Volume Bragg Gratings Research Articles

The single‐wavelength 561 nm laser based on reflective volume Bragg grating

AbstractThis article presents 561 nm single‐wavelength laser by laser diode end‐pumped Nd:YAG crystal. The reflective volume Bragg grating is used as the output coupling mirror to effectively select the center wavelength of 1123 nm. The single wavelength laser output at 561 nm is obtained by intracavity frequency doubling via LBO crystal cut in accordance with Type I phase matching condition. When the pump power is 4 W, the output power reaches 229.6 mW, and the power instability is less than 3% within 5 min. Experiments show that the reflective volume Bragg grating acts as both a frequency selection element and an output coupling mirror, which simplifies the resonator structure and improves the laser output power.

Wavelength stabilization and spectra narrowing of a 405 nm external-cavity semiconductor laser based on a volume Bragg grating.

We introduce a 405 nm external-cavity semiconductor laser using a volume Bragg grating (VBG) as the feedback element. By decreasing the length of the external cavity and reducing the wavelength difference between the output wavelength of the laser diode during free running and Bragg wavelength of the VBG, the emission wavelength of the semiconductor laser is stably locked at 405.1 nm with a spectral linewidth of 0.08 nm. The output power reaches 292 mW, and the wavelength drift with temperature reduces to 0.0006 nm/°C. These results are helping for the spectroscopy applications of a blue-violet laser diode. In contrast to traditional external-cavity semiconductor lasers, this laser is less expensive and more compact, in addition to having a narrow linewidth and good wavelength stability. These advantages would facilitate the development of associated areas of research, including optical data storage, laser display, and laser medicine.

High-power continuous-wave Tm3+:Ho3+-codoped fiber laser operation from 2.1 µm to 2.2 µm

A Tm3+:Ho3+-codoped free-space single-oscillator fiber laser is under investigation with special focus on the power scalability of emission wavelengths from 2.1 µm to 2.2 µm. Using a tunable diffraction grating, a 200-nm tunable laser source is built. Laser output powers above 10 W are delivered from 1990 nm up to 2190 nm, demonstrating the range for stable high-power laser operation. By replacing the diffraction grating by a highly reflective, narrow linewidth volume Bragg grating, power scaling is performed at 2.1 µm and is even enabled at a wavelength of 2.2 µm. Using a volume Bragg grating (VBG) at 2.1 µm, a slope efficiency of 49% is measured with an output power of 262 W. Using another VBG with a center wavelength of 2.2 µm, the fiber laser delivers a record power of 77 W with a slope efficiency of 29%.

High-power tunable dual-wavelength diode laser with a composite external cavity

A high-power tunable dual-wavelength composite external cavity architecture obtained by means of a holographic grating and a volume Bragg grating is proposed and demonstrated. The tunable frequency difference of the dual-wavelength output is from 0.41 THz to 3.89 THz. We obtain an output power of 2.1 W when the frequency difference is 1.86 THz. The side-mode suppression ratio of more than 29 dB is suppressed over the entire tunable dual-wavelength output range. The two corresponding wavelengths of the dual-wavelength output basically maintain the same intensity with the smallest power difference of only 0.10%.

High-efficiency 940- and 969-nm brightness-maintaining wavelength-multiplexed LD-pumped 240-W thin-rod Yb:YAG amplifier

A high-efficiency ultrafast laser amplifier based on thin-rod Yb:YAG was demonstrated, featuring a 940- and 969-nm brightness-maintaining wavelength-multiplexed laser diode (LD)-pumping method. Two high-brightness LDs (940 nm and 969 nm) were spectrally combined into one beam spatially with a dichroic mirror, thus enabling twice pump power while maintaining high brightness. A maximum signal power of 240 W was obtained at a repetition rate of 1 MHz, with a power extraction efficiency (PEE) of ∼51%. To the best of the authors' knowledge, this is the first report of >50% efficiency as well as the highest average power operating at the fundamental mode for thin-rod Yb:YAG amplifiers. The beam quality factors (M2) of the amplified signal were measured to be 1.72 and 1.12 for the horizontal and vertical directions, respectively. A preliminary pulse compression was conducted at a signal power of 80 W with a chirped volume Bragg grating (CVBG) compressor. The compressed pulse duration was 744 fs with an average power of 66.5 W, corresponding to a compression efficiency of 83.1%.

Volume Bragg Grating Locked Alexandrite Laser

We report the first ever demonstration of a wavelength-locked Alexandrite laser using a volume Bragg grating (VBG) as a wavelength-selective mirror. Output power of 3.3 W with a diffraction limited beam quality of M2=1.1 was obtained at a lasing wavelength of 762.2 nm and a linewidth (FWHM) of 2.5 GHz.

