Intensity-dependent Transmission Research Articles

Cerium oxide/polydimethylsiloxane polymer deposited onto tapered fiber for 1.97 µm mode-locked thulium-doped fiber laser

This work presents the use of cerium oxide (CeO2) as saturable absorber (SA) in the mode-locked pulse fiber laser generation. The CeO2 materials were prepared through sonication which then incorporated into a polydimethylsiloxane (PDMS) polymer to be coated on a tapered fiber. The CeO2/PDMS-SA exhibited saturation intensity of 44 MW/cm2 and modulation depth of 3.4%, confirming its nonlinear intensity-dependent transmission characteristics. Integrated into a thulium-doped fiber laser cavity, the CeO2/PDMS-SA enabled mode-locking at a pump power of 420 mW, leading to a fundamental mode-locking operation up to 1484 mW within the net anomalous dispersion regime. The laser emitted pulses at wavelength of 1973 nm with signal-to-noise ratio of 54 dB, repetition rate of 9.04 MHz and pulse width of 1.49 ps. The CeO2/PDMS-SA mode-locked fiber laser demonstrated stability of an average 3 dB bandwidth of 2.89 ± 0.08 nm, average central wavelength of 1973.05 ± 0.14 nm and average pulse width of 1.5 ± 0.01 ps for the duration of 120 min. Notably, the CeO2/PDMS-SA demonstrated a high pulse energy of 10.62 nJ and average output power of 96 mW. This investigation suggests their viability for use in future studies. The results also demonstrated the potential of CeO2 for use in various photonics applications in the “eye-safe” region.

Single- and two-photon absorption induced all-optical control of gallium selenide integrated silicon nitride photonic devices in the 700–800 nm wavelength range

In this work, we report single- and two-photon absorption (TPA) induced transmission and resonance modulation in a multilayer gallium selenide (GaSe) integrated silicon nitride (Si3N4) waveguide and ring resonator operating in the 700–800 nm wavelength range. Intensity dependent saturable absorption at low optical powers followed by TPA at higher power levels in GaSe integrated Si3N4 waveguides is observed at 785 nm pulsed laser excitation. A TPA coefficient of 0.117 cm/GW for the GaSe–Si3N4 composite waveguide and a three-photon absorption coefficient of 7.876 × 10−6 cm3/GW2 for the bare Si3N4 waveguide are extracted from intensity dependent transmission measurements. The single-photon absorption process induced by a blue laser incident on the multilayer GaSe transferred on top of the Si3N4 ring resonator is used for all-optical resonance tuning through the free-carrier refraction effect. A strong blue shift of the resonance by ∼12.3 pm/mW combined with resonance broadening is observed due to the free-carrier induced refractive index and absorption modulation. The TPA in the GaSe integrated Si3N4 ring resonator is also shown to result in a blue shift of the resonances excited using a 785 nm pulsed laser. This work demonstrates the all-optical control of 2D material integrated Si3N4 guided-wave structures operating in the shorter near-infrared wavelength range with potential applications in integrated quantum photonics, miniaturized sensing devices, and biomedical imaging.

SnS2 as a Saturable Absorber for Mid-Infrared Q-Switched Er:SrF2 Laser.

Two-dimensional (2D) materials own unique band structures and excellent optoelectronic properties and have attracted wide attention in photonics. Tin disulfide (SnS2), a member of group IV-VI transition metal dichalcogenides (TMDs), possesses good environmental optimization, oxidation resistance, and thermal stability, making it more competitive in application. By using the intensity-dependent transmission experiment, the saturable absorption properties of the SnS2 nanosheet nearly at 3 μm waveband were characterized by a high modulation depth of 32.26%. Therefore, a few-layer SnS2 was used as a saturable absorber (SA) for a bulk Er:SrF2 laser to research its optical properties. When the average output power was 140 mW, the passively Q-switched laser achieved the shortest pulse width at 480 ns, the optimal single pulse energy at 3.78 µJ, and the highest peak power at 7.88 W. The results of the passively Q-switched laser revealed that few-layer SnS2 had an admirable non-linear optical response at near 3 μm mid-infrared solid-state laser.

