Laser Gain Research Articles

Monolithic edge-cladding process for the elliptical disk of N31-type Nd-doped high-power laser glass

Abstract This paper investigates the monolithic edge-cladding process for the elliptical disk of N31-type Nd-doped phosphate laser glass, which will be utilized under liquid cooling conditions for high-power laser systems. The thermal stress, interface bubbles and residual reflectivity, which are due to high-temperature casting and bonding during the monolithic edge-cladding process, are simulated and determined. The applied mould is optimized to a rectangular cavity mould, and the casting temperature is optimized to 1000°C. The resulting lower bubble density makes the mean residual reflectivity as low as 6.75 × 10−5, which is enough to suppress the amplified spontaneous emission generated in the Nd-glass disk, and the resulting maximum optical retardation is converged to 10.2–13.3 nm/cm, which is a favourable base for fine annealing to achieve the stress specification of less than or equal to 5 nm/cm. After fine annealing at the optimized 520°C, the maximum optical retardation is as low as 4.8 nm/cm, and the minimum transmitted wavefront peak-to-valley value is 0.222 wavelength (632.8 nm). An N31 elliptical disk with the size of 194 mm × 102 mm × 40 mm can be successfully cladded by the optimized monolithic edge-cladding process, whose edge-cladded disk with the size of 200 mm × 108 mm × 40 mm can achieve laser gain one-third higher than that of an N21-type disk of the same size.

Low-loss and Small-cross-section Waveguide for Compact Terahertz Free -electron Laser

The demand for terahertz (THz) sources that can be used in practical implementations has not yet been fully met. Through the development of small-scale free-electron lasers (FEL), we investigated a solution to overcome the low lasing gain of the FEL, which is caused by a small-current accelerator and a short-length undulator. To enhance the FEL gain, the FEL interaction between the electron beam and radiation was increased by reducing the cross-sectional area of the FEL oscillator mode. We developed a waveguide for the FEL oscillator, which has a remarkably small eye-shaped cross section with low wave loss. The mode cross-sectional area was calculated to be just 4 mm2 (full width at half maximum), which is considerably smaller than those of the free-space Gaussian mode and other waveguide modes. Using COMSOL Multiphysics simulation code, we calculated and analyzed the attenuation loss and mode cross-section area for waveguides of different shapes and sizes, and we observed that the dielectric coated eye-shaped waveguide has the lowest attenuation loss, that is, less than 2.5% for 1-m propagation at an operating wavelength of 300–600 µm. These results are in good agreement with our requirements for the tabletop THz FEL. Finally, we showed that the calculated operating wavelength range of the waveguide-mode THz FEL is 300–600 µm using the machine parameters of an accelerator and undulator that were developed by considering the dispersion relations of the eye-shaped waveguide and undulator radiation.

Optical gain in a free-electron laser with laser wiggler in the presence of a magnetized ion-channel

Optical gain in a free-electron laser with laser wiggler in the presence of a magnetized ion-channel

NV\u2013 diamond laser

For the first time, lasing at NV− centers in an optically pumped diamond sample is achieved. A nanosecond train of 150-ps 532-nm laser pulses was used to pump the sample. The lasing pulses have central wavelength at 720 nm with a spectrum width of 20 nm, 1-ns duration and total energy around 10 nJ. In a pump-probe scheme, we investigate lasing conditions and gain saturation due to NV− ionization and NV0 concentration growth under high-power laser pulse pumping of diamond crystal.

Luminescence Characteristics of Ho3+/Tm3+Co-Doped Bi2O3–GeO2–Ga2O3–Na2O Laser Glasses

The Ho3+/Tm3+co-doped 35Bi2O3–40GeO2–15Ga2O3–10Na2O laser glass was synthesized by high temperature melting method. The 2.0μm luminescence characteristics of system glass under central wavelength of 808 nm pumping light excitation were researched, and the fluorescence intensity changing with the doping concentration of Ho3+was analyzed at Tm3+content of 3-mol%. The results indicate that the 2.0μm luminescence of Ho3+is remarkably changed due to the doping ratio of Ho3+/Tm3+. The fluorescence intensity was achieved for the lase glass in the system when the sensitization ratio of Ho3+/Tm3+is 0.5:3 mol%. Its maximum emission cross-section at 2.05μm reaches 7.32 × 10−21cm2, while the gain characteristic ofσemi×τradvalue can get 35.50 × 10−21cm2· ms, and the gain coefficient achieves 2.30 cm−1. Furthermore, the Gaussian decomposition method was employed to fit the spectrum of host glass, maximum phonon energy is 812 cm−1. It indicates that the host gets small phonon energy which is benefit to the 2.0μm laser emission. In summary, results purport that Ho3+/Tm3+co-doped bismuth-germanate system laser glass is a good ~2μm laser gain glass material with good gain characteristics.