Radiation source based on an optical parametric oscillator with MgO : PPLN crystal and volume Bragg grating, tunable in ranges of 2050 – 2117 and 2140 – 2208 nm

A tunable radiation source based on an optical parametric oscillator (OPO) with a periodically poled lithium niobate (PPLN), generating light with a wavelength of ∼2.1 μm in a near-degenerate regime, is developed. The use of a volume Bragg grating (VBG) as a selective element makes it possible to reduce significantly (by a factor of more than 180) the radiation linewidth. Wavelength tuning from 2050 to 2117 nm for the signal wave and from 2140 to 2208 nm for the idler wave is demonstrated. The maximum average OPO output power is found to be 617 mW (123 μJ), which corresponds to a conversion efficiency of 10 %. The use of a VBG provides a fundamental possibility of designing a pump source with a small linewidth (of no more than 1 nm) and smooth wavelength tuning in a desired range for frequency converters based on non-oxide nonlinear crystals generating mid-IR light (5 – 15 μm).

Compact pulse shaper based on a tilted volume Bragg grating.

A compact optical layout of a pulse shaper for strongly chirped laser pulses of nanosecond time scale exploiting a tilted chirped volume Bragg grating and a programmable spatial light modulator is proposed. The setup has a non-zero frequency dispersion; thus it may be used for stretching or compressing the pulse and controlling its shape simultaneously. The feasibility of spectral shaping with a resolution of 0.16 nm, corresponding to a time resolution of 150 ps, and a contrast ratio of 102 is demonstrated experimentally.

Stable-Spectrum and Narrow-Bandwidth Operation of a High-Brightness Fiber-Coupled Laser Diode Using a Volume Bragg Grating

Stable-Spectrum and Narrow-Bandwidth Operation of a High-Brightness Fiber-Coupled Laser Diode Using a Volume Bragg Grating

A large dispersion-managed monolithic all-fiber chirped pulse amplification system for high-energy femtosecond laser generation

A high energy monolithic all fiber chirped pulse amplification (CPA) system composed of fiber-silicate glass fiber is demonstrated in this work. In this compact system, the optical stretcher consists of two large dispersion chirped fiber Bragg gratings that offer a high stretch ratio to lower the nonlinearity that accumulates in the fiber amplifier. By employing high gain silicate glass fiber with a length of 20 cm as the main amplifier, an optimal balance of pulse energy and the B-integral is achieved. An amplified power of 42.6 W, corresponding to a pulse energy of 213 µJ, is obtained with an injection power of 100 mW. After compression, the laser pulses are compressed by a novel polarization-controlled double-pass configuration of a chirped volume Bragg grating (CVBG) to match the dispersion of the fiber stretcher. A total compression efficiency of 79.8% is obtained. The dispersion between the stretcher and the compressor is matched with the self-made temperature tuning device of the fiber grating. An optimized pulse width of 781 fs and a pulse energy of 170 µJ are obtained with the single-mode beam profile. To the best of our knowledge, this is the highest energy ever obtained with a single-mode beam profile for a spliced monolithic all-fiber CPA system. This compact high-energy fiber system will be useful in scientific research and industrial applications.

Versatile approach to laser beam shaping and analyzing by holographic phase masks

A new technology that includes a flexible holographic recording system using photo-thermo-refractive (PTR) glass is demonstrated for creating so-called ‘holographic phase masks’ or HPMs. These diffractive optical elements are permanently recorded in the PTR glass, can be multiplexed as with normal holograms, do not require electric power to operate, and can perform near arbitrary beam phase transformations. The holographic setup includes a spatial light modulator that enables recording transmitting volume Bragg gratings (VBGs) with arbitrary phase patterns encoded into them. As a result, the desirable phase pattern is introduced in a beam that is diffracted by the VBG. These HPMs are tunable within the visible and near IR spectral regions and can be made achromatic, with spectral widths of up to 50 nm. Furthermore, they tolerate laser power densities on the order of several kW cm−2. Applications of HPMs include high-power, broadband laser beam shaping and modal analysis of complex beams profiles, both of which are demonstrated here.

Generation of adjustable femtosecond double pulses by diffraction of a chirped femtosecond pulse from volume Bragg gratings

Based on Kogelnik’s coupled wave theory, femtosecond double pulses with adjustable pulse shape and interval are generated by diffraction of a chirped femtosecond pulse from transmitted volume Bragg gratings (VBGs). It is found that duration of the diffracted double pulses depends on the chirped parameter of the incident femtosecond pulse. When the chirp parameter is larger than zero, the double pulses are compressed. However, when it is less than zero, the double pulses are broadened. The degree of compression and broadening of the double pulses are different from each other. Moreover, the pulse interval between the double pulses is determined mainly by thickness and refractive index modulation of the VBG. This paper provides a theoretical foundation to design proper VBGs for generating femtosecond double pulses with different waveform.