Evolution of a Gaussian pulse in negative index Kerr medium about threshold intensity: A numerical simulation

Temporal evolution of a pico-sec Gaussian pulse is numerically simulated in a representative intensity dependent negative index Kerr medium about a threshold intensity to study the effect of Kerr non-linearity on a propagating pulse, using the non-linear Schroedinger equation solved by split step Fourier method. Results are obtained for both dispersive and non-dispersive cases, that show the pulse to bear characteristics of a fundamental soliton. Moreover, the speed of the soliton i.e., the group velocity may be controlled by tuning the intensity of the pulse around the threshold. Overall, the analysis indicates prospect of an intensity dependent non-linear transmission material with the provision to control transmission speed through intensity tuning.

Gap solitons on an integrated CMOS chip.

Nonlinear propagation in periodic media has been studied for decades, yielding demonstrations of numerous phenomena including strong temporal compression and slow light generation. Gap solitons, that propagate at frequencies inside the stopband, have been observed in optical fibres but have been elusive in photonic chips. In this manuscript, we investigate nonlinear pulse propagation in a chip-based nonlinear Bragg grating at frequencies inside the stopband and observe clear, unequivocal signatures of gap soliton propagation, including slow light, intensity-dependent transmission, intensity-dependent temporal delay and gap soliton compression. Our experiments which are performed in an on-chip ultra-silicon-rich nitride (USRN) Bragg grating with picosecond time scales, reveal slow light group velocity reduction to 35%-40% of the speed of light in vacuum, change in the temporal delay of 7ps at low peak powers between 15.7 W-36.6W, which is accompanied by up to 2.7× temporal compression of input pulses. Theoretical calculations using the nonlinear coupled mode equations confirm the observations of intensity-dependent temporal delay. Of fundamental importance, this demonstration opens up on-chip platforms for novel experimental studies of gap solitons as the basis of all-optical buffers, delay lines and optical storage.

Cerium oxide/polydimethylsiloxane composite tapered fiber saturable absorber for mode-locked pulsed erbium-doped fiber laser

This investigation demonstrates the nonlinear saturable absorption of cerium oxide (CeO2) nanoparticles; a lanthanide allotrope oxide material and its employment as a saturable absorber (SA) for the generation of stable mode-locked pulsed fiber laser. The CeO2 nanoparticles were simply prepared by sonicating its bulk solution and incorporating a polydimethylsiloxane (PDMS) polymer to form a thin film nanocomposite on tapered optical fiber for the fabrication of CeO2/PDMS-SA. The nonlinear intensity-dependent transmission characteristic of this material was confirmed with a modulation depth of 0.66% and saturation intensity of 77 MW/cm2. The SA was integrated into an erbium-doped fiber laser cavity, where mode-locking operation self-started at 35 mW pump power with stable soliton operation up to 250 mW in net anomalous dispersion regime. The pulse laser centered at 1561 nm wavelength with a constant repetition rate of 7.64 MHz, shortest pulse width of 800 fs, and highest signal to noise ratio of 54.2 dB. The highest pulse energy of 0.645 nJ and average power of 4.92 mW were measured at 250 mW pump power. Based on its simple preparation and stable pulse generation, this investigation suggests the viability of CeO2 nanoparticles as a new SA material for ultrashort pulse generation.

Generation of sub-nanosecond pulse in dual-wavelength praseodymium fluoride fibre laser

In this paper, we demonstrate a stable dual-wavelength pulsed output from a praseodymium doped fluoride fiber laser. These dual functions are induced by intensity-dependent transmission and loss due to the cavity birefringence. The two lasing wavelengths centered at 1298.26 nm and 1299.91 nm with a spacing of 1.65 nm and a 3 dB bandwidth of 0.11 nm and 0.13 nm respectively are obtained. The pulse width induced by the dual-wavelength laser falls within the picosecond region, with a repetition rate of 389 KHz and a maximum average pulse energy of 5.4 nJ.