Structural and spectroscopic analyses on the multi former glasses developed with Eu3+ ions in favor of red laser gain medium applications

Certainly, multi glass formers are reckoned to be prospective efficient laser hosts due to their collective influence. Lead by this idea, a batch of Eu3+ ions activated modifier incorporated multi former glasses were fabricated via conventional melt quenching technique in favor of red laser applications. The produced glasses were collectively coded as BTPEM (M = Zn, Ba, Li, Sr & Ca) and their structural, spectroscopic traits were analyzed through XRD, SEM-EDX, FTIR, Raman, absorption and luminescence studies. The calculated positive bonding parameter (+δ) from the absorption study revealed the covalent nature of the titled glasses. The radiative parameters were derived from the JO intensity parameters (Ωλ) utilizing the emission spectra. Moreover, the luminescence spectra of the samples exposed a striking red peak at 614 nm (5D0→7F2 transition) with an elevated stimulated emission cross-section (σsec) and high luminescence branching ratio (βR) for the zinc containing glass (BTPEZn). Additionally, the acquired color coordinates (x, y) that lie close to the standard red color (x = 0.67, y = 0.33) accompanied with the high quantum efficiency (73%) affirmed the adequacy of the BTPEZn glass as a solid potential red laser host.

Heat-induced wavelength-switchable high-power CW orange Pr3+:YLF lasers

Pr3+:YLF crystal has been demonstrated as a successful laser gain medium in the visible region. However, the 604-nm laser based on this crystal is limited in power due to the intrinsic reabsorption in the orange region. We propose a laser diode (LD) pumped continuous-wave (CW) 604-nm laser based on a lowly-doped Pr3+:YLF crystal. A maximum output power of 3.28 W with a total slope efficiency of 34.2% is achieved at 604 nm. The slope efficiency increases to 43.0% under high-power pumping conditions. To the best of our knowledge, the output power of the 604-nm laser we achieved is the highest to date and no output power saturation is observed. Besides, dual-wavelength operations at 604 and 607 nm are achieved by tuning the thermal lensing effects of the crystal, without any additional wavelength-selection elements. The dual-wavelength laser at 604 and 607 nm with a maximum output power of 3.30 W (highest to date as well) is obtained by this approach. Since no extra intracavity losses are introduced by intracavity wavelength-selection elements, the method we proposed will be promising to obtain high-power wavelength-switchable lasers.

Review of the Yb3+:ScBO3 Laser Crystal Growth, Characterization, and Laser Applications

Passive Q-switching is an effective approach for generating pulsed lasers, owing to its compact and additional modulation-free design. However, to compare favorably with active Q-switching and multi-stage amplification, the output energy needs to be enhanced for practical applications. Kramers Ytterbium ion (Yb3+)-doped borate crystals, with their excellent energy storage capacity, have been proven to be high-potential laser gain mediums for achieving pulsed lasers with moderate and high output energy using passive Q-switching technology. In this study, the growth, characterization, and laser generation of one Yb3+-doped borate crystal, the Yb3+:ScBO3 crystal, are systematically reviewed. The continuous-wave and passive Q-switching laser characteristics are presented in detail, and the self-pulsations derived from intrinsic ground-state reabsorption are also demonstrated. The specific characteristics and experiments confirm the potential of the Yb3+:ScBO3 crystal for future pulsed laser applications with moderate or even high energy output.

Ultrafast Lasing Dynamics in a CsPbBr3 Perovskite Microplate

The perovskite microlaser is a competitive candidate of light source for optical communication and integrated photonic circuits. Understanding the fundamental mechanism of lasing is crucial for the upcoming devices. The ultrafast establishment of lasing modes within a 2D perovskite microplate cavity at room temperature is investigated. The transient red shift of lasing modes can be found. Analysis based on electron–hole plasma (EHP) and a dynamic Drude‐like model are carried out with simulations on the transient dielectric response and the mode shift. In addition, the integrated lasing gain profile have a red shift at high excitation intensity, which is explained with EHP‐induced bandgap renormalization. The conflict between the experimental phenomena and the exciton–polariton theory confirms that under intense excitation, the exciton–polariton is ruled out for the origin of lasing. These results provide a direct understanding of the lasing evolving in a 2D perovskite microcavity. Suppressing the EHP‐related transient shift of cavity modes will advance the lasing applications.

