise and intensity noise of the reflected probe light prove that acoustic wave generation and detection is based on a four wave mixing process, which enhances our understanding of the wave coupling in random fiber lasers and gives us a new perspective to understand the fundamental physics of the random lasing process and its noise property.

All-fiberized very-large-mode-area Yb-doped fiber based high-peak-power narrow-linewidth nanosecond amplifier with tunable pulse width and repetition rate* *Project supported by the National Natural Science Foundation of China (Grant No. 61675009) and the Beijing Natural Science Foundation Program, China, and Scientific Research Key Program of Beijing Municipal Education Commission, China (Grant No. KZ201910005006).

We demonstrate an all-fiberized narrow-linewidth nanosecond amplifier with high peak power, tunable pulse width, and repetition rate. A fiber-coupled narrow-linewidth laser diode operating at 1064.1 nm is employed as the seed source, which is gain-switched to generate nanosecond pulses with tunable pulse widths of 1–200 ns and tunable repetition rates of 10 Hz–100 kHz. By utilizing a very-large-mode-area Yb-doped fiber with a core diameter of 50 μm in the power amplifier, thresholds of the stimulated Brillouin scattering at different pulse widths and repetition rates are increased. The maximum average power reaches 30.8 W at the pulse width of 4 ns and a repetition rate of 100 kHz, corresponding to an optical-to-optical conversion efficiency of ∼55.2%. Pulse energy and peak power are calculated to be 0.2 mJ and 50 kW, respectively, which are limited by stimulated Brillouin scattering. The 3-dB spectral linewidth remains around 0.05 nm during the power scaling process. The stimulated Brillouin scattering limited output powers at different pulse widths and repetition rates are investigated. Peak power of 47.5 kW (0.19 mJ) is obtained for the 4 ns pulses at a repetition rate of 50 kHz, which is nearly the same as that of 4 ns pulses at 100 kHz. When the pulse width of the seed source is increased to 8 ns, peak powers/pulse energies are decreased to 19.6 kW/0.11 mJ and 13.3 kW/0.08 mJ at repetition rates of 50 kHz and 100 kHz, respectively.

Suppression of stimulated Brillouin scattering in optical fibers by tilted fiber Bragg gratings.

Stimulated Brillouin scattering (SBS) has significant influence on optical fiber communication (OFC), optical fiber sensing (OFS), and narrow-linewidth fiber laser (NLFL) systems. How to effectively suppress it has always been a challenge. In this Letter, we propose and demonstrate a versatile solution, for the first time, to the best of our knowledge, by using tilted fiber Bragg gratings (TFBGs). A specially designed and fabricated TFBG can be used as an ultra-narrow spectral filter, precisely matching with the operation laser wavelength and the tiny frequency shift due to SBS. Experimental results show that the backward Stokes can be strongly rejected with a filtering ratio of >10dB; meanwhile, an obvious increasing of SBS threshold is observed with a maximum value of 1.7 times that without the TFBG, which enhances the effective transmission power by 33%. The operation stability of this method also is validated. This work opens new opportunities for SBS suppression in OFC, OFS, and high-power NLFL systems.

Fiber Bragg gratings fabricated by femtosecond lasers for narrow-linewidth fiber laser

A narrow-linewidth erbium-doped fiber laser based on the fiber Bragg gratings (FBGs) fabricated was proposed and demonstrated experimentally. A pair of FBGs with reflectivity of 92 % and 10 %, respectively, at the center wavelength of 1580 nm were inscribed in standard SMF-28 fibers without hydrogen loading by femtosecond laser point-by-point directly writing method, which was used as mirrors for wavelength selection of fiber laser, gratings full width at half maximum (FWHM) was less than 0.15 nm. A simple linear-cavity fiber laser was realized based on gain medium, pump source and FBGs. When gain medium of Er-doped fiber is selected 8 m, a stable single wavelength of 1579 nm laser with a 3 dB linewidth of 0.06 nm was obtained. Its output power is 24.1 mW when the maximum pump power is 600 mA, and its single wavelength of 1579 nm is consistent with the central wavelength of the fabricated FBGs. This type of laser source shows real promise for applications in coherent optical frequency domain reflectometry and high resolution fiber sensor systems.