High efficiency multiplexed volume Bragg gratings for angle deflection at 1064nm

The volume Bragg grating (VBG) recorded in the photo-thermo-refractive (PTR) glass has high diffraction efficiency (DE), excellent angle selectivity, multiplexed and flexible design, which is an ideal device to achieve the angle magnification of beam scanning. In this study, a 4-channel multiplexed VBGs with the average relative diffraction efficiency (RDE) greater than 96% at 1064 nm and the maximum discrete angle deflection of 12° was designed and fabricated. And the angle deviation of experiments and design schemes were controlled less than 0.3°. The laser damage of PTR glass and multiplexed VBGs were also tested at 1064 nm. Laser damage tests were performed with a beam diameter of 0.3 mm by 1-on-1 mode. It showed that the laser damage threshold of PTR glass and multiplexed VBGs were 44.33J/cm2 and 30.15 J/cm2 respectively.

Fabrication and analysis of broadband chirped volume Bragg grating

In this work, based on the double cylindrical wave holographic interference method, a broadband chirped volume Bragg grating (CVBG) in photo-thermo-refractive glass (PTR) has been fabricated and studied, which has a diffraction bandwidth of about 23.8 nm. The transmittance and diffraction efficiency (DE) of the prepared CVBG were measured. The results showed that the DE and refractive index modulation (RIM) of the CVBG samples increased first and then decreased with the increase of the heat treatment time. And the transmittance of all samples decreased, indicating that the losses (sum of absorption and scattering) of the samples increased. It was found that the absorption coefficients of all samples did not exceed 0.1cm-1 even after prolonged heating, and the scattering losses accounted for the main part losses of the CVBG. Then, the thermostatic duration of the sample was shorter during heat treatment at the first time, the starting point of the RIM of CVBG became higher, which is expected to obtain higher RIM and DE of the CVBG in the subsequent heat development process. Although increasing the dose of UV exposure can reduce the losses of the CVBG, it is not conducive to the improvement of the RIM and DE of the CVBG. Therefore, we have made restrictions on both the dose of UV exposure and the duration of heat treatment. Finally, based on the fundamental matrix (F-matrix) method, the influence of the key structural parameters of the CVBG on its diffraction characteristics was analyzed, and a guidance scheme for making high-efficiency CVBG was proposed.

Dual-wavelength Rapid Excitation Raman Difference Spectroscopy System for Direct Detection of Ethanol in Illegal Beverages

A dual-wavelength rapid excitation Raman difference spectroscopy (DWRERDS) system for separation of Raman signals from background interference was developed. In the DWRERDS system, a dual-wavelength diode laser at 784.59 and 785.22 nm with volume Bragg gratings was used as the excitation light source. For each laser line, optical power could reach as high as 550 mW. Under dual-wavelength excitation mode, the DWRERDS system could switch each laser instantaneously and the excitation time of each laser was as short as 1 s. Two Raman spectra were measured with slightly shifted excitation wavelengths and subtracted from each other, resulting in a background-free difference spectrum. The DWRERDS system was applied to complex matrix background suppression, which could improve a very good signal-to-noise ratio (SNR) for Raman spectra. Although ethanol in Coca-Cola could not be detected by conventional Raman spectroscopy, the DWRERDS system could be used for the direct detection of ethanol in illegal alcoholic beverages. The experimental results showed that the limit of detection (LOD) of ethanol in Coca-Cola was volume percentage of 4.76% and the correlation coefficient for the linear regression was as high as 0.984. The DWRERDS system could be used to determine the concentration of ethanol in illegal alcoholic beverages within 1 min without any sample pretreatment.

Sub‐nanosecond, single longitudinal mode laser based on a VBG‐coupled EOQ Nd:YVO4 oscillator for remote sensing

AbstractHigh energy, sub‐nanosecond, single longitudinal mode (SLM) pulse is especially preferred for LiDAR application. In this paper, a kHz‐level sub‐nanosecond electro‐optical Q‐switched Nd:YVO4 laser of SLM operation for remote sensing was demonstrated. By utilizing a compact resonator with a Volume Bragg Grating as output coupler, maximum SLM output power of 1.08 W and pulse width of 919 ps was obtained at 10 kHz. Moreover, we proposed a theoretical model to calculate the multimode threshold of the VBG‐coupled cavity. The longitudinal mode characteristics at four different cavity lengths were investigated experimentally and the results match well with the numerical simulations, which validates the theory. We expected the method could help to guide VBG‐coupled cavity design and construction.

