Tunable Laser Output Research Articles

Wavelength tunable single-frequency ring cavity fiber laser utilizing external injection locking signal

The ring cavity fiber laser with saturable absorption fibers is a simple and practical technical way of achieving single-frequency laser output. However, this structure cannot obtain wavelength-tunable laser output. In this work, an external injection locking signal launched by tunable LD is used to optimize the tunability in single-frequency ring cavity fiber lasers. By aligning the external injection signal, tunable laser output ranging from 1544.13 to 1546.51 nm is achieved. The maximum output is 16.5 mW and the linewidth is less than 6.15 kHz. The operation process and the linewidth compression effect of external Injection signals are investigated. The results indicate that there is a correlation between the output signal-to-noise ratio and the output linewidth, and this wavelength-tuning structure has the potential for rapid tuning. This work provides a technical solution for achieving a tunable single-frequency laser in the ring cavity structure.

Broadband tunable single-frequency Yb-doped fiber laser with dual ring subcavity and Sagnac ring filter

We demonstrate a broadband tunable ultra-narrow linewidth single-frequency (SF) Yb-doped fiber laser (YDFL). A dual ring subcavity (DRS) as a filtering component to eliminate the dense longitudinal mode inherent in YDFL and theoretically analyzed its filtering characteristics. By inserting a 0.8 m unpumped polarization-maintaining Yb-doped fiber in the Sagnac ring, a dynamic narrowband grating (DNG) based on the saturable absorption (SA) effect is constructed to ensure SF laser output over the entire wavelength tuning range. We achieve a wide wavelength tunable SF laser output in the range of ∼74 nm, covering 1021.06 nm to 1094.98 nm. The laser has an optical signal-to-noise ratio (OSNR) of more than 62.5 dB, a slope efficiency of 3.57 %, and an ultra-narrow linewidth of 622 Hz over the entire tunable range. In addition, the stability of the wavelength and power fluctuations below 0.018 nm and 0.67 dB, respectively. This tunable SF fiber laser has a compact structure, lower cost, and excellent performance, and is expected to be used in the fields of coherent communications, precision metrology, and so on.

Switchable and tunable multi-wavelength random distributed feedback fiber laser based on parallel Lyot filter

A switchable and tunable multi-wavelength random distributed feedback fiber laser based on a parallel Lyot filter is designed, and the transmission characteristics of the filter are deduced in detail by using the transmission matrix theory. The randomly distributed backward Rayleigh scattering generated in single-mode fiber (SMF) serves as a feedback mechanism. The number of output wavelength increases linearly with the increase of pump power. By adjusting the polarization controller (PC) to control the polarization state of the beam into the polarization maintaining fiber (PMF), laser channels with wavelength interval of 0.88 nm or 0.46 nm can be output. Where the wavelength interval depends on the wavelength phase difference caused by the corresponding branch PMF. The intensity-dependent loss (IDL) caused by the nonlinear optical loop mirror (NOLM) in the experimental structure is not only used to suppress the intense mode competition caused by EDF, but also to control the spectral range of the output wavelength to achieve tunable laser output. When the wavelength interval is 0.88 nm, the wavelength tuning ranges of the three and four laser channels are 4.4 nm and 3.7 nm, respectively. When the wavelength interval is 0.46 nm, the wavelength tuning ranges of the three and four laser channels are 3.7 nm and 4 nm, respectively. Subsequently, the influence of using different lengths of SMF on the output wavelength is analyzed. It is found that longer SMF can increase the linewidth of the laser channel, and the side mode suppression ratio (SMSR) will be improved to 39 dB. In addition, the output power fluctuation of the laser is less than 0.32 dB within 30 min, and the power amplitude and SMSR are stable at −4.47dBm and 34.5 dB, respectively, indicating that the laser can achieve flat and stable multi-wavelength output at room temperature.