Acoustical and [formula omitted] noises in narrow linewidth lasers

The acoustical effects on the 1∕f noise spectrum of narrow linewidth lasers are experimentally evaluated employing Mach–Zehnder fiber interferometer with homodyne detection. The analysis includes an external cavity laser (ECL), a He–Ne Laser, and an integrated external cavity laser (IECL). It is shown that the 1∕f noises of very narrow linewidth lasers have a Gaussian behavior if the acoustical noises in the fiber and laser are reduced employing a vacuum environment over an optical table.

Narrow-linewidth passively Q-switched fibre laser based on microsphere resonator

We experimentally demonstrate a stable narrow-linewidth passively Q-switched fibre laser based on a microsphere resonator (MSR) and graphene saturable absorber (SA). The MSR made by the arc discharge method has the characteristics of easy fabrication, broad free spectral range (FSR) and flexibility. And it acts as a narrow band-pass filter to ensure narrow-linewidth operation. A stable passively Q-switched pulse with 0.016 nm narrow spectral linewidth is successfully achieved. The output pulse has the pulse width of 5.2 µs, repetition frequency of 28 kHz and high signal to noise ratio (SNR) of ∼60 dB. The results demonstrate that our works may provide an effective way to achieve narrow-linewidth pulsed fibre lasers.

Real-time observation of vortex mode switching in a narrow-linewidth mode-locked fiber laser

Temporal and spatial resonant modes are always possessed in physical systems with energy oscillation. In ultrafast fiber lasers, enormous progress has been made toward controlling the interactions of many longitudinal modes, which results in temporally mode-locked pulses. Recently, optical vortex beams have been extensively investigated due to their quantized orbital angular momentum, spatially donut-like intensity, and spiral phase front. In this paper, we have demonstrated the first to our knowledge observation of optical vortex mode switching and their corresponding pulse evolution dynamics in a narrow-linewidth mode-locked fiber laser. The spatial mode switching is achieved by incorporating a dual-resonant acousto-optic mode converter in the vortex mode-locked fiber laser. The vortex mode-switching dynamics have four stages, including quiet-down, relaxation oscillation, quasi mode-locking, and energy recovery prior to the stable mode-locking of another vortex mode. The evolution dynamics of the wavelength shifting during the switching process are observed via the time-stretch dispersion Fourier transform method. The spatial mode competition through optical nonlinearity induces energy fluctuation on the time scale of ultrashort pulses, which plays an essential role in the mode-switching dynamic process. The results have great implications in the study of spatial mode-locking mechanisms and ultrashort laser applications.

High-Power Broadband Frequency Chirped Intensity-Modulated Single-Frequency 1064-nm Laser

Optical carried microwave radar (OCMR), combining the advantages of lidar and microwave radar, uses an intensity-modulated laser beam as the medium to detect the distance and velocity of objects. In order to achieve a high ranging resolution at long distance, a high-power frequency chirped intensity-modulated light source at a 1064-nm wavelength was developed. A low-power narrow-linewidth 1064-nm laser beam was input to a Mach-Zehnder electro-optic modulator (EOM), and the frequency of the EOM was tuned from 10 MHz to 2.1 GHz. The output from the EOM was coupled to a laser-diode-pumped ytterbium-doped fiber amplifier (YDFA). A maximum output power of 29.5 W was achieved from the YDFA. We measured the frequency characteristics of the modulated laser beams before and after amplification, respectively. The amplification process did not bring any obvious degeneration of the modulation in terms of the bandwidth, frequency stability, chirping linearity, and modulation depth. Our research presents a new approach of obtaining a high-power broad-band intensity-modulated light source for OCMR.

Mode dynamics in high-power Yb-Raman fiber amplifier.

