Volume Bragg Gratings Research Articles

Angular filtering by volume Bragg grating in photothermorefractive glass for nanosecond laser pulse

The two-dimensional angular filter based on volume Bragg gratings in photothermorefractive glass for a nanosecond (ns) laser pulse is demonstrated. The experimental results show that the near-field beam quality of the laser pulse was effectively improved. The near-field modulation and contrast ratio were improved by 1.75 and 4.48 times, respectively. The power spectral density curves showed that the spatial frequencies more than 0.9 mm−1 in the x direction and 1.2 mm−1 in the y direction were effectively suppressed.

啁啾体布拉格光栅的脉冲响应特性

啁啾体布拉格光栅的脉冲响应特性

Band-stop angular filtering with hump volume Bragg gratings

A band-stop angular filter (BSF) based on hump volume Bragg gratings (HVBGs) is proposed. Band-stop filtering in a two-stage amplifier laser system is discussed and simulated. Simulation results show that small-scale self-focusing effects in the laser system can be effectively suppressed with the BSF due to the control of fast nonlinear growth in a specific range of spatial frequencies in the laser beam. Near-field modulation of the output beam from the laser system was decreased from 2.69 to 1.37 by controlling the fast nonlinear growth of spatial frequencies ranging from $0.6~\text{mm}^{-1}$ to $1.2~\text{mm}^{-1}$ with the BSF. In addition, the BSF can be used in a plug-and-play scheme and has potential applications in high-power laser systems.

Combined dual-wavelength laser diode beam end-pumped single longitudinal mode Pr<sup>3+</sup>:LiYF<sub>4</sub> 360 nm ultraviolet laser

In recent years, all-solid-state ultraviolet lasers have had widely potential applications in the fields of spectroscopy, biological analysis, precision manufacturing, optical data storage, high-resolution printing, medicine and lithography. The good monochrome of all-solid-state ultraviolet laser can improve the accuracy of spectral absorption measurement when used to detect specific proteins and reduce the laser spot diameter when used for high density data storage or acousto-optic deflector. In this paper, a combined dual-wavelength laser diode (LD) beam end-pumped single longitudinal mode Pr<sup>3+</sup>:LiYF<sub>4</sub> all-solid-state UV laser at 360 nm is presented. A V-folded cavity structure is used in the laser, which consists of a reflective volume Bragg grating (RBG) and a Fabry-Perot (F-P) etalon. The RBG is used as a wavelength selection and resonator reflector to narrow the width of spectral line. The F-P etalon is hybrid in the cavity, serving as a narrow-band filter, to achieve the single longitudinal mode. The lithium triborate crystal with critical type-I phase matching at room temperature is used for implementing the second-harmonic generation of the fundamental 720 nm laser and obtaining an efficient and compact ultraviolet laser at 360 nm. The optical resonator is simulated and analyzed by MATLAB software. Two experiments are conducted to compare the accuracy of central wavelength tuning by changing the temperature of F-P etalon and the angle of F-P etalon. The result shows that the change temperature of F-P etalon can achieve 0.165 pm/℃, showing that it is a better method. The structure of the laser is simplified and the anti-interference capability is improved in this way. It is different from mode competition method and the stability of single longitudinal mode laser output is increased. When the output power of LD at 444 nm is 1200 mW and that of LD at 469 nm is 1400 mW, a single longitudinal mode CW UV laser at 360 nm with output power as high as 112 mW is achieved. The optical-to-optical conversion efficiency is 4.3%, and the longitudinal linewidth of laser is 30 MHz. The measurements show that the edge suppression ratio is greater than 60 dB, the stability of root mean square (RMS) of output power in 4 h is better than 0.5%, the frequency shift in 1h is better than 220 MHz, and amplitude noise is less than 0.5%.

