Narrow Spectral Linewidth Research Articles

1.7-[formula omitted]m thulium fiber laser with all-fiber ring cavity

A 1.7-μm thulium-doped all-fiber laser based on a ring-cavity configuration is demonstrated. The long-wavelength lasing near the 1.9-μm thulium emission peak was suppressed using a wavelength division multiplexer and single-mode–multimode–single-mode (SMS) fiber device, both of which served as a short-pass filter instead of the grating devices usually used in 1.7-μm thulium fiber lasers. A stable hundred-milliwatt-level 1720-nm laser output with a narrow spectral linewidth on the order of gigahertz was obtained after the optimization of the output coupling, active fiber length, and SMS device. With a cascaded SMS filter, the output power was further scaled to 227 mW under a launched 1570-nm pump power of 2.8 W, while the slope efficiency was 10.3%. The wavelength coverage could be further extended by using different SMS devices and applying tension on them.

Laser diagnostics for the electron density of helium low temperature plasmas using saturated absorption spectroscopy

Helium (He) low temperature plasma with an electron density higher than 1011cm−3 was produced using an inductively coupled plasma source, and the electron density of this plasma was determined using a high-resolution laser spectroscopic technique. To remove the Doppler broadening which is the most dominant line broadening in low temperature plasmas, a saturated absorption spectroscopy system was configured. The Lamb dip was successfully separated from the Doppler broadening for the 21S−41P transition of the He plasma using a high-resolution laser beam with a narrow spectral line width (below 1 MHz) at 396.5 nm. The estimated spectral line widths of natural broadening, van der Waals broadening, resonance broadening, and Stark broadening were 40.14 MHz, 0.93 MHz, 0.15 MHz, and 12.60 MHz, respectively, when the He gas pressure was 30 mTorr and 1 kW RF power was applied. The electron density of the He plasmas was determined to be ∼3×1011cm−3, which was comparable with those determined by a Langmuir probe and optical emission spectroscopy combined with a collisional-radiative model.

Passively Q-Switched 3-$\mu$ m $\text{Ho}^{3+}$ -Doped Fiber Laser Based on Nonlinear Polarization Rotation

We report a 3- $\mu \text{m}$ -waveband passively Q-switched $\text{Ho}^{\mathbf {3+}}$ -doped ZBLAN fiber laser based on nonlinear polarization rotation (NPR) in the ring cavity for the first time, to the best of our knowledge. The laser can operate in stable single- and dual-wavelength regimes. The stable single-wavelength pulse train is obtained with a ∼104 pm spectral linewidth at 2960.7 nm for 1.064-W launched pump power. By further increasing the launched pump power, the dual-wavelength pulsed laser can be observed, which is caused by the combination of intra-cavity birefringence and polarizer. The resulting dual-wavelength pulse has a narrow spectral linewidth of ∼87 pm and ∼131 pm at the central wavelength of 2966.8 nm and 2971.7 nm, respectively, under the maximum launched pump power of 3.86 W. The corresponding maximum output power and shortest pulse duration are 86.6 mW and $1.6~\mu \text{s}$ , respectively.

2.05 kW all-fiber high-beam-quality fiber amplifier with stimulated Brillouin scattering suppression incorporating a narrow-linewidth fiber-Bragg-grating-stabilized laser diode seed source.

We experimentally demonstrate an all-fiber high-power fiber amplifier with high beam quality and a slope efficiency of 81.8%, using a fiber-Bragg-grating-stabilized laser diode as a narrow spectral linewidth (0.08 nm) seed source. During amplification, the spectral linewidth of the laser output is broadened from 0.08 to 0.24 nm due to nonlinear phenomena. To the best of our knowledge, we report the first experimental observation of the suppression of stimulated Brillouin scattering (SBS), with increased output power. In addition, we investigated the SBS suppression by simultaneously measuring the optical backscattered power, backscattered spectrum, and output spectrum at different values of output power. The beam quality, M2, was measured to be ∼1.28 at the maximum output power of 2.05 kW, and modal instability was not observed.