Yb-Raman fiber amplifier (YRFA) is a compact setup that can be applied to achieve high-power narrow linewidth or special wavelength lasers. In this Letter, we realized a high-power YRFA with seed wavelengths of 1090 nm and 1150 nm, tandem-pumped by a 1018 nm fiber laser. The dynamic of mode interaction has been carefully studied. The beam cleanup effect in the large mode area, step-index fiber has been observed for the first time, to the best of our knowledge, when the pump power ranges from 800 W to 1700 W. A model taking into account the Raman mode interaction is proposed to explain this phenomenon, which agrees well with the experiments. The mode instability (MI) effect is also observed in the amplifier, and the threshold is about 2 kW, which is lower than the conventional Yb-doped fiber amplifier. Stimulated Raman scattering is attributed to the onset of MI. Finally, the 1338 W 1150 nm laser is achieved by this YRFA, which we believe to be the highest power reported at this wavelength.

Narrow linewidth erbium-doped fiber laser incorporating with photonic crystal fiber based Fabry–Pérot interferometer for temperature sensing applications

We propose a narrow linewidth ring cavity erbium-doped fiber laser based on a photonic crystal fiber (PCF) based Fabry–Pérot interferometer (F–P). A 5.2-m-long erbium-doped fiber was used in the resonant cavity as gain medium. The F–P was fabricated by cascaded with single-mode fiber and 2 mm length PCF as the interferometer filter, and the interferometer period was 0.58 nm. The laser threshold was 57 mW in the experiment, and 1573.99 nm narrow linewidth lasing was realized and 3 dB linewidth was 0.2 nm. Single wavelength laser temperature sensing was implemented by heating and cooling the F–P filter. In the experiment, single-wavelength lasing could be tuned from 1573.25–1574.27 nm within 40–120 ℃, with 12.6 pm/℃ sensitivity, power fluctuation was lower than 1.78 dB, and linearity was more than 0.996. The single-wavelength laser temperature stability was tested, and power fluctuation was less than 0.819 dB within 25 min scan time. In addition, four different dual-wavelength lasers could be obtained with power shift less than 1.187 dB.

Coherent narrow-linewidth optical frequency synthesis across the optical telecommunication band.

Broadband, coherent narrow-linewidth optical frequency synthesis is of crucial importance in dual-comb interferometric measurement. Here we present a detailed description of the construction and performance characterization of a hertz-level linewidth coherent optical frequency synthesizer across the optical telecommunication band. A narrow-linewidth cavity-stabilized laser at 1565.00 nm is built and coherently transferred through a fiber link with an additional fractional frequency instability of 2.0×10-16 at 1 s averaging time. Broadband, coherent optical frequency synthesis is then achieved by steering one mode of a laser frequency comb with the transferred optical frequency oscillation. By beating with a 1542.14 nm ultra-stable cavity-stabilized laser, the evaluated fractional frequency stability and absolute linewidth of the nearest synthesized optical oscillation are 3.5×10-15 at 1 s averaging time and 1.8 Hz, respectively. According to the ultra-low-noise feature of the utilized laser frequency comb of 4.7×10-17 at 1 s averaging time, the synthesized optical frequency oscillations could maintain the high coherence across the comb's output bandwidth.

Linewidth-related residual intensity modulation in lithium niobate phase modulators.

We present a modified model for residual intensity modulation (RIM) observed in lithium niobate phase modulators, which is suitable for both narrow linewidth and wide linewidth lasers. This model is based on two key points leading to RIM: one is the optical propagation loss, which is proportional to the applied voltage, and the other is the interference between an injected wave and its reflection from the lithium niobate substrate. In order to verify the model, the RIM is measured accurately with different linewidths of input lasers respectively. The experimental results are in good agreement with the theoretical model as the values of fitting determination coefficient R-square are all above 0.995. The results have revealed that the chief reasons causing RIM are different. When using a narrow linewidth laser, the interference is the dominant reason leading to RIM as the ratio of the reflection-related coefficient including linewidth effects to optical loss reaches 34.33. However, the optical loss is the dominant reason leading to RIM with the ratio mentioned above reaching 0.31 when using a wide linewidth laser.