FM-to-AM conversion in angular filtering based on transmitted volume Bragg gratings

FM-to-AM conversion for angular filtering based on transmitted volume Bragg gratings (TBGs) is discussed. Simulation results show that a narrower spectral selectivity of TBGs led to stronger FM-to-AM conversion. Good angular selectivity and a wide bandwidth for the TBGs can be obtained by controlling the grating period and thickness. FM-to-AM conversion can be effectively suppressed and the distortion criterion for the filtered beam reduces to less than 5%. FM-to-AM conversion of TBGs is demonstrated in the ‘Shenguang’ facility, and the results are in good agreement with the simulation.

Modeling the Diffraction Efficiency of Reflective-Type PQ-PMMA VBG Using Simplified Rate Equations

A simulation based on the rate equations was performed to analyze the concentration distribution of molecules in phenanthrenequinone (PQ) doped polymethyl methacrylate (PMMA) polymers (PQ-PMMAs) exposed using a two-beam interference configuration. The concentration distributions were used to predict the diffraction efficiency of PQ-PMMA-based volume Bragg gratings (VBGs) by using the transfer matrix method. The simulation results suggest that an intermittent exposure method can promote the diffusion of PQ molecules and increase the final diffraction efficiency. A series of experiments validated our hypothesis that the diffraction efficiency of reflective-type PQ-PMMA VBG can be predicted.

Wavelength-locked continuous-wave and Q-switched Ho:CaF2 laser at 2100.5 nm.

In this paper, we demonstrated a linewidth-narrowed continuous-wave (CW) and acousto-optical Q-switched Ho:CaF2 laser for the first time. With a volume Bragg grating, a maximum CW output power of 6.94 W at 2100.5 nm, and slope efficiency of 57.9%, an FWHM linewidth of 0.31 nm was obtained. When absorbed pump power was 13.2 W, the maximum average output powers of 6.08 W, 5.9 W, and 5.71 W were achieved in a Q-switched Ho:CaF2 laser under pulse repetition frequencies of 10 kHz, 5 kHz, and 3 kHz, corresponding to the slope efficiencies of 51.2%, 49.6%, and 48.5%, respectively. The minimum pulse width of 54 ns was achieved at pulse repetition frequency of 3 kHz.

Design and fabrication of multiplexed volume Bragg gratings as angle amplifiers in high power beam scanning system.

In this study, the realization of multiplexed volume Bragg gratings (VBGs) working as angle amplifiers in high power beam scanning system is theoretically and experimentally investigated. The design of the multiplexed VBG for the working wavelength of 1064 nm is described. We propose a cascaded multiplexed VBGs scheme that consists of 12 grating channels. Three 4-channel multiplexed VBGs were fabricated inside photo-thermo-refractive (PTR) glasses by multiple exposures and subsequent heat treatment. The test results show that this angle amplifier can achieve discrete angle deflection ranging from -45° to + 45°. The relative diffraction efficiency of all the grating channels is more than 80% and is almost polarization independent.

KW-class fiber-coupled diode laser source based on dense spectral multiplexing of an ultra-narrow channel spacing.

A dense spectral multiplexing structure based on the reflective volume Bragg gratings is introduced to realize the diode laser multiplexing with an ultra-narrow channel spacing of 1.5 nm. With the combination of polarization multiplexing and coarse spectral multiplexing, a diode laser source producing 2045 W power from an output fiber with a core diameter of 105 µm and NA of 0.2 is achieved at an injection current of 10 A. The electro-optical and optical-optical efficiency of the laser source is 42% and 76%, respectively. Experimental results demonstrate the ability of dense spectral multiplexing based on VBG to realize high output power and brightness.