Electroluminescence enhancement in mid-infrared InAsSb resonant cavity light emitting diodes for CO2 detection

In this work, we demonstrated a mid-infrared resonant cavity light emitting diode (RCLED) operating near 4.2 μm at room temperature, grown lattice-matched on a GaSb substrate by molecular beam epitaxy, suitable for CO2 gas detection. The device consists of a 1λ-thick microcavity containing an InAs0.90Sb0.1 active region sandwiched between two high contrast, lattice–matched AlAs0.08Sb0.92/GaSb distributed Bragg reflector (DBR) mirrors. The electroluminescence emission spectra of the RCLED were recorded over the temperature range from 20 to 300 K and compared with a reference LED without DBR mirrors. The RCLED exhibits a strong emission enhancement due to resonant cavity effects. At room temperature, the peak emission and the integrated peak emission were found to be increased by a factor of ∼70 and ∼11, respectively, while the total integrated emission enhancement was ∼×33. This is the highest resonant cavity enhancement ever reported for a mid-infrared LED operating at this wavelength. Furthermore, the RCLED also exhibits a superior temperature stability of ∼0.35 nm/K and a significantly narrower (10×) spectral linewidth. High spectral brightness and temperature stable emission entirely within the fundamental absorption band are attractive characteristics for the development of next generation CO2 gas sensor instrumentation.

Narrow-linewidth all-fiber amplifier with up to 3.01 kW output power based on commercial 20/400 μm active fiber and counterpumped configuration.

A multikilowatt all-fiber amplifier with narrow spectral linewidth is investigated by using commercial 20/400μm active fiber due to its more extensive application prospects. Stimulated Brillouin scattering (SBS) is suppressed effectively by combining multiple approaches including optimizing linewidth-broadened seed, shortening fiber length, and a counterpumped configuration. Consequently, the output power is pushed up to 3.01kW with M2=1.17, and the linewidth is maintained at 48GHz. The detailed characteristics of SBS are presented and analyzed in the frequency and time domains as output power is gradually raised. A high optical-to-optical efficiency is obtained with 85.6% and amplified spontaneous emission is reduced to 40dB with respect to a laser signal. The proposed architecture reaches a relatively high output power with excellent beam quality under the condition of the equivalent linewidth for the commercial 20/400μm active fibers in robustly all-fiberized structures.

Fourier-domain mode-locked laser combined with a master-oscillator power amplifier architecture

Originally introduced in 2005 for high-speed optical coherence tomography, the rapidly wavelength-swept Fourier-domain mode-locked (FDML) laser still, to this day, enables highest imaging speeds through a very high-speed spectral tuning capability. The FDML laser achieves a tuning bandwidth of over 1/10th of its center wavelength and can sweep this entire bandwidth in less than a microsecond. Interestingly, even though it covers a very broad spectral range, instantaneously it has a narrow spectral linewidth that puts it in a unique space compared to other high-speed broadband laser sources, e.g., mode-locked lasers or supercontinuum sources. Although it has been applied for nonlinear Raman spectroscopy and imaging, a current drawback of this continuous wave laser is the relatively low instantaneous power of 10-100mW. Here, we report the combination of an FDML laser with a master oscillator power amplifier (MOPA) architecture to increase the instantaneous power of the FDML for nonlinear optical interactions. The output of an FDML laser around 1060nm is modulated to short pulses by using an electro-optic amplitude modulator and subsequently amplified using ytterbium-doped fiber amplifiers (YDFAs). This generates a spectral rainbow of 65 picosecond pulses, where each pulse has a distinct, monochromatic wavelength. The instantaneous power can be adjusted by the YDFAs to reach nonlinear optical excitation regimes. This wavelength-swept FDML-MOPA laser will have a vast range of applications in, e.g., nonlinear optics, spectroscopy, imaging, and sensing.

Narrowing spectral linewidth in passively mode-locked solid-state lasers.

A novel scheme to realize a mode-locked laser with a narrow spectral linewidth is demonstrated by exploiting a reflected Fabry-Perot (FP) cavity to introduce an intense FP effect. Stable continuous-wave mode-locked operation is achieved with the repetition rate of 48MHz and the maximum average output power of 2.6W under an incident pump power of 11.9W. The mode-locked pulse width is systematically investigated by varying the optical thickness of the reflected FP cavity. The pulse duration is experimentally found to be in a range of 0.8 to 2.6ns. The experimental results reveal that the reflected FP cavity is the key factor leading to the pulse generation in the nanosecond regime. This Letter is believed to provide a promising method for generating optical pulses with narrow spectral linewidths.