Narrow linewidth characteristics of interband cascade lasers

Narrow-linewidth mid-infrared laser sources are highly demanding for high-resolution gas spectroscopy applications. Interband cascade lasers (ICLs) are power-efficient laser sources emitting in the mid-infrared range. This work unveils the low phase noise characteristics of distributed feedback ICLs driven by a battery source. We show that the measured spectral linewidth of ICLs is as narrow as 284 kHz (at a 1 ms observation time), which is smaller than those of common quantum cascade lasers. On the other hand, raising the pump current reduces the intrinsic linewidth down to 12 kHz. The linewidth broadening factor is in the range of 2.0–3.0, leading to a Schawlow–Townes linewidth as narrow as 1.6 kHz. This work suggests the high potential of developing battery-driven, high-resolution gas spectroscopy instruments using ICLs.

Effect of linewidth on intensity noise induced by stimulated Brillouin scattering in single-mode fibers.

Non-linear effects and intensity noise characteristics are critical factors that influence the performance of fiber communication systems as well as fiber-based sensors. It is observed in our experiment that relative intensity noise (RIN) subsequent to fiber transmission has a strong dependence on laser linewidth. Over a short transmission distance, RIN decreases with a narrowing laser linewidth. For longer distances, a narrower laser linewidth will result in a smaller RIN in a frequency range higher than 1 MHz and a larger RIN in a low-frequency range. In this study, the Brillouin linewidth parameter is introduced into a stimulated Brillouin scattering (SBS) three-wave coupling equation to simulate RIN variation phenomenon. Excellent agreement between the theoretical and experimental RIN spectra was obtained. We initially prove that the phenomenon is primarily owing to SBS. It is believed that the experimentally observed phenomena and theoretical justification presented in this study is significant in improving the performance of communication systems and fiber-based sensors.

Plug-and-play continuous-variable quantum key distribution for metropolitan networks.

We report a plug-and-play continuous variable quantum key distribution system (CV-QKD) with Gaussian modulated quadratures and a true local oscillator. The proposed configuration avoids the need for frequency locking two narrow line-width lasers. To minimize Rayleigh back-scattering, we utilize two independent fiber strands for the distribution of the laser and the transmission of the quantum signals. We further demonstrate the quantum-classical co-existing capability of our system by injecting high-power classical light in both fibers. A secret key rate up to 0.88 Mb/s is obtained by using two fiber links of 13 km and up to 0.3 Mb/s when adding 4 mW of classical light in the optical fiber used for transmitting the quantum signal. The reported performance indicates that the proposed QKD scheme has the potential to become an effective low-cost solution for metropolitan optical networks.

Chemical kinetics of hydroxyl reactions with cyclopentadiene and indene

Oxidation reactions of highly unsaturated cyclic hydrocarbons are very important in combustion, competing with their participation in PAH and soot growth. One of the key oxidation pathways at combustion conditions is the reaction with hydroxyl radicals. Cyclopentadiene (C5H6) and indene (C9H8) are typical PAH precursors, which have highly unsaturated penta-ring structure. In this study, rate coefficients of the reactions of hydroxyl radical with cyclopentadiene and indene were measured behind reflected shock waves over the temperature range of 828–1390 K and pressure near 1 atm. Hydroxyl radicals were monitored by a narrow line-width laser absorption near 306.7 nm. The measured rate coefficients may be expressed as kC5H6+OH=3.68−0.23+0.27×1013e−(1742.5−77+75)/T and kC9H8+OH=1.44−0.14+0.10×1013e−(1497.8−72+130)/T cm3 mol−1 s−1. Our experimental results showed that the reaction of hydroxyl radicals with cyclopentadiene is about two times faster than that of indene, and the indene + OH reaction exhibited a relatively weaker temperature dependence. Chemical kinetic simulations, carried out with a detailed model, showed the sensitivity of model performance to these reactions and the potential of model improvement with our measured rate coefficients.