Recent advances in IR nonlinear optical chalcogenides with well-balanced comprehensive performance

Infrared nonlinear optical (IR-NLO) crystals have attracted many attentions due to the frequency-conversion capability to generate coherent tunable IR laser output for military and civil applications. Chalcogenides as a most popular branch of IR-NLO materials are the competitive candidates owing to their large second-harmonic generation (SHG) coefficient (deff) and broad transmission regions. Currently, how to obtain the non-centrosymmetric (NCS) structure and realize the coexistence of wide energy gap (Eg ≥ 3.0 eV), sufficient deff (≥ 1.0 × AgGaS2), and beneficial phase-matching (PM) feature are the two major challenges faced in this field. The former can be effectively solved through the combination of local asymmetric units and chemical substitution strategy, while for the latter, there is no systematic survey and lack of universally applicable methodology. In this review, 40 IR-NLO chalcogendides are selected based on the aforementioned two challenges and divided into 7 types according to the different chemical components. The composition-structure-performance relationships and the theoretical origin of their IR-NLO properties are summarized systematically. At length, some useful information and development prospects were briefly discussed, which may offer new ideas and promote the discovery of IR-NLO materials with the ideal comprehensive performance.

Organic Synthetic Photonic Systems with Reconfigurable Parity-Time Symmetry Breaking for Tunable Single-Mode Microlasers.

Synthetic photonic materials exploiting the quantum concept of parity-time (PT) symmetry lead to an emerging photonic paradigm-non-Hermitian photonics, which is revolutionizing the photonic sciences. The non-Hermitian photonics dealing with the interplay between gain and loss in PT synthetic photonic material systems offers a versatile platform for advancing microlaser technology. However, current PT-symmetric microcavity laser systems only manipulate imaginary parts of the refractive indices, suffering from limited laser spectral bandwidth. Here, an organic composite material system is proposed to synthesize reconfigurable PT-symmetric microcavities with controllable complex refractive indices for realizing tunable single-mode laser outputs. A grayscale electron-beam direct-writing technique is elaborately designed to process laser dye-doped polymer films in one single step into microdisk cavities with periodic gain and loss distribution, which enables thresholdless PT-symmetry breaking and single-mode laser operation. Furthermore, organic photoisomerizable compounds are introduced to reconfigure the PT-symmetric systems in real-time by tailoring the real refractive index of the polymer microresonators, allowing for a dynamically and continuously tunable single-mode laser output. This work fundamentally enhances the PT-symmetric photonic systems for innovative design of synthetic photonic materials and architectures.

Growth and spectroscopic properties investigation of Er-doped BGSO single crystal: A potential gain medium for eye-safe laser

Erbium-doped crystals are well-suited for eye-safe solid-state laser systems around 1.5 μm. Broadband and high-gain Er-doped laser crystals can achieve high-power tunable laser output, which plays a great role in promoting commercial eye-safe laser research. Herein, the 2 at% Er-doped bismuth germanium silicate (BGSO) laser crystal is successfully grown by the vertical Bridgman method. The X-ray Rietveld refinement was used to obtain the structural parameters. The spectroscopic parameters of Er ions in BGSO crystal are calculated and analyzed with Judd-Ofelt theory. Furthermore, the broadband luminescence is observed in the wavelength range of 1450–1600 nm. The stimulated emission cross-section is calculated around 1500 nm, which largest value is 5.31 × 10−20 cm2 at 1543 nm. Meanwhile, the luminous efficiency of BGSO crystal in the eye-safe band is further characterized and explored by studying the energy transfer process and anomalous thermal quenching in the 550 nm and 660 nm bands. All results indicate that the Er: BGSO crystal is a promising broadband eye-safe laser material.

Comparison of back conversion and power differential in extra-cavity multi-optical parametric oscillator using MgO:APLN

The extra-cavity multi-optical parametric oscillator (MOPO) using MgO-doped aperiodically poled lithium niobite (MgO:APLN) is a technique to generate multi-wavelength tunable laser output, in which the back conversion and power differential can reduce the output power and conversion efficiency. A comparison of the back conversion and power differential between the two methods is presented in this paper. One approach is to adjust the mirror transmittance to change the parametric light's power density involved in the oscillation. With 1.47 μm transmittance of 20 % and 3.30 μm transmittance of 60 %, the 3.30 μm output power is 0.95 W, the 3.84 μm output power is 0.94 W, and the power ratio is 1.01. The other approach is to add two electro-optical polarization mode converters (EO PC) to change the parametric light's polarization. When the electric fields of two EO PCs are 66 V/mm and 15 V/mm, the 3.30 μm and 3.84 μm output powers are 2.54 W and 2.49 W, the conversion efficiencies are 10.3 % and 10.0 %, and the power ratio is 1.02. The comparison results confirm that both methods have the capability of back conversion suppression and power differential reduction. Compared with the adjusting transmittance, adding EO PCs has higher output power and conversion efficiency, which is the first reported method for better balanced multi-wavelength laser output.