Compact narrow-linewidth optical negative feedback laser with Si optical filter

A compact narrow-linewidth semiconductor laser is demonstrated by hybridly integrating an optical filter constructed from Si waveguides with a single-mode semiconductor laser, whose spectral linewidth is narrowed through optical negative feedback. We design the optimum structure of a reflective ring filter for an optical negative feedback laser, and a narrow spectral linewidth of 160 kHz is realized by the hybridly integrated laser source whose effective device length is less than 1.5 mm.

A wavelength-tunable narrow-linewidth all-fiber laser with cylindrical vector beam outputs

Abstract In this paper, we experimentally demonstrated wavelength-tunable cylindrical vector beams of very narrow spectral linewidth at ∼ 1 μ m with an authentic all-fiber structure for the first time to the best of our knowledge. An ytterbium-doped fiber laser was utilized to initiate the continuous-wave single-longitudinal-mode laser emission, and an off-set splice spot together with a few-mode fiber Bragg grating were used as the mode converter as well as mode selecting output coupler. By adjusting the built-in polarization controllers, radially and azimuthally polarized CVB outputs were both obtained. The central wavelength was at 1053.4 nm with a tuning range of 1.1 nm with thermal control, and two fiber Mach–Zehnder interferometers were cascaded to achieve a 20-dB linewidth of 14.7 kHz.

Nonpolarizing guided-mode resonance filter with high tolerance of conical angle

A tunable one-dimensional (1D) nonpolarizing guided-mode resonance filter (GMRF) with high peak efficiencies, narrow spectral linewidths, and low sideband reflections is experimentally demonstrated. The GMRF composed of a 1D grating and two waveguide layers on a glass substrate is designed with rigorous coupled-wave theory. The grating structure of the GMRF is patterned by interference lithography with two ultraviolet laser beams. The reflection resonance wavelengths of nonpolarizing spectra of the GMRF sample with period of 481.9 nm are tuned from 836.6 to 866.3 nm by rotating the angle of incidence. The measured full width at half-maximums of the reflection peaks located at 836.6, 844.1, 851.8, 858.4, and 866.3 nm are 8.1, 8.9, 9.0, 8.6, and 9.4 nm, respectively. The corresponding sidebands of the measured spectra are below 0.2. Moreover, the dependence of the nonpolarizing effect on the conical angle is studied to consider the mounting tolerance. The measured results show that the transmission and reflection spectra of two nonpolarizing GMRFs are insensitive to the conical angle. Compared with previous reported 1D nonpolarizing GMRFs, the tolerances of conical angle of the fabricated structures are remarkably enhanced.

Tunable narrow linewidth picosecond pulses from a single grating gain-switched Ce:LiCAF laser

We report a gain-switched laser oscillator configuration for generating tunable short and narrow spectral linewidth 290 nm laser pulses from a Ce:LiCAF crystal. Continuous tuning of the laser emission over a range of 18 nm from 281 nm to 299 nm is achieved using a single grating in a low-Q short-cavity oscillator. The shortest pulse duration achieved is 450 ps at 281 nm and the narrowest spectral linewidth is 0.17 nm at 287 nm. The average output energy ranged between 0.5 mJ and 0.8 mJ, resulting to conversion efficiencies of 8%–10%, depending on the output wavelength. The simplicity of our configuration makes aligning easier as well as reduces the loss in laser output energy, thereby leading to improved laser efficiency.

A putative G-quadruplex structure in the proximal promoter of VEGFR-2 has implications for drug design to inhibit tumor angiogenesis

Tumor angiogenesis is mainly regulated by vascular endothelial growth factor (VEGF) produced by cancer cells. It is active on the endothelium via VEGF receptor 2 (VEGFR-2). G-quadruplexes are DNA secondary structures formed by guanine-rich sequences, for example, within gene promoters where they may contribute to transcriptional activity. The proximal promoter of VEGFR-2 contains a G-quadruplex, which has been suggested to interact with small molecules that inhibit VEGFR-2 expression and thereby tumor angiogenesis. However, its structure is not known. Here, we determined its NMR solution structure, which is composed of three stacked G-tetrads containing three syn guanines. The first guanine (G1) is positioned within the central G-tetrad. We also observed that a noncanonical, V-shaped loop spans three G-tetrad planes, including no bridging nucleotides. A long and diagonal loop, which includes six nucleotides, connects reversal double chains. With a melting temperature of 54.51 °C, the scaffold of this quadruplex is stabilized by one G-tetrad plane stacking with one nonstandard bp, G3-C8, whose bases interact with each other through only one hydrogen bond. In summary, the NMR solution structure of the G-quadruplex in the proximal promoter region of the VEGFR-2 gene reported here has uncovered its key features as a potential anticancer drug target.

