Reflective Volume Bragg Gratings Research Articles

Research on self phase-locked two-channel narrow linewidth laser

We have designed and constructed a self phase-locked two-channel narrow linewidth solid-state laser based on reflective volume Bragg grating (RVBG) as the output mirror. The resonant cavity of the laser is designed as a U-shaped structure, and two independent laser diode (LD)-side-pumped modules achieve passive phase locking of the output laser through mutual injection of photon energy. Compression of the linewidth is achieved by the RVBG. When the LD pump power was 70 W, the output powers of the two-channel lasers were 1.59 W and 1.61 W, the measured central wavelengths of the spectra were 1064.23 nm and 1064.24 nm, and the linewidths were 0.10 nm and 0.11 nm, respectively. The slope efficiencies of the two lasers are 9.5 % and 9.56 %, respectively, and their powers stability were both better than 3 %. The beam quality factors were Mx2=1.29, My2=1.31 and Mx2=1.32, My2=1.36, respectively. The interference fringes formed by the coherent superposition of two laser beams in the far field were observed through CCD, which verified the phase-locked coherence characteristics of the two lasers. The output power of the coherent combination laser is 2.64 W, with a corresponding efficiency of coherent combination is 82.5 %. The experimental results show that the laser can directly obtain two-channel narrow-linewidth continuous wave lasers output with phase locking and consistent laser intensity. Our research provides a new idea for obtaining a new type of high-quality narrow-linewidth coherent laser source

Comparison of spectral beam combining based on reflecting volume Bragg grating, with and without collimating lenses

AbstractOn the basis of the principle of spectral beam combining (SBC), a simple combining system using a reflecting volume Bragg grating as a combiner was designed in which two focusing lenses were used to control the incident angles of input beams, and two concave lenses were used to solve the problem of beam divergence caused by focusing lenses. The effects of the beam divergence angle (DA) on diffraction efficiency and beam quality were analyzed theoretically. The simulation results showed that a larger DA will reduce diffraction efficiency and lead to deterioration of beam quality. In the SBC experiment, the performances of the output beam with and without concave lenses were compared. The experimental results showed that, when the concave lenses were not inserted, the measured output power was 503 mW with a combining efficiency of 85.3% and the beam quality factor of 1.68. The experiment further showed that the combined power was 566 mW with a combining efficiency of 96% and a beam quality factor of 1.31 when concave lenses were used.

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.

202 W dual-end-pumped Tm:YLF laser with a VBG as an output coupler

Abstract We demonstrated a 202 W Tm:YLF slab laser using a reflecting volume Bragg grating (VBG) as an output coupler at room temperature. Two kinds of active heat dissipation methods were used for the VBG to suppress the shift of wavelength caused by its increasing temperature. The maximum continuous wave (CW) output power of 202 W using the microchannel cooling was obtained under the total incident pump power of 553 W, the corresponding slope efficiency and optical-to-optical conversion efficiency were 39.7% and 36.5%, respectively. The central wavelength was 1908.5 nm with the linewidth (full width at half maximum) of 0.57 nm. Meanwhile, with the laser output increasing from 30 to 202 W, the total shift was about 1.0 nm, and the wavelength was limited to two water absorption lines near 1908 nm. The beam quality factors M2 were measured to be 2.3 and 4.0 in x and y directions at 202 W.

Development and Thermal Management of kW-Class High-Power Diode Laser Source Based on the Structure of Two-Stage Combination

In this paper, we present an optical structure for two-stage beam combination to realize a high-power fiber-coupled diode laser source. In the first stage, dense spectral combination based on reflecting volume Bragg gratings is implemented for combining five diode laser blocks with a spectral separation of 1.5 nm, to a high output power submodule. In the second stage, submodules are further coaxially multiplexed by polarization beam combination and coarse spectral combination to obtain a higher output power. In the process of the beam combination, thermal effects of combining elements are also investigated. By using a temperature control, the diode laser source can steadily produce 3120-W power from an optical fiber with 105-μm core diameter and 0.2 numerical aperture. This paper demonstrates the benefits of this combination structure and the effectiveness of the temperature control.

Fabrication of high-precision reflective volume Bragg gratings.

Reflecting Bragg gratings (RBGs) recorded in photo-thermo-refractive (PTR) glasses have been widely used in narrowing and stabilization of the laser emission spectrum. As the center wavelength of RBGs determines the final output wavelength of lasers, it is necessary to carefully control the center wavelength of RBGs during the fabrication process. In this paper, the fabrication process of high-precision RBGs was investigated. We developed a two-step method and demonstrated both theoretically and experimentally that it is effective and can be used to guide the fabrication process of high-precision RBGs. The experimental results show that the center wavelength of the fabricated RBG deviates from the target center wavelength within ±10 pm.

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%.

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.

Wavelength- and OAM-tunable vortex laser with a reflective volume Bragg grating.

Vortex beams carrying orbital angular momentum (OAM) have been recently investigated intensely in optical communication systems, as using OAM mode multiplexing simultaneously with other conventional multiplexing techniques is the key to further expand data capacity. This article demonstrates a wavelength- and OAM-tunable vortex laser at 1.6 µm in an Er:YAG system. For the first time to the best of our knowledge, a reflective volume Bragg grating (VBG) was theoretically and experimentally proved to be an effective OAM-preserving wavelength selector inside the laser cavity. A z-shaped laser cavity employing a VBG as a folding mirror was constructed for the direct generation of vortex beams, and we finally obtained wavelength-tunable beams of five OAM states (0, ± ħ, and ± 2ħ) with a narrow bandwidth less than 0.04 nm. This laser supplies a new way for optical communication by combining the spatial degree of freedom for multiplexing information channels with the conventionally used wavelength domains in packable and robust resonant cavity.

Stretching of picosecond laser pulses with uniform reflecting volume Bragg gratings

This study shows that a uniform reflecting volume Bragg grating (VBG) can be used as a compact monolithic stretcher of high-power picosecond laser pulses, which is important for cases in which chirped Bragg gratings with the required chirp rate are difficult to fabricate. When an incident short pulse propagates along a grating and experiences local Bragg diffraction, a chirp-free reflected stretched pulse with an almost rectangular shape is generated. The increase in the duration of the reflected pulse is approximately equal to twice the propagation time along the grating. We derive an analytic expression for the diffraction efficiency, which incorporates the incident pulse duration, grating thickness, and amplitude of the refractive index modulation, enabling selection of the optimum grating for pulse stretching. Theoretical models of the extended pulse profiles are found to be in good agreement with experimental autocorrelation measurements. We also propose a simple and reliable method to control the temporal parameters of high-power picosecond pulses using the same laser source and a VBG of variable thickness, which can simplify experiments requiring different pulse durations significantly.

Long-pulse, high-energy, narrow-linewidth Nd:LGGG laser at 1336.63 nm with reflecting Bragg grating

A long-pulse, high-energy, narrow-linewidth 1336.63 nm laser based on Nd:(LuxGd1−x)3Ga5O12 crystal is demonstrated for the first time. A reflecting volume Bragg grating is used as a cavity mirror of the standing-wave cavity to select the desired wavelength. In order to narrow the linewidth, an 8 mm thick etalon is inserted in the cavity. A maximum output energy of 0.66 J at an operation pulse repetition rate of 5 Hz with a pulse width of 770 µs is obtained. The beam quality factor M2 is about 1.2. The tuning range is more than 32 pm from 1336.613 to 1336.645 nm through changing the temperature of RVBG. The standard deviation of wavelength stability is about 1.6 pm over 20 min. At the output energy of about 0.59 J, the linewidth is estimated to be 6.79 pm.

Spectral Beam Combining of Fiber Lasers by Using Reflecting Volume Bragg Gratings**Supported by the National Natural Science Foundation of China under Grant Nos 11474257 and 61605183.

By employing three reflecting volume Bragg gratings, a near-infrared 4-channel spectral-beam-combining system is demonstrated to present 720 W combined power with a combining efficiency of 94.7%. The combined laser beam is near-diffraction-limited with a beam factor M2 ∼ 1.54. During this 4-channel beam-combining process, no special active cooling measures are used to evaluate the volume Bragg gratings as combining elements are under the higher power laser operation. Thermal expansion and period distortion are verified in a 2 kW 2-channel beam-combining process, and the heat issue in the transmission case is found to be more remarkable than that in the diffraction case. Transmitted and diffracted beams experience wave-front aberrations with different degrees, thus leading to distinct beam deterioration.

Volume-chirped Bragg gratings: monolithic components for stretching and compression of ultrashort laser pulses

An innovative type of optical component—a volume Bragg grating—has recently become available commercially and has found wide applications in optics and photonics due to its unusually fine spectral and angular filtering capability. Reflecting volume Bragg gratings, with the grating period gradually changing along the beam propagation direction (chirped Bragg gratings—CBGs) provide stretching and recompression of ultrashort laser pulses. CBGs, being monolithic, are robust devices that have a footprint three orders of magnitude smaller than that of a conventional Treacy compressor. CBGs recorded in photo-thermo-refractive glass can be used in the spectral range from 0.8 to 2.5 μm with the diffraction efficiency exceeding 90%, and provide stretching up to 1 ns and compression down to 200 fs for pulses with energies and average powers exceeding 1 mJ and 250 W, respectively, while keeping the recompressed beam quality M 2 <1.4 , and possibly as low as 1.1. This paper discusses fundamentals of stretching and compression by CBGs, the main parameters of the gratings including the CBG effects on the laser beam quality, and currently achievable CBG specifications.

Effect of aberrations in a holographic system on reflecting volume Bragg gratings

The effect of aberrations in the recording beams of a holographic setup is discussed regarding the deterioration of properties of a reflecting volume Bragg grating. Imperfect recording beams result in a spatially varying grating vector, which causes broadening, asymmetry, and washed out side lobes in the reflection spectrum as well as a corresponding reduction in peak diffraction efficiency. These effects are more significant for gratings with narrower spectral widths.

Thermal limitations of volume Bragg gratings used in lasers for spectral control

In this paper we present experiments and calculations of the property changes of a highly reflecting volume Bragg grating (VBG) when it is used as a laser cavity mirror. A small absorption of the reflected laser beam resulted in a laser output power roll-off, increased coupling through the VBG, and a change of the spectrum from a single to a double peak at high power. The simulations revealed that an inhomogeneous temperature distribution deformed the grating such that the diffraction efficiency was reduced and the light penetrated deeper into the VBG, which accelerated the deteriorating effects. We extrapolated the power limit found in our investigations for various beam radii and absorption coefficients.

High power single-longitudinal-mode Nd:YAG CW laser using reflective volume Bragg grating as output coupler

The single-longitudinal-mode (SLM) laser system has been demonstrated using reflective volume Bragg grating (RVBG) as output coupler based on short oscillator. The laser is laser diode (LD) side pumped and the oscillator is composed of one surface of Nd:YAG and RVBG. The RVBG has good angle and wavelength selectivity in SLM laser system. The maximum SLM output power is 622.15 mW. The average opticsoptics efficiency is 17.2% and slope efficiency is 25.6%. The M2 factor is smaller than 1.3.

Passive frequency stabilization in Nd:YAG pulse laser using reflective volume Bragg grating

We demonstrate a stable Q-switched single-longitudinal-mode (SLM) Nd:YAG laser using a volume