Effective sensitization of Yb3+ ions on Yb3+/Nd3+ co-doped fluoroborate glasses for NIR luminescence applications

Nd3+/Yb3+ co-doped fluoroborate glass samples were fabricated by melt quenching. The near infrared luminescence (NIR) spectral features of Nd3+/Yb3+ co-doped fluoroborate glasses were characterized by XRD, SEM, FT-IR, optical absorption, photoluminescence (PL) and PL decay spectral studies. An effective energy transfer process has been observed from Nd3+ to Yb3+ in the titled glasses under 808 nm excitation wavelength. Broad emission ranging from 850 to 1400 nm due to both Nd3+ (4F3/2→4I9/2, 4F3/2→4I11/2 and 4F3/2→ 4I13/2) and Yb3+ (2F5/2→2F7/2) transitions has been observed in all the glasses. The dependence of NIR emission, decay lifetime, and the energy transfer efficiency (ηETE) with the concentration of Yb3+ was studied in detailed. The PL spectra along with donor decay curves have been used to establish the energy transfer mechanism between Nd3+/Yb3+ ions. The results represent the possible sensitization of Yb3+ through 4f-4f transition of Nd3+ ions in titled glasses. The PL of the Nd3+ singly doped and Nd3+/Yb3+ co-doped titled glasses indicate their suitability as multiple pump channel sources for Yb3+ fiber laser systems. The results finally prove that the Nd3+/Yb3+ co-doped fluoroborate titled glasses are quite useful to fabricate infrared laser gain materials.

Recycling of Triplets into Singlets for High‐Performance Organic Lasers

AbstractAchieving continuous‐wave (CW) lasing in organic semiconductors is known to be a difficult task because long‐lived triplets quench radiative singlets via singlet–triplet annihilation (STA). To avoid STA and operate organic lasers in CW or long‐pulse photoexcitation, the triplets need to be removed from an organic laser gain medium. However, this triplet removal leads to a loss of excitons. In addition to removing the detrimental triplets, here it is reported a triplet recycling process, which includes triplet scavenging and successive triplet upconversion via triplet–triplet annihilation (TTA) to regenerate emissive singlet excitons in a laser medium. An anthracene derivative of 9‐(1‐naphthalenyl)‐10‐(4‐(2‐naphthalenyl)phenyl)anthracene (NaNaP‐A) and a laser dye of 4,4′‐bis[4‐(diphenylamino)styryl]biphenyl (BDAVBi) are used as the triplet recycling sensitizer and the emitting laser dye, respectively. In this laser system, NaNaP‐A can efficiently scavenge the triplets formed on BDAVBi because the triplet level is deeper for NaNaP‐A than for BDAVBi, and then NaNaP‐A successively recycles the triplets into the BDAVBi's singlet state via TTA. The TTA compensates and overcomes the STA in this laser system. Hence, these laser devices can be operated with long pulse widths of up to 10 ms. This unique triplet recycling behavior is confirmed by transient photoluminescence (PL) and electroluminescence (EL) studies.

Improved optical confinement in ambipolar field-effect transistors toward electrical injection organic lasers

Increasing optical confinement is critical to lowering laser thresholds and increasing modal gain in semiconductor lasers. Here, mode-solver calculations are used to demonstrate that improvements to optical confinement are possible in organic field-effect transistor geometries by using high refractive index cladding layers. Optical experiments show that the proposed structure increases the efficiency of amplified spontaneous emission (ASE) and lowers ASE thresholds without incurring additional losses. The results suggest that the structure can be used to improve optical confinement for both optically pumped and electrical injection organic lasers where thin, low refractive index active materials are required.

Development of a compact x-ray laser in the ‘water window’ at 4.0 nm

A narrow spectral range between 2.3 nm and 4.4 nm wavelengths, the so-called ‘water window’ (WW), provides a unique opportunity for time-resolved imaging of proteins in their natural environment, as water is semi-transparent, while carbon is mostly opaque at these wavelengths. In this work we are presenting experimental developments toward high laser gain in CV ions at 4.03 nm, in a table-top device, and discuss possible application of such an x-ray laser to high-resolution microscopy, as well as generation of attosecond pulses.

Performance Evaluation of Solid-State Laser Gain Module by Measurement of Thermal Effect and Energy Storage

The optimization of solid-state laser cavities requires a deep understanding of the gain module, the most critical laser component. This study proposes a procedure for evaluating the performance of the solid-state laser gain module. The thermal effect and energy storage characteristics are the performance criteria. A normalized heating parameter was calculated as a quantitative indicator of the performance criteria. We proposed a method to quantify the heat dissipated into the gain medium using the wavefront distortion, thermal deformation theory of the gain medium, and the ray transfer matrix method. The suggested procedure was verified by evaluating the flashlamp type Nd:YAG rod gain module, but it can also even be extended to other solid-state laser gain modules by applying the appropriate thermal deformation theory.

Mutually induced soliton polarization instability in a bidirectional ultrafast fiber laser.