The sensitization and deactivation effects of Nd3+ ion in Nd,Ho:CaYAlO4 disordered crystal for 2.86 µm tunable and ultrafast laser

Nd,Ho:CaYAlO4 (Nd,Ho:CYA) crystals with two different Nd3+ doping concentrations for ∼3 µm mid-infrared (MIR) laser were grown successfully by the Czochralski method. The sensitization and deactivation effects of Nd3+ ions were analyzed based on the polarized absorption spectra, mid-infrared and upconversion fluorescence spectra and fluorescence decay curves in detail. Excited by 808 nm laser diode (LD), strong 2.86 µm emission was observed. The maximum emission cross section was calculated to be 1.93 × 10-20 cm2 in 1.0 at.% Nd, 2.5 at.% Ho:CYA, and the FWHM was about 115 nm. The ∼3 µm emission was enhanced by 10 times compared to the 2.5 at.% Nd, 2.5 at.% Ho:CYA crystal. Due to the dual function of Nd3+ as effective sensitizing and deactivating ion, 1.0 at.% Nd, 2.5 at.% Ho:CYA had a relatively longer lifetime 0.240 ms for the upper level 5I6, while the lifetime of the lower level 5I7 reduced to 0.723 ms, which was greatly advantageous to the population inversion. The calculated deactivation efficiency from Ho3+: 5I7 to Nd3+: 4F13/2 was estimated to be 89.66%. All results indicated that 1.0 at.% Nd, 2.5 at.% Ho:CYA crystal is a promising gain medium for ∼3 µm tunable and ultrafast laser output under 808 nm LD pump.

Supercontinuum comb generated by soliton molecule pulse laser injecting into a nonlinear amplifying loop mirror

Very first time, we experimentally demonstrated the generation of supercontinuum (SC) comb by pumping a tunable soliton molecule fiber laser output signal into a nonlinear amplifying loop mirror (NALM). SC signals with different comb-teeth structures were obtained by controlling the pump power and waveplate orientation of the polarization-maintained soliton molecule fiber laser. The maximum achieved output powers of dual SC combs from two ports were 21.95 (output 1) and 3.75 mW (output 2), respectively. The spectral and temporal profiles of the SC comb were compared with the simulation results and the first-order coherence was evaluated. Our experiment provides a novel technique for the generation of SC signal with various comb-teeth structures that can be utilized in broadband spectrometers and full-field 3D metrology.

Theoretical and experimental analysis of second order Bragg resonance of 45° TFG and its fiber laser application

We report the observation of second order Bragg resonance (2nd-OBG) produced by tilted fiber gratings (TFGs) fabricated using phase mask UV inscription. The theoretical analysis has revealed that the generation of high order Bragg resonance of gratings is induced by a square-shape refractive index profile, which is caused by over-saturated UV exposure. In the experiment, we have studied the TFGs with different tilt angles under over-saturated UV exposure, in which all gratings have showed the 2nd-OBR, and the larger tilt angle of the grating has the stronger 2nd-OBR. When the tilt angle of the grating is ∼45°, the Bragg resonance exhibits very strong polarization dependence, because the 2nd-OBR wavelength is located within the polarizing bandwidth of 45° TFG. Finally, we have demonstrated an erbium-doped fiber laser with >99.9% degree of polarization, and, by applying mechanical stretching on the grating, a wavelength tunable laser output has been achieved. The output laser shows ∼0.2 dB amplitude variation within 1 h continuous monitoring of the laser.