A Chip-Scale Oscillation-Mode Optomechanical Inertial Sensor Near the Thermodynamical Limits.

Modern navigation systems integrate the global positioning system (GPS) with an inertial navigation system (INS), which complement each other for correct attitude and velocity determination. The core of the INS integrates accelerometers and gyroscopes used to measure forces and angular rate in the vehicular inertial reference frame. With the help of gyroscopes and by integrating the acceleration to compute velocity and distance, precision and compact accelerometers with sufficient accuracy can provide small-error location determination. Solid-state implementations, through coherent readout, can provide a platform for high performance acceleration detection. In contrast to prior accelerometers using piezoelectric or capacitive readout techniques, optical readout provides narrow-linewidth high-sensitivity laser detection along with low-noise resonant optomechanical transduction near the thermodynamical limits. Here an optomechanical inertial sensor with an 8.2 μg Hz-1/2 velocity random walk (VRW) at an acquisition rate of 100 Hz and 50.9 μg bias instability is demonstrated, suitable for applications, such as, inertial navigation, inclination sensing, platform stabilization, and/or wearable device motion detection. Driven into optomechanical sustained-oscillation, the slot photonic crystal cavity provides radio-frequency readout of the optically-driven transduction with an enhanced 625 μg Hz-1 sensitivity. Measuring the optomechanically-stiffened oscillation shift, instead of the optical transmission shift, provides a 220× VRW enhancement over pre-oscillation mode detection.

Rate Equation Modeling of Interband Cascade Lasers on Modulation and Noise Dynamics

This work presents a rate equation model for interband cascade lasers, which is used for investigating modulation dynamics and optical noise properties. Through standard small-signal analysis of the rate equations, we analytically derive the resonance frequency, the damping factor, the relative intensity noise (RIN), and the frequency noise (FN) of the interband cascade lasers. It is shown that more cascading stages are beneficial for reaching a broad modulation bandwidth at a low pump current. On the other hand, fewer cascading stages are favorable to achieve low-RIN and low-FN (or narrow-linewidth) laser emission.

Recent progress on narrow-linewidth crystalline bulk Raman lasers

Narrow-linewidth, solid-state Raman lasers are of significant importance for numerous applications. In last decade, variety of methods have been utilized to suppress the spectral linewidth of Raman laser output. In this review we summarize the recent works of narrow-linewidth and/or single-frequency solid-state Raman lasers. The mechanisms and results are addressed while the positive and negative aspects of each technique are also discussed to give the readers a reference to choose a proper narrow-linewidth method for certain applications.

Kilowatt-level ytterbium-Raman fiber amplifier with a narrow-linewidth and near-diffraction-limited beam quality.

By focusing on a typical emitting wavelength of 1120 nm as an example, we present the first, to the best of our knowledge, demonstration of a high-efficiency, narrow-linewidth kilowatt-level all-fiber amplifier based on hybrid ytterbium-Raman (Yb-Raman) gains. Notably, two temporally stable, phase-modulated single-frequency lasers operating at 1064 nm and 1120 nm, respectively, were applied in the fiber amplifier, to simultaneously alleviate the spectral broadening of the 1120 nm signal laser and suppress the stimulated Brillouin scattering effect. An over 1 kW narrow-linewidth 1120 nm signal laser was obtained with slope efficiency of ${\sim}{77}\% $∼77% and beam quality of ${\rm M}_x^2\sim {1.25}$Mx2∼1.25, ${\rm M}_y^2 \sim {1.17}$My2∼1.17. The amplified spontaneous emission (ASE) noise in the fiber amplifier was effectively suppressed by incorporating an ASE-filtering system between the seed laser and the main amplifier. Furthermore, the experimental results demonstrate that the spectral linewidth broadening effect is tightly related to the injected power ratios between the two seed lasers. Overall, this setup could provide a reference on obtaining and optimizing high-power narrow-linewidth fiber lasers operating in the long wavelength extreme of the Yb gain spectrum.