Ultrafast nonlinear absorption and carrier relaxation in ReS2 and ReSe2 films

As two important members of two-dimensional (2D) transition metal dichalcogenides, ReS2 and ReSe2 have gained interest for optoelectronic and photonic applications. The key to the application of the 2D materials to the optoelectronic devices is to understand the interaction between light and matter. Here, we report that the chemical vapor deposition-grown few-layer ReS2 and ReSe2 display saturable absorption under 400 nm pulse excitation, measured through intensity dependent transmission and confirmed by ΔT/T spectroscopy. ΔT/T spectroscopy substantiates the coexistence of saturable absorption and excited state absorption at 800 nm for both ReS2 and ReSe2. Two time constants are extracted from time-resolved spectroscopy; the short time constant of 10–20 ps is associated with the relaxation of hot carriers and exciton formation, and the long time constant of 70–100 ps is assigned to exciton lifetime. The polarization dependence of ΔT/T reveals that the initial distribution of photoexcited carriers centered at excitation state is anisotropic, and this initial anisotropy loses rapidly with carrier relaxation. The nonequilibrium carriers scattered far away from excitation state are fully isotropic in the entire relaxation process. These findings provide fundamental information for using the two materials in ultrafast optoelectronic and photonic devices.

Experimental research on an L-band multi-wavelength erbium-doped fiber laser based on a cascaded Sagnac loop and M–Z filters

In this paper, we proposed and experimentally demonstrated an L-band multi-wavelength erbium-doped fiber (EDF) laser with a cascaded Sagnac loop, M–Z interferometer filters and a non-linear optical loop mirror (NOLM). The NOLM acted as the intensity-dependent transmissivity; it can effectively alleviate the mode competition and beating caused by homogeneous broadening. The transfer characteristic of the Sagnac loop was investigated firstly and got the optimized parameters. Then the laser output spectrum with different kinds of filters in the cavity was experimentally tested when the pump power was fixed at 450 mW. The experimental results showed that the EDF laser with the cascaded filters in the cavity had a better performance, such as a better flat lasing bandwidth within 3 dB, higher side-mode-suppression ratio and more comb lines generated. The peak power fluctuation was less than 0.35 dB within a 30 min monitor time.

A pulsewidth measurement technology based on carbon-nanotube saturable absorber

We demonstrate a proof-of-concept saturable absorption based pulsewidth measurement (SAPM) by exploring the intensity dependent nonlinear transmission (i.e., saturable absorption) of low-dimensional material (LDM) carbon nanotubes. A minimum pulse energy of 75 fJ is experimentally detected with an average-power-peak-power product (Pav⋅Ppk) of 5.44×10-7 W2 near 1550 nm. A minimum detectable pulse energy of 10 fJ with a Pav⋅Ppk of 1.3×10-9 W2 is estimated with further optimization. The nanometer-level thickness and femtosecond-level decay time of LDMs allow ultrafast light interaction on a very small footprint, which potentially supports chip-scale characterization of ultrafast pulses with minimum distortion.

Two-photon absorption and non-resonant electronic nonlinearities of layered semiconductor TlGaS2.

The ultrafast nonlinear optical properties of bulk TlGaS2 crystal, a semiconductor with a layered structure, are studied by combining intensity dependent transmission, time-resolved transient absorption, and optical Kerr effect coupled to optical heterodyne detection. TlGaS2 demonstrates obvious two-photon absorption and electronic nonlinearities at 800 nm. The two-photon absorption coefficient and the nonlinear refractive index are determined to be of the order of 10-10 cm/W and 10-14 cm2/W, respectively. Furthermore, both the real and imaginary parts of the complex third-order susceptibility tensor elements are extracted. The large ultrafast optical nonlinearities make TlGaS2 a promising material for application in photonic techniques.

Intensity-dependent resonant transmission of x-rays in solid-density aluminum plasma

X-ray free-electron lasers (XFELs) provide unique opportunities to generate and investigate dense plasmas. The absorption and transmission properties of x-ray photons in dense plasmas are important in characterizing the state of the plasmas. Experimental evidence shows that the transmission of x-ray photons through dense plasmas depends greatly on the incident XFEL intensity. Here, we present a detailed analysis of intensity-dependent x-ray transmission in solid-density aluminum using collisional-radiative population kinetics calculations. Reverse saturable absorption (RSA), i.e., an increase in x-ray absorption with intensity has been observed for photon energies below the K-absorption edge and in the intensity range of 1016–1017 W/cm2 for XFEL photons with 1487 eV. At higher intensities, a transition from RSA to saturable absorption (SA) is predicted; thus, the x-ray absorption decreases with intensity above a threshold value. For XFEL photon energies of 1501 eV and 1515 eV, the transition from RSA to SA occurs at XFEL intensities between 1017–1018 W/cm2. Electron temperatures are predicted to be in the range of 30–50 eV for the given experimental conditions. Detailed population kinetics of the charge states explains the intensity-dependent absorption of x-ray photons and the fast modulation of XFEL pulses for both RSA and SA.