Narrow spectral linewidth in InAs/InP quantum dot distributed feedback lasers

This paper reports on the spectral linewidth of InAs/InP quantum dot distributed feedback lasers. Owing to a low inversion factor and a low linewidth enhancement factor, a narrow spectral linewidth of 160 kHz (80 kHz intrinsic linewidth) with a low sensitivity to temperature is demonstrated. When using anti-reflection coatings on both facets, narrow linewidth operation is extended to high powers, believed to be due to a reduction in the longitudinal spatial hole burning. These results confirm the high potential of quantum dot lasers for increasing transmission capacity in future coherent communication systems.

Room-Temperature Lasing Action in GaN Quantum Wells in the Infrared 1.5 μm Region

Large-scale optoelectronics integration is strongly limited by the lack of efficient light sources, which could be integrated with the silicon complementary metal-oxide-semiconductor (CMOS) technology. Persistent efforts continue to achieve efficient light emission from silicon in the extending the silicon technology into fully integrated optoelectronic circuits. Here, we report the realization of room-temperature stimulated emission in the technologically crucial 1.5 micron wavelength range from Er-doped GaN multiple-quantum wells on silicon and sapphire. Employing the well-acknowledged variable stripe technique, we have demonstrated an optical gain up to 170 cm-1 in the multiple-quantum well structures. The observation of the stimulated emission is accompanied by the characteristic threshold behavior of emission intensity as a function of pump fluence, spectral linewidth narrowing and excitation length. The demonstration of room-temperature lasing at the minimum loss window of optical fibers and in the eye-safe wavelength region of 1.5 micron are highly sought-after for use in many applications including defense, industrial processing, communication, medicine, spectroscopy and imaging. As the synthesis of Er-doped GaN epitaxial layers on silicon and sapphire has been successfully demonstrated, the results laid the foundation for achieving hybrid GaN-Si lasers providing a new pathway towards full photonic integration for silicon optoelectronics.

Toward ultimate efficiency: progress and prospects on planar and 3D nanostructured nonpolar and semipolar InGaN light-emitting diodes

Nonpolar and semipolar III-nitride-based blue and green light-emitting diodes (LEDs) have been extensively investigated as potential replacements for current polar c-plane LEDs. High-power and low-efficiency-droop blue LEDs have been demonstrated on nonpolar and semipolar planes III-nitride due to the advantages of eliminated or reduced polarization-related electric field and homoepitaxial growth. Semipolar (202¯1) and (202¯1¯) LEDs have contributed to bridging “green gap” (low efficiency in green spectral region) by incorporating high indium compositions, reducing polarization effects, and suppressing defects. Other properties, such as low thermal droop, narrow spectral linewidth, small wavelength shift, and polarized emission, have also been reported for nonpolar and semipolar LEDs. In this paper we review the theoretical background, device performance, material properties, and physical mechanisms for nonpolar and semipolar III-nitride semiconductors and associated blue and green LEDs. The latest progress on topics including efficiency droop, thermal droop, green-gap, and three-dimensional nanostructures is detailed. Future challenges, potential solutions, and applications will also be covered.

InP-Based Monolithically Integrated Photonic Devices for Digital Coherent Transmission

We comprehensively review InP-based monolithically integrated photonic devices for digital coherent transmission. A single-stripe tunable laser consisting of the tunable distributed amplification-chirped sampled grating-distributed reflector structure has yielded the high fiber output power of more than +17 dBm and the narrow spectral linewidth of less than 200 kHz, which achieved sufficient properties for 16-ary quadrature amplitude modulation (16QAM). A dual-polarization in-phase and quadrature modulator module with the Mach–Zehnder modulator array accomplished 16QAM modulation for 224 Gb/s at a low differential driving voltage of 2.5 ${\rm{V}}_{{\rm{pp}}}$ . A photodetector monolithically integrated with a 90° hybrid presented a high responsivity (> 0.130 A/W over C-band) and the stable operation with low dark current over 5000 h in the accelerated aging test (test condition: −7.5 V at 175 °C). Demodulation of 224 Gb/s DP-16QAM modulated signals was demonstrated as the compact coherent receiver using this photodetector. A wide 3-dB bandwidth of more than 40 GHz for 400 Gb/s DP-16QAM by 64 GBaud operation was also realized as the receiver thanks to p-i-n-photodiodes with a high-speed response.