Linear cavity erbium-doped fiber laser employing PM-PCF based Sagnac loop comb filter for wavelength tunable laser generation

A wavelength tunable linear cavity erbium-doped fiber (EDF) laser based on Sagnac loop incorporating with polarization-maintaining photonic crystal fiber (PM-PCF) was proposed and demonstrated experimentally. The Sagnac loop was composed of 0.5 m length PM-PCF, 1 × 2 optical coupler with 50:50 splitting ratio, and one polarization controller (PC). For the proposed fiber laser, the working threshold was 35 mW, the 3 m length EDF was used as gain medium, and 0.5 m length EDF was selected as saturable absorber to improve lasers stability. In the experiment, the comb filter effect was generated by Sagnac interferometer, and period was 2.52 nm. When the pump power was 150 mW, single-wavelength tunable laser output was realized within the range of 1528.73–1531.45 nm through adjusting PC, wavelength tuning interval was more than 0.58 nm, power difference was less than 3.681 dB, and signal to noise ratio (SNR) were higher than 31.101 dB. For the designed fiber laser, different groups of dual- and triple-wavelength lasers output were obtained, the power difference was lower than 7.349 and 7.724 dB, and SNR was more than 31.752 and 38.623 respectively. The single-, dual- and triple-wavelength laser stability was test within 20 min, and power and wavelength fluctuation were 3.325 dB and 0.07 nm respectively.

Tunable erbium-doped-fiber laser based on the Double-Superimposed Fiber Bragg Gratings

Multi-wavelength D-SIFBG has the advantages of small size, wavelength interval and flexible reflectivity, so it is suitable for tunable EDFL. Therefore, a single wavelength tunable EDFL based on multi-wavelength D-SIFBG is proposed. The pump source of the laser is a 980 nm semiconductor laser, and the resonator is composed of SIFBG1 with a reflectivity higher than 91% and SIFBG2 with a reflectivity higher than 87%. The gain medium is erbium-doped fiber. Based on gain equalization technology, stable single-wavelength tunable laser output is obtained at room temperature. The test results show that the 3 dB line width is less than 0.020 nm, the optical signal-to-noise ratio is more than 60.4 dB, the central wavelength shift and maximum output power fluctuation are less than 0.004 nm and 1.25 dBm respectively, and the maximum tunable range is 5.796 nm. The single-wavelength laser output from this multi-wavelength laser has good stability.

Multi- and dual-wavelength Thulium-doped fluoride fiber laser assisted by four-wave mixing in S-band region

In this work, multi-wavelength lasing in the S-band region using a Sagnac loop filter in a thulium-doped fluoride fiber laser (TDFFL) was demonstrated for the first time. The Sagnac loop served as a comb-like wavelength filter, while a 10-km long dispersion compensating fiber (DCF) was utilized to induce the four-wave mixing (FWM) effect in the cavity. The FWM effect was used to suppress mode competition in the TDFFL as well as to enhance the number of lasing lines. At its optimum polarization state, 17 stable lasing lines at peak powers of less than 10 dB from the highest peak power were observed, with these lasing lines having a uniform wavelength spacing of 0.23 nm. The multi-wavelength operation exhibited minimal wavelength drifts and power fluctuations of less than 0.01 nm and 1.2 dB respectively. A tunable dual-wavelength laser output was also demonstrated, with a constant wavelength spacing throughout the tuning range.

Growth and characterization of Ti:MgAl<sub>2</sub>O<sub>4</sub> laser crystal by Czochralski method

The melting point of Ti:MgAl<sub>2</sub>O<sub>4</sub> crystal is as high as 2130 °C, it is a challenge to obtain a large-sized and high-quality laser crystal. By optimizing the crystal growth process, Ti:MgAl<sub>2</sub>O<sub>4</sub> crystal with a size of 30 mm× 70 mm is successfully grown by the Czochralski method under the condition of weak reducing atmosphere. The X-ray diffraction pattern is studied, and the x-ray rocking curve indicates that the grown crystal has a high crystalline quality in terms of the lower full width at half maximum(FWHM) intensity, which provides a material basis for the next laser output experiment. In a range of 100–1000 cm<sup>–1</sup>, there are four Raman vibration peaks located at 312, 410, 675 cm<sup>–1</sup> and 771 cm<sup>–1</sup> respectively. The grown crystal has an absorption cutoff range of 250–318 nm and two wide absorption bands of 395–495 nm and 550–1100 nm. Excited by 271 nm, the grown crystal shows a strong broadband emission ina range of 340–650 nm with a peak centered at 480 nm. After annealing in hydrogen atmosphere, shape of the transmittance spectrum and emission spectrum are both unchanged, but the fluorescent emission intensity is significantly reduced. After annealing in air atmosphere, the original two absorption bands disappear while none of the characteristics of fluorescence emission in a 340–650 nm range changes significantly. In addition, a new fluorescence emission peak near 725 nm is observed. Combining with the ESR spectrum, what we canconfirm is that the Ti:MgAl<sub>2</sub>O<sub>4</sub> as-grown crystal contains Ti<sup>3+</sup> and Ti<sup>4+</sup> ions, and no ESR signal of Ti<sup>3+</sup> is observed after annealing in air atmosphere. Moreover, excitationspectrum is also recorded. The fluorescence lifetime is 14 μs at room temperature, which is 4–5 times that of Ti:Al<sub>2</sub>O<sub>3</sub> crystal and Ti:BeAl<sub>2</sub>O<sub>4</sub> crystal. Furthermore, the emission cross section of the grown Ti:MgAl<sub>2</sub>O<sub>4</sub> crystal is calculated from the Füchtbauer-Ladenburg (F-L) formula and its value is 2 × 10<sup>–20</sup> cm<sup>2</sup>, large emission cross section which is beneficial for realizing laser oscillation. All the above results show that the Ti:MgAl<sub>2</sub>O<sub>4</sub> crystal is a potential crystal material for realizing broadband tunable blue laser output.