A Yb:YAG dual-crystal regenerative amplifier

An Yb:YAG dual-crystal regenerative amplifier was developed. The thermal effects were largely decreased by the dual-crystal configuration and optimized cavity design. The chirped pulse was amplified to 3.5 mJ with excellent energy stability (root mean square ∼0.95%) and beam quality (M2 < 1.1) at a repetition rate of 1 kHz using chirped pulse amplification (CPA) systems. A single space-saving simple-to-align chirped volume Bragg grating (CVBG) was used as a stretcher/compressor, which yielded 2-mJ pulses with a pulse duration of 1.9 ps after compression with an efficiency of 94.3%. A low-cost highly compact kilohertz high-energy ultrafast laser system solution was obtained.

Preclinical Imaging and Spectroscopy in the NIR-II Window with Indocyanine Green (ICG) and Single-Walled Carbon Nanotubes

We developed a near-infrared small animal imaging system, IR VIVO, that provides real-time images, videos and spectral imaging in the NIR-I and shortwave infrared (SWIR or NIR-II) regions. We found that SWIR wavelengths can give optimal resolution for in vivo optical imaging of deep organs in mice up to 3 cm due to the low tissue autofluorescence, scattering and absorption of light at these wavelengths. We also showed how this higher penetration depth can enable the detection of small wavelength changes in the emission of carbon nanotube-based sensors implanted in vivo using a continuously tunable filter.First, we demonstrated superior image clarity and penetration depth at NIR-II wavelengths (1000-1700 nm) in vivo using an FDA approved dye, indocyanine green (ICG) in a mouse. We used 780 nm excitation, detection using a Zephir 1.7 InGaAs camera and 1250 nm long-pass emission filter. We found low tissue autofluorescence at 1250 nm. The ICG dye imaging resulted in superior visualization of microvasculature, perfusion, measurement of hearst rate, respiratory rate, hepatobiliary and intestinal contractions. Further anatomical and functional imaging was carried out by looking at the kinetics of ICG, resulting in the identification of different organs.Second, In vivo near-infrared hyperspectral imaging of carbon nanotubes was conducted using a continuously-tunable filter in the imager. A diffraction volume Bragg grating (VBG), was used to provide narrow-band wavelength selection. Coupled with a homogeneous global illumination, spectrally-defined images were acquired in the entire field of view. The result was a dataset containing both 2D spatial information and the full spectrum for each point in the image. We conducted near-infrared hyperspectral imaging of single-walled carbon nanotubes to measure small wavelength changes of the nanotubes implanted into live mice. Hyperspectral measurements of carbon nanotube sensors for lipids in the liver as well as implantable sensors for doxorubicin were conducted, facilitating liver disease monitoring and drug pharmacokinetics measurements.Finally, we believe wide-area NIR-II imaging and spectral/hyperspectral measurements have broad potential applications for the use of carbon nanotubes and other NIR-I/II materials in basic materials/biology, translational, and clinical work, including sensor arrays, point-of-care measurements, implants, whole small animal imaging and intraoperative/surgical imaging.

Quantum-inspired terahertz spectroscopy with visible photons

Terahertz technology offers solutions in nondestructive testing and spectroscopy for many scientific and industrial applications. While direct detection of photons in this frequency range is difficult to achieve, quantum optics provides a highly attractive alternative: it enables the characterization of materials in hardly accessible spectral ranges by measuring easily detectable photons of a different spectral range. Here we report on the application of this principle to terahertz spectroscopy, measuring absorption features of chemicals at sub-terahertz frequencies by detecting visible photons. To generate the needed correlated signal-idler photon pairs, a periodically poled lithium niobate crystal and a 660 nm continuous-wave pump source are used. After propagating through a single-crystal nonlinear interferometer, the pump photons are filtered by narrowband volume Bragg gratings. An uncooled scientific CMOS camera detects the frequency-angular spectra of the remaining visible signal and reveals terahertz-spectral information. Neither cooled detectors nor expensive pulsed lasers for coherent detection are required.

Compact millijoule Yb3+:CaF2 laser with 162 fs pulses.

We report on a compact diode-pumped, chirped pulse regenerative amplifier system with a pulse duration of 162 fs and an output pulse energy of 1 mJ before as well as 910 µJ after compression optimized for the probing of ultrafast relativistic laser-plasma processes. A chirped volume Bragg grating (CVBG) acts as a combined pulse stretcher/compressor representing a robust solution for a CPA laser system in the millijoule range. Yb3+:CaF2 is used as gain medium to support a large bandwidth of 16 nm (FWHM) when spectral gain shaping is applied. Chirped mirrors compensate for any additional dispersion introduced to the system.