The bidirectional ultrafast fiber laser is a promising light source for dual-comb applications. The counter-propagating geometry could lead to soliton interaction through gain sharing, as well as the possible outcome of polarization instability. However, the polarization dynamics hidden behind the soliton interaction process in bidirectional fiber lasers were rarely investigated. Herein, we report on the polarization instability induced by the mutual soliton interactions through fiber gain in a bidirectional mode-locked fiber laser. Depending on the adjustment of the intracavity birefringence, the polarization states of two counter-propagating solitons can exhibit similar periodical polarization switching behaviors with a polarization-rotating transition state. The successive interactions of the bidirectional solitons mediated by the polarization cross-saturation effect of gain fiber could be responsible for the soliton polarization instability. These findings, in addition to the fundamental interest of the soliton nonlinear dynamics in dissipative optical systems, also open up new possibilities for creating dynamical control of the soliton polarization state and performance improvement in bidirectional ultrafast fiber lasers.

Anti-resonant hollow-core fiber fusion spliced to laser gain fiber for high-power beam delivery.

We present the selective excitation of the fundamental mode in an anti-resonant hollow-core fiber (ARHCF) fusion-spliced with a commercial large mode area (LMA) fiber. By designing and fabricating a single-ring ARHCF that is mode-matched to a LMA fiber and by splicing the two using a CO2 laser-based splicer, we achieve a coupling efficiency of 91.2% into the fundamental mode. We also demonstrate an all-fiber integration of an ARHCF with a commercial ytterbium-doped fiber in a laser cavity for beam delivery application. Coupling of the single-mode laser output beam into the fundamental mode of the ARHCF is demonstrated with 90.4% efficiency (<0.45dB loss) for up to 50 W continuous wave beam in a stable and alignment-free all-fiber laser setup.

Widely tunable, high-power, femtosecond noncollinear optical parametric oscillator in the visible spectral range

Ultrafast visible radiation is of great importance for many applications ranging from spectroscopy to metrology. Because some regions in the visible range are not covered by laser gain media, optical parametric oscillators offer an added value. Besides a high-power broadband laser source, the ability to rapidly tune the frequency of pulses with high-power spectral density offers an extra benefit for experiments such as multicolor spectroscopy or imaging. Here, we demonstrate a broadband, high-power, rapidly tunable femtosecond noncollinear optical parametric oscillator with a signal tuning range of 440–720 nm in the visible range. The oscillator is pumped by the third harmonic of an Yb-fiber laser at 345 nm with a repetition rate of 50.2 MHz. Moreover, the signal wavelength is tuned by changing the cavity length only, and output powers up to 452 mW and pulse durations down to 268 fs are achieved. This is, to the best of our knowledge, the first demonstration of a quickly tunable femtosecond optical parametric oscillator that covers nearly the entire visible spectral range with high output power.

Modulated Noncollinear Optical Parametric Amplifier Output Induced by Stimulated Raman Scattering

We studied spectra of the amplified signal from a noncollinear optical parametric amplifier (NOPA) based on a Ti:sapphire laser and BBO crystal gain medium. The signal shows characteristic structures with periodic intensity variations in the signal during the parametric amplification processes in a BBO crystal. This phenomenon is attributed to the stimulated Raman process excited by the pump pulse affected by the phase-matching condition of four-wave mixing. The effect of this stimulated Raman process on optical parametric amplification and on final pulse compression of the signal was analyzed. The results demonstrate that caution must be taken when constructing a NOPA. Specifically, great care must be taken not to use mirrors with non-uniform spectral reflectivity for obtaining a stable pulse in terms of both temporal and spectral shapes.

Theory and experiments of a power-ratio tunable dual-wavelength Nd:YVO4/Nd:GdVO4 laser by varying the pump wavelength

We demonstrate the theory and experiments of a power-ratio tunable dual-wavelength laser with coaxial diode end-pumping configuration by varying the pump wavelength, which is realized by controlling the working temperature of the pump laser diode (LD). Composite laser gain media containing an Nd : YVO4 crystal and an Nd : GdVO4 crystal was used for example. The dynamics of the dual-wavelength laser generation based on practical input operating parameters were simulated, and in the experiment, a total power of 3.72 W was obtained under the maximum LD pump power of 8 W, corresponding to the optical–optical conversion efficiency of 46.5%. The characteristics of the power-ratio tuning and output power agreed well with the theoretical predictions.

Active Q-switching operation of a Tm:SrF2 single crystal fiber laser near 2 µm

Single crystal fiber (SCF) lasers have attracted extensive attention recently. In this paper, Tm:SrF2 SCF was successfully grown by the temperature gradient method. With a compact resonator, the acousto-optically Q-switched properties of a diode-pumped Tm:SrF2 SCF laser was investigated first. At the repetition rate of 500 Hz, a highest pulse energy of 1.32 mJ and a narrowest pulse width of 51 ns were acquired with the absorbed pump power of 2.60 W, resulting in the peak power of 25.85 kW. These results indicate that Tm:SrF2 SCF is a promising laser gain material for generating ∼2 µm region pulsed lasers.