High-power, near-diffraction-limited, narrowband ring optical parametric oscillator with counterdirectional mode coupling at 1.53 µm

A continuous-wave (CW) singly resonant optical parametric oscillator (OPO) employing a volume Bragg grating (VBG) in a ring resonator is demonstrated. Similar to a linear cavity, the minute absorption induces surface distortion and frequency shift in both nonlinear crystal and grating. The joint thermal effects are proved to account for the unconventional OPO performance in regard to power stability, spectrum and beam quality. However, counterdirectional mode coupling of the signal at ~1.53 µm with considerable power is first observed from the VBG-based resonator and what theory suggests would be a unidirectional ring. This is believed to originate from the grating structure and has some influence on threshold and stability of the OPO. Up to 4.6 W of narrowband (~0.09 nm), diffraction limited (M2 ⩽ 1.2) radiation at 1.53 µm has been obtained in our experiment, together with 2.8 W of output idler at 3.49 µm.

Passively Q switched dual channel Tm:YLF laser by intracavity spectral beam combination with volume Bragg gratings.

A novel dual channel Tm:YLF laser system was developed where two degenerate laser cavities were coupled by spectrally beam combining their emission and by implementing a common output coupler. Under continuous wave running conditions, each channel's slope efficiency was greater than 45% and the maximum combined output power was 11 W. Passive Q-switching was achieved using an 80%, Cr:ZnSe saturable absorber. The output pulses had a maximum energy of 5.8 mJ and duration of 90 ns (~65 kW of peak power) at 5.7 W of absorbed pump power. Each channel showed less than 1 nm of spectral width with central wavelengths around 1880 nm and 1908 nm correspondingly. The system had adjustable spectral difference between the channels ranging from 5 to 20 nm which corresponds to 0.4 - 1.7 THz if the system is used for nonlinear difference frequency generation.

Narrowband, tunable, 2\xa0\xb5m optical parametric master-oscillator power amplifier with large-aperture periodically poled Rb:KTP

A high-energy, ns, narrow-linewidth optical parametric oscillator and amplifier system based on large-aperture periodically poled Rb:KTP is presented. The 2 µm seed source is a singly resonant OPO locked with a transversely chirped volume Bragg grating, allowing a wavelength tuning of ~ 21 nm and output linewidth of 0.56 nm. A maximum output energy of 52 mJ and conversion efficiency of 36% was obtained from the amplifier for a pump energy of 140 mJ. The high-energy and the robust and narrow dual-wavelength spectra obtained make this system an ideal pump source for difference frequency generation-based THz generation schemes.

Widely Wavelength-Tunable Mid-Infrared Fluoride Fiber Lasers

We demonstrate widely wavelength-tunable conti-nuous-wave (CW) and Q-switched Er3+-doped ZBLAN fluoride fiber lasers operating around 3 μm enabled by a volume Bragg grating (VBG). In the CW operation regime, a total wavelength tuning range of over 160-nm spanning from 2694 to 2854 nm has been achieved. For the Q-switched mode of operation, a slightly modified resonator configuration, incorporating a passive Q-switcher, topological insulator Bi2Te3 nanosheets, can produce stable pulse trains with a pulse width of 880 ns at a repetition rate of 81 kHz, while maintaining a wavelength tuning range of 62 nm from 2762 to 2824 nm through adjusting the VBG. In both operation regimes, the output spectral width is measured to be <0.3 nm (full-width at half-maximum) over the whole tuning range. Our work both demonstrates the great wavelength-tuning potential of the Er3+ -doped fluoride fiber laser, and also paves a way for the development of a range of high-performance midinfrared laser sources.

Off-axis digital holography with multiplexed volume Bragg gratings.

We report on an optical imaging design based on common-path off-axis digital holography, using a multiplexed volume Bragg grating. In the reported method, a reference optical wave is made by deflection and spatial filtering through a volume Bragg grating. This design has several advantages, including simplicity, stability, and robustness against misalignment.

Stable Simultaneous CW 1052, 1071, and 1083 nm Narrow Spectral Width Nd-Based Laser Output Using Precise Gain Control With VBGs

A continuous wave Nd-based laser with simultaneous narrow spectral 1052, 1071, and 1083 nm output was achieved by using temperature-controlled volume Bragg gratings to balance the gain competition between each wavelength and stabilize the laser output. Each output wavelength was characterized thoroughly and showed good stability in both frequency and power.

