Chirped Volume Bragg Grating Research Articles

Diagnostics of First Samples of Chirped Volume Bragg Gratings Inscribed Using Femtosecond Laser Pulses

Diagnostics of First Samples of Chirped Volume Bragg Gratings Inscribed Using Femtosecond Laser Pulses

Crackless high-aspect-ratio processing of a silica glass with a temporally shaped ultrafast laser.

In this Letter, we propose a crackless high-aspect-ratio processing method based on a temporally shaped ultrafast laser. The laser pulse is temporally split into two sub pulses: one with smaller energy is used to excite electrons but without ablation so that the applied pressure to the sample is weak, and the other one is used to heat the electrons and achieve material removal after it is temporally stretched by a chirped volume Bragg grating (CVBG). Compared with the conventional ultrafast laser processing, the crack generation is almost suppressed by using this proposed method. The hole depth increases more than 3.3 times, and the aspect ratio is improved at least 2.2 times. Moreover, processing dynamics and parameter dependence are further experimentally studied. It shows that the processing highly depends on the density of electrons excited by the first pulse (P1) and the energy of the second pulse (P2). This novel, to the best of our knowledge, method provides a new route for the precise processing of wide-bandgap materials.

450 W pulsed laser system with M2 < 1.2 based on a 969-nm laser diode end-pumped Yb:YAG rod amplifier.

We demonstrated a compact and power-efficient multi-stage pulsed end-pumped amplifier with stabilized output power of 450 W and near-diffraction-limited beam quality (M2 < 1.2) at a repetition rate of 1 MHz. The pulsed amplifier produced an exceptional average power and optimal beam quality achieved in laser diode (LD) end-pumped Yb:YAG thin rod configuration at room temperature. A preliminary pulse compression with a chirped volume Bragg grating (CVBG) was performed reducing pulse duration to ∼730 fs at a compression efficiency of 90%. With the combined features, including compactness, reliability, and efficiency, of the end-pumped scheme, the demonstrated laser system would be of great value in both industry and scientific research.

Compact dual-band spectral analysis via multiplexed rotated chirped volume Bragg gratings.

Chirped Bragg volume gratings (CBGs) offer a useful alternative for spectral analysis, but increasing the bandwidth necessitates increasing the device area. In contrast, recently developed rotated CBGs (r-CBGs), in which the Bragg structure is rotated by 45° with respect to the device facets, require increasing only the device length to extend the bandwidth, in addition to the convenience of resolving the spectrum at normal incidence. Here, we multiplex r-CBGs in the same device to enable spectral analysis in two independent spectral windows without increasing the system volume. This new, to the best of our knowledge, device, which we term an X-CBG, allows for compact multi-band spectroscopy in contiguous or separated spectral windows for the visible and near-infrared applications in nonlinear microscopy and material identification and sensing.

First steps in the development of next-generation chirped volume Bragg gratings by means of fs laser inscription in fused silica.

The first steps towards the development and characterization of next-generation chirped volume Bragg gratings (CVBGs) by means of fs laser inscription were made. Based on the phase mask inscription technique we realized CVBGs in fused silica with a 3 × 3 mm2 aperture and a length of almost 12 mm with a chirp rate of ∼190 ps/nm around a central wavelength of 1030.5 nm. Strong mechanical stresses induced serious polarization and phase distortions of the radiation. We show a possible approach to solution of this problem. The change in the linear absorption coefficient associated with local modification of fused silica is quite small, enabling utilization of this type of gratings in high average power lasers.

Rotated chirped volume Bragg gratings for compact spectral analysis.

We introduce a new, to the best of our knowledge, optical component-a rotated chirped volume Bragg grating (r-CBG)-that spatially resolves the spectrum of a normally incident light beam in a compact footprint and without the need for subsequent free-space propagation or collimation. Unlike conventional chirped volume Bragg gratings in which both the length and width of the device must be increased to increase the bandwidth, by rotating the Bragg structure we sever the link between the length and width of a r-CBG, leading to a significantly reduced device footprint for the same bandwidth. We fabricate and characterize such a device in multiple spectral windows, we study its spectral resolution, and confirm that a pair of cascaded r-CBGs can resolve and then recombine the spectrum. Such a device can lead to ultracompact spectrometers and pulse modulators.