Broadband Oscillator-Free THz Pulse Generation and Radiation Based on Direct Digital-to-Impulse Architecture

Broadband 0.03–1.1 THz signal generation and radiation are demonstrated based on an oscillator-free direct digital-to-impulse architecture with a 1.9-ps full width at half maximum and 130-GHz 3-dB bandwidth (BW) (200-GHz 10-dB BW) centered at 160 GHz. The radiated pulse achieves a peak pulse effective isotropic-radiated power of 19.2 dBm and peak pulse-radiated power of 2.6 mW. An ON/OFF impulse-shaping technique is introduced and implemented to suppress undesired ringing and to increase dc-to-radiated efficiency. The frequency-comb spectrum of the radiated pulse train with 5.2-GHz repetition rate is measured up to 1.1 THz. At a distance of 4 cm, the measured received SNR at 1 and 1.1 THz is 28 and 22 dB, respectively. A 1.1-THz tone is measured with a 10-dB spectral width of 2 Hz, demonstrating an extremely narrow spectral line width (two parts per trillion). Time-domain picosecond pulses are characterized using a custom femtosecond-laser-based terahertz time-domain spectroscopy system. Coherent spatial combining from two widely spaced chips is demonstrated. It is shown that the starting time of the radiated pulses is locked to the edge of the input digital trigger with a timing jitter of 270 fs. The chip is fabricated in a 130-nm SiGe BiCMOS process technology.

Ultrathin GaAsN matrix-induced reduced full width at half maximum of GaAsN/InAs/GaAsN dot-in-a-well heterostructures with extended emission wavelength

We grew InAs quantum dots (QDs) embedded in a GaAs0982N0.018 quantum well—a dot-in-a-well (DWELL) structure—that exhibits emission at wavelengths of up to 1.31µm at 19K and the full width at half maximum (FWHM) of 59meV which are lower those of the InAs/GaAs QDs (1.09µm and 82meV, respectively), the reference sample. Our results can be explained as follows: modified strain field at the GaAsN matrix/InAs QDs due to insertion of ultrathin GaAsN matrix, or nitrogen-induced point defects formed in the ultrathin GaAsN matrix during growth, could promote the homogeneous distribution of InAs QDs on the surface. The GaAsN capping layer evidently enhances the emission wavelength by reducing the overall compressive strain within the QDs. DWELL with ultrathin matrix layer annealed at 800°C exhibited 130-fold improvement in PL intensity, as compared to the as-grown sample, attributed to annihilation of N-related point defects. The results of this study are expected to be useful in fabricating DWELL-based infrared photodetectors with narrow spectral linewidth.

A Faraday laser lasing on Rb 1529\u2009nm transition

We present the design and performance characterization of a Faraday laser directly lasing on the Rb 1529 nm transition (Rb, 5P3/2 − 4D5/2) with high stability, narrow spectral linewidth and low cost. This system does not need an additional frequency-stabilized pump laser as a prerequisite to preparing Rb atom from 5S to 5P excited state. Just by using a performance-improved electrodeless discharge lamp-based excited-state Faraday anomalous dispersion optical filter (LESFADOF), we realized a heterogeneously Faraday laser with the frequency corresponding to atomic transition, working stably over a range of laser diode (LD) current from 85 mA to 171 mA and the LD temperature from 11 °C to 32 °C, as well as the 24-hour long-term frequency fluctuation range of no more than 600 MHz. Both the laser linewidth and relative intensity noisy (RIN) are measured. The Faraday laser lasing on Rb 1529 nm transition (telecom C-band) can be applied to further research on metrology, microwave photonics and optical communication systems. Besides, since the transitions correspongding to the populated excited-states of alkali atoms within lamp are extraordinarily rich, this scheme can increase the flexibility for choosing proper wavelengths for Faraday laser and greatly expand the coverage of wavelength corresponding to atomic transmission for laser frequency stabilization.