Intravital multiphoton microscopic imaging platform for ocular surface imaging

The purpose of this study is to provide an intravital noninvasive multiphoton microscopic platform for long-term ocular imaging in transgenic fluorescent mice with subcellular resolution. A multiphoton microscopic system with tunable laser output was employed. We designed a mouse holder incorporated with stereotaxic motorized stage for in vivo three-dimensional imaging of ocular surface in 3 transgenic mouse line with fluorescent protein (FP) expression to visualize distinct structures. With our imaging platform and the expression of FPs, we obtained the three-dimensional images across the whole cornea from epithelium to endothelium and in conjunctiva with subcellular resolution in vivo. Specified EGFP expression in corneal epithelium of K5-H2B-EGFP mice helped to identify both corneal and limbal epithelial cells while ubiquitous nuclear FP expression in R26R-GR mice allowed us to visualized nuclei of all cell types. Universal membrane-localized FP in mT/mG mice outlined all cell boundaries, nerve fibers, and capillaries. The simultaneously collected second harmonic generation signals from collagenous stroma provided architectural contrast. Time-lapsed recording enabled monitoring the mitotic activity of corneal epithelial cells and limbal epithelial cells. We developed an intravital multiphoton microscopic stereotaxic imaging platform and showed that, by incorporating FP-expressing transgenic mice, this platform enables in vivo 4-dimensional ophthalmic study at subcellular resolution.

Measurement of photoionization cross-section of Li (2p, 3d) excited states using thermionic diode detector

Studies on photoionization cross-section measurement of lithium 2p and 3d excited states selectively populated by single and two photons are reported. The photoionization is carried out in a heat-pipe cum thermionic diode detector using nitrogen laser pumped pulsed dye laser. Rhodamine perchlorate (Rh-640) dye is used to generate the desired tunable laser output in the range of 635 to 675 nm. Both, the thermionic diode detector and dye laser are developed in-house. A negative space charge region formed around diode cathode filament is illuminated by dye laser exciting pulse followed by ionizing UV radiation from nitrogen laser. The effect of ionizing laser pulse energy and precise tuning of dye laser on photoionization signal, across 670.780 nm (2S1/2 → 2P1/2,3/2) single-photon transition and 639.145 nm (2S1/2 → 2D3/2,5/2) two-photon transition are studied. The results obtained are compared with model calculations of photoionization signal saturation curve. The ionization cross-section for both the states (2P1/2, 2D5/2) are determined as 15.40 ± 2.8 Mb and 8.04 ± 1.6 Mb, respectively and atomic number density N ∼ 5.6 × 108 (cm−3).