Experimental study on the fluctuation–magnification effect of chirped volume Bragg grating compressor in high-energy, ultrafast, thin-disk chirped pulse amplification system

The fluctuation–magnification effect on the peak intensity of a laser pulse caused by the nonlinear Kerr effect in the chirped volume Bragg grating (CVBG) compressor is investigated experimentally for a high-energy, thin-disk, chirped pulse amplification system. The nonlinear Kerr effect occurs at the blue end, and the accumulated nonlinear phase shift (B-integral) in the CVBG rises with the increase of laser pulse energy. Experiments show that small fluctuations in peak power of uncompressed pulses cause increasing of the temporal fluctuation and spatial fluctuation due to high Kerr-nonlinearity in the CVBG when B-integral is larger than π. Thus the initial fluctuation would be magnified by the CVBG compressor.

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%.

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.

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.

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.

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.

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.

Numerical analysis of beam distortion induced by thermal effects in chirped volume Bragg grating compressors for high-power lasers

We conducted a study on the thermal effects in chirped volume Bragg gratings (CVBGs) used as optical compressors in high-power lasers. A simulation code was developed to predict the distortion of the pulses by the thermal effects. The types and significance of the thermal distortion were analyzed using an Yb:YAG laser with an average power of 250 W. The temperature distribution in the CVBG, the thermal lens effect, the beam propagation path changes due to the thermal lens, and the compression efficiency decrease due to the phase delay were analyzed. As a result, we predict that the peak power decreases to 63.9% due to the thermal effects under the operating conditions of 250 W. We also suggest new designs for the CVBG mount that minimize the peak power loss. We expect the peak power loss to be reduced only by 1.1% when using our new mount design for the CVBG.

Development of a 67.8 W, 2.5 ps ultrafast chirped-pulse amplification system based on single-crystal fiber amplifiers.

We demonstrate a high-power, high-energy chirped-pulse amplification (CPA) system based on three Yb:YAG amplifiers and a chirped-volume Bragg grating (CVBG). With an all-fiber laser as the seed light, a Yb:YAG rod amplifier and two Yb:YAG single-crystal fiber (SCF) amplifiers as the amplification stages, a laser power of 96 W at 200 kHz repetition rate corresponding to a pulse energy of 0.48 mJ has been generated. The stability of different output power has been measured and compared. To the best of our knowledge, this is the first report on a stable 100 W-level laser with sub-mJ pulse energy based on SCF. The beam quality M2 of different output lasers has also been measured, which is below 1.55 when the output power is 92 W. The amplified laser is seeded into the CVBG to be compressed, and a compression efficiency of 0.724 has been obtained with an output power of 67.8 W and pulse duration of 2.5 ps. The ultrafast CPA system delivering high pulse energy (sub-mJ) with hundreds of kHz repetition rate is expected to be used as the driving source of high-flux high-harmonic generation after further compression.

Experimental study on compression of 216-W laser pulses below 2 ps at 1030 nm with chirped volume Bragg grating.

We report on the characterization of a high-power, chirped volume Bragg grating (CVBG) pulse compressor. It includes measurements of the CVBG's diffraction efficiency, beam profile, beam quality (M2 parameter), pulse spectrum, the CVBG's temperature, and the thermal lens. These parameters were monitored for a wide range of input laser powers and with different clamping forces applied on the CVBG. This analysis was performed with a CPA-based Yb:YAG thin-disk laser system operating at a wavelength of 1030 nm, a 92 kHz repetition rate, 2 ps pulse duration, and an average output power after compression of 216 W (270 W uncompressed), which is, to the best of our knowledge, the highest value reported to date using this pulse compression technique.