Holographic lens for 532 nm in photo-thermo-refractive glass

Volume Bragg gratings (VBGs) recorded in photo-thermo-refractive (PTR) glass are extensively used for laser beam control applications. As planar holographic optical elements, VBGs operate in the entire spectral window of PTR glass transparency – from near UV, through visible, and up to IR spectral regions. However, complex holographic elements, e.g. lenses or curved mirrors, cannot be fabricated in conventional Ce3+ doped PTR glass for applications in the visible spectral region due to its photosensitivity being limited to the UV region. Recently developed PTR glass doped with Tb3+ ions demonstrated sensitivity to visible light. The photo-ionization involves two-step absorption of UV and visible photons via metastable levels of Tb3+. Holograms in this glass were recorded by simultaneous exposure to UV radiation from an LED and visible radiation of the second harmonic from a Nd:YAG laser at 532 nm. For the first time holographic lenses operating at 532 nm were demonstrated in PTR glass. This result paves the way for complex holographic optical elements for operation with high power visible lasers.

Moiré apodized reflective volume Bragg grating

Volume Bragg grating (VBG) with refractive index modulation (RIM) apodized with sinusoidal semiperiod profile is studied theoretically and experimentally. An apodized VBG of this type was fabricated with a sequential recording of two VBGs with slightly different resonant Bragg wavelengths in the same glass wafer. As a result, a moire pattern was produced with a constant average refractive index and a slow sinusoidal envelope of RIM. Modeling showed that an apodized VBG with a sinusoidal semiperiod has provided a suppression of the sidelobes in the reflection spectrum. The experimental measurements are in good agreement with the theoretical predictions. This type of VBG is suitable for high-resolution spectroscopy applications due to a significant reduction of sidelobes.

Thermal stability of volume Bragg gratings in chloride photo-thermo-refractive glass after femtosecond laser bleaching.

We demonstrate that the Joule heating of the volume Bragg grating recorded in chloride photo-thermo-refractive glass can be suppressed by bleaching the silver nanoparticles with intense ultrashort laser pulses. Measurement of the bleached grating angular selectivity showed that, at the signal wavelength at 972nm, the spectral drift is 0.5nm at the CW laser diode beam intensity as high as 145 W/cm2. Thus, the bleaching of silver nanoparticles results in the improved thermal stability of transmission gratings, allowing one to employ them to control the powerful CW laser radiation.

Coherent addition of high power broad-area laser diodes with a compact VBG V-shaped external Talbot cavity

We introduced a compact V-shaped external Talbot cavity for phase locking of high power broad-area laser diodes. The length of compact cavity is ∼25 mm. Near diffraction-limit coherent addition of 10 broad-area laser diodes indicated that high quality phase locking was achieved. We measured the near-field emission mode of each individual broad-area laser diode with different feedback, such as a volume Bragg grating and a high reflection mirror. We found out that the best result of phase locking broad-area laser diodes was achieved by the compact V-shaped external Talbot cavity with volume Bragg grating feedback.

Method for precise evaluation of refractive index modulation amplitude inside the volume Bragg grating recorded in photo-thermo-refractive glass.

In this paper, we present a method for achieving precise evaluation of amplitude of refractive index modulation (RIM) inside the volume Bragg grating (VBG) recorded in photo-thermo-refractive (PTR) glasses. The Gaussian divergence characteristics of the incident beam is theoretically considered when calculating the angular selectivity of VBG, and the profiles of experimental angular selectivity curves are utilized to determine the value of RIM with one step. The effectiveness of our proposed method is experimentally verified. This method is applicable even if the full width at half maximum (FWHM) of VBG's angular selectivity curve has the same order of magnitude as or is less than the beam divergence.