Electron paramagnetic resonance and optical absorption study of acceptors in CdSiP2 crystals

Cadmium silicon diphosphide (CdSiP2) is a nonlinear material often used in optical parametric oscillators (OPOs) to produce tunable laser output in the mid-infrared. Absorption bands associated with donors and acceptors may overlap the pump wavelength and adversely affect the performance of these OPOs. In the present investigation, electron paramagnetic resonance (EPR) is used to identify two unintentionally present acceptors in large CdSiP2 crystals. These are an intrinsic silicon-on-phosphorus antisite and a copper impurity substituting for cadmium. When exposed to 633 nm laser light at temperatures near or below 80 K, they convert to their neutral paramagnetic charge states (SiP0 and CuCd0) and can be monitored with EPR. The corresponding donor serving as the electron trap is the silicon-on-cadmium antisite (SiCd2+ before illumination and SiCd+ after illumination). Removing the 633 nm light and warming the crystal above 90 K quickly destroys the EPR signals from both acceptors and the associated donor. Broad optical absorption bands peaking near 0.8 and 1.4 μm are also produced at low temperature by the 633 nm light. These absorption bands are associated with the SiP0 and CuCd0 acceptors.

A novel crystal Tm: Lu0.8Gd0.8Y0.4SiO5 with potential for broadband tunable lasing

A novel Tm3+ -doped crystal Lu0.8Gd0.8Y0.4SiO5 (Tm: LGYSO) was grown by the Czochralski (Cz) method. The full width half maximum (FWHM) of the Tm:LGYSO crystal around 2 μm was as broad as 280 nm, which is the broadest FWHM among the Tm3+ doped crystals have been reported, to the best of our knowledge. The crystal growth, structure, absorption spectra, J-O parameters, emission spectra and fluorescence lifetime were discussed in details. The absorption cross-section is 1.429 × 10−20 cm2 and the full width half maximum is 8.337 nm at 791 nm. The spectroscopic parameters were calculated using the Judd-Ofelt theory, and the J-O parameters Ω2, Ω4 and Ω6 were found to be 1.615 × 10−20, 2.773 × 10−20 and 0.695 × 10−20 cm2, respectively. An intensity and broad 2 μm emission of the 3F4 → 3H6 transition in Tm: LGYSO crystal was observed with an emission cross-section of 0.73672 × 10−20 cm2 at 1972 nm. The fluorescence decay time of the 3F4 → 3H6 transition was measured to be 1.669 ms. Results suggest that Tm: LGYSO crystal is a promising laser crystal for tunable and ultrafast laser output around the 2 μm wavelength.

Off-surface optic axis birefringent filters for smooth tuning of broadband lasers.

Transition metal-doped gain media, such as Ce:LiCAF, Ti:sapphire, Cr:ZnSe, and Fe:ZnSe, possess wide gain bandwidths that could provide ultra broadly tunable laser output with the usage of adequate intracavity tuning elements. Birefringent filters (BRFs) are a low-cost and easy to use solution for tuning. However, for ultrabroad gain media, regular on-surface optic axis BRFs could not provide smooth tuning of laser wavelength in the whole emission range. Basically, regular BRFs could not accommodate a large enough free spectral range with acceptable modulation depth variation while tuning, due to their slow tuning rates. Motivated by this, in this study, we have numerically investigated the effect of optic axis orientation on filter parameters for magnesium fluoride birefringent tuning plates. We have shown that a magnesium fluoride BRF with an optic axis diving by 30° into the plate could provide smooth tuning of ultra-broad laser gain media. A similar analysis has shown that for broadband tuning applications, the optimum optic axis diving angle lies around 25° for crystal quartz BRFs. The proposed filters have the potential to be useful in tuning of broadband lasers in continuous-wave, long-pulsed, and femtosecond operation regimes.

采用可调Mach-Zehnder滤波的波长可切换掺铒光纤激光器

In order to realize a highly stable and tunable laser output, an Er3+-doped fiber laser based on Mach-Zehnder filter structure and grating array made by femtosecond laser was proposed, and an optical delay line(ODL) was set into one interference arm to tune the interference interval flexibly. In the designed configuration, one 976 nm laser diode (LD) was used for pumping; an Er3+-doped fiber (EDF) of 6 m length was selected as the gain medium; The grating array will selete the femtosecond light reflection loop cavity specific wavelength oscillation form; the M-Z structure, which was used as comb filter, was adopted to select the wavelength reflected by the grating array; Tuning laser output can be realized by adjusting the ODL. In the experiment, when the pump power was 100 mW, a stable and tunable single, dual and triple wavelength output laser was observed. The single wavelength tunable interval was 2 nm within 1 570-1 596 nm, and the side-mode suppression ratios (SMSRs) of all were more than 50 dB.