Influence of 2.09-μm pulse duration on through-silicon laser ablation of thin metal coatings

A robust Ho:YAG chirped pulse amplifier with a simple dispersion management was developed for material processing applications. The amplifier produces 3.2 ps pulses centered at 2.09 μm with energies up to 1.6 mJ. At 10 kHz repetition rate this results in 16 W of average power. Both pulse stretching and compression in the amplifier is realized within a single chirped volume Bragg grating. With the developed amplifier we study laser ablation of a gold layer through a carrier substrate and, particularly, the effect of pulse duration on the process. A balance between nonlinear effects within the carrier and interaction with the thin ablation layer makes few picosecond pulses most suitable for non-thermal ablation. Furthermore, since only a few-microjoules of pulse energy is required for de-bonding, the developed 16 W amplifier can be potentially operated at MHz repetition rates for high throughput.

Hybrid CPA system comprised by fiber-silicate glass fiber-single crystal fiber with femtosecond laser power more than 90 W at 1 MHz

A high power and hybrid chirped pulse amplification (CPA) system comprised by fiber-silicate glass fiber-single crystal fiber amplifier is demonstrated in this work. In this hybrid system, the high gain compact silicate glass fiber amplifier is used to boost the power, amplified power more than 92 W is obtained with the injected power of 400 mW at 1 MHz, corresponded amplification gain as high as 23.6 dB. To our best knowledge, this is the highest power obtained in CPA amplification with silicate glass fiber. The single crystal fiber amplifier is used to boost the energy with low nonlinearity in high pulse energy amplification. Amplified power of 122 W at 1 MHz is obtained, corresponding to a pulse energy of 122 μJ. After amplification, the laser pulses are compressed by chirped volume Bragg grating (CVBG), the total compression efficiency greater than 73% is obtained by control the spectrum width with a fiber filter with pass band of 5 nm in the fiber pre-amplifier to match the reflected bandwidth of the CVBG compressor. To match the dispersion between the chirped fiber Bragg grating (CFBG) stretcher and the CVBG compressor, the self-made temperature tuning device of CFBG is employed to make precision dispersion match. The optimized pulse width of 660 fs at compressed power of 90 W is obtained. This compact and high power ultrafast laser will find various applications in scientific research and industrial applications.

41.8 W output power, 200 kHz repetition rate ultra-fast laser based on Yb:YAG single crystal fiber(SCF)amplifier

We demonstrate a high repetition rate, high energy chirped pulse amplification (CPA) system based on single crystal fiber (SCF) amplifier. With home-made all-fiber laser as the seed laser and Yb:YAG rod, SCF as the amplification gain medium, an output power of 61.8 W at 200 kHz repetition rate has been obtained. To the best of our knowledge, this is the highest power based on SCF at hundreds of kHz repetition rate. The measured M2 values are approximately 1.241 in the horizontal and 1.186 in the vertical direction. The spectrum width is 3.8 nm, which support Fourier transform-limited pulse duration of 411 fs. An average power of 41.8 W with pulse duration of 1.5 ps has been obtained after the chirped volume Bragg grating (CVBG) compressor, which will be compressed to hundreds of fs with accurate chirp compensation.

Compact pulsed thulium-doped fiber laser for topographical patterning of hydrogels

We report the generation of high energy 2 μm picosecond pulses from a thulium-doped fiber master oscillator power amplifier system. The all-fiber configuration was realized by a flexible large-mode area photonic crystal fiber (LMA-PCF). The amplifier output is a linearly-polarized 1.5 ns, 100 kHz pulse train with a pulse energy of up to 250 μJ. Pulse compression was achieved with (2+2)-pass chirped volume Bragg grating (CVBG) to obtain a 2.8 ps pulse width with a total pulse energy of 46 μJ. The overall system compactness was enabled by the all-fiber amplifier design and the multi-pass CVBG-based compressor. The laser output was then used to demonstrate high-speed direct-writing capability on a temperature-sensitive biomaterial to change its topography (i.e. fabricate microchannels, foams and pores). The topographical modifications of biomaterials are known to influence cell behavior and fate which is potentially useful in many cell and tissue engineering applications.