Nonlinear terahertz metamaterials with active electrical control

We present a study of an electrically modulated nonlinear metamaterial consisting of an array of split-ring resonators fabricated on n-type gallium arsenide. The resonant metamaterial nonlinearity appears as an intensity-dependent transmission minimum at terahertz frequencies and arises from the interaction between local electric fields in the split-ring resonator (SRR) capacitive gaps and charge carriers in the n-type substrate. We investigate the active tuning range of the metamaterial device as the incident terahertz field intensity is increased and conversely the effect of an applied DC bias on the terahertz field-induced nonlinear modulation of the metamaterial response. Applying a DC bias to the metamaterial sample alters the nonlinear response and reduces the net nonlinear modulation. Similarly, increasing the incident terahertz field intensity decreases the net modulation induced by an applied DC bias. We interpret these results in terms of DC and terahertz-field-assisted carrier acceleration, scattering, and multiplication processes, highlighting the unique nature of this DC-field modulated terahertz nonlinearity.

Switchable dual-wavelength SLM narrow linewidth fiber laser based on nonlinear amplifying loop mirror

We demonstrate a stable switchable dual-wavelength single longitudinal mode (SLM) narrow linewidth ytterbium-doped fiber (YDF) laser using a nonlinear amplifying fiber loop mirror (NALM) at 1064nm. The NALM of intensity-dependent transmission acts as a saturable absorber filter and an amplitude equalizer to suppress mode competition and the fiber Bragg grating (FBG) pair is used as one wavelength selection component. By properly adjusting the polarization controllers (PCs), the switchable dual-wavelength SLM fiber laser can be operated steadily at room temperature. The optical signal-to-noise ratio (OSNR) is better than 50dB for both lasing wavelengths. Meanwhile, the linewidth of the fiber laser for each wavelength is approximate 17.07kHz and 18.64kHz with a 20dB linewidth, which means the laser linewidth is approximate 853Hz and 932Hz FWHM. Correspondingly, the measured relative intensity noise (RIN) is less than −120dB/Hz at frequencies over 5.0MHz.

L-Band multi-wavelength erbium-doped fibre laser based on non-linear optical loop mirror and dual-channel Mach–Zehnder interferometer

In this letter, we propose and demonstrate an L-Band linear cavity tunable multi-wavelength erbium-doped fibre laser based on non-linear optical loop mirror (NOLM) and dual-channel Mach–Zehnder interferometer (MZI) . The NOLM provides the intensity-dependent transmissivity, can effectively alleviate the mode competition and beating caused by the homogeneous gain broadening, so that the multi-wavelength lasing can be achieved at room temperature. The dual-channel MZI, configured by linking the two outputs of the single-channel MZI, serves as comb filter. By adjusting the polarization controller in NOLM and pump power, the tunable multi-wavelength output at 1600 nm can be achieved. Moreover, the output stability of the laser has also been accomplished .

Improvement of the temporal and spatial contrast of high-brightness laser beams

A novel method is suggested for temporal and spatial cleaning of high-brightness laser pulses, which seems more energy-scalable than that based on crossed polarizers and offers better contrast improvement compared to the plasma mirror technique. The suggested arrangement utilizes nonlinear modulation of the beam in the Fourier-plane leading both to directional and to temporal modulation. By the use of a ‘conjugate’ aperture arrangement before and after the nonlinear spatial selector, intensity dependent transmission is obtained; simultaneous temporal and spatial filtering can be realized both for amplitude and phase modulation. In the case of phase modulation introduced by plasma generation in noble gases the experimental observations are in good agreement with the theory; demonstrating >103 improvement in the temporal contrast, ~40% throughput, associated with effective spatial filtering. Due to the broad spectral and power durability of the optical arrangement used here, the method is widely applicable for energetic beams even of UV wavelengths, where most of the former techniques have limited throughput.

High-power, high-repetition-rate performance characteristics of β-BaB₂O₄ for single-pass picosecond ultraviolet generation at 266 nm.

We report a systematic study on the performance characteristics of a high-power, high-repetition-rate, picosecond ultraviolet (UV) source at 266 nm based on β-BaB2O4 (BBO). The source, based on single-pass fourth harmonic generation (FHG) of a compact Yb-fiber laser in a two-crystal spatial walk-off compensation scheme, generates up to 2.9 W of average power at 266 nm at a pulse repetition rate of ~80 MHz with a single-pass FHG efficiency of 35% from the green to UV. Detrimental issues such as thermal effects have been studied and confirmed by performing relevant measurements. Angular and temperature acceptance bandwidths in BBO for FHG to 266 nm are experimentally determined, indicating that the effective interaction length is limited by spatial walk-off and thermal gradients under high-power operation. The origin of dynamic color center formation due to two-photon absorption in BBO is investigated by measurements of intensity-dependent transmission at 266 nm. Using a suitable theoretical model, two-photon absorption coefficients as well as the color center densities have been estimated at different temperatures. The measurements show that the two-photon absorption coefficient in BBO at 266 nm is ~3.5 times lower at 200°C compared to that at room temperature. The long-term power stability as well as beam pointing stability is analyzed at different output power levels and focusing conditions. Using cylindrical optics, we have circularized the generated elliptic UV beam to a circularity of >90%. To our knowledge, this is the first time such high average powers and temperature-dependent two-photon absorption measurements at 266 nm are reported at repetition rates as high as ~80 MHz.

A nonlinear optical loop mirror-based linear cavity switchable multi-wavelength erbium-doped fiber laser

A nonlinear optical loop mirror (NOLM)-based linear cavity switchable multi-wavelength erbium-doped fiber (EDF) laser is proposed and experimentally demonstrated. Due to the characteristics of the intensity-dependent transmissivity, the NOLM can effectively mitigate the mode competition of the homogenous broadening gain medium, so that the multi-wavelength lasing can be achieved at room temperature. By adjusting the states of the polarization controllers (PCs), the number of the lasing wavelengths in the proposed laser can be adjusted flexibly from 11 to 13 with a wavelength spacing of 0.4 nm around the wavelength of 1 530 nm.

Mode-locking of Er-doped fiber laser using a multilayer MoS2 thin film as a saturable absorber in both anomalous and normal dispersion regimes.

Application of a multilayer Molybdenum Disulfide (MoS2) thin film as a saturable absorber was experimentally demonstrated by realizing a stable and robust passive mode-locked fiber laser via the evanescent field interaction between the light and the film. The MoS2 film was grown by chemical vapor deposition, and was then transferred to a side polished fiber by a lift-off method. Intensity-dependent optical transmission through the MoS2 thin film on side polished fiber was experimentally observed showing efficient saturable absorption characteristics. Using erbium doped fiber as an optical gain medium, we built an all-fiber ring cavity, where the MoS2 film on the side polished fiber was inserted as a saturable absorber. Stable dissipative soliton pulse trains were successfully generated in the normal dispersion regime with a spectral bandwidth of 23.2 nm and the pulse width of 4.98 ps. By adjusting the total dispersion in the cavity, we also obtained soliton pulses with a width of 637 fs in the anomalous dispersion regime near the lasing wavelength λ = 1.55 μm. Detailed and systematic experimental comparisons were made for stable mode locking of an all-fiber laser cavity in both the normal and anomalous regimes.

Nonlinear high-temperature superconducting terahertz metamaterials

We report the observation of a nonlinear terahertz response of split-ring resonator arrays made of high-temperature superconducting films. Intensity-dependent transmission measurements indicate that the resonance strength decreases dramatically (i.e. transient bleaching) and the resonance frequency shifts as the intensity is increased. Pump–probe measurements confirm this behaviour and reveal dynamics on the few-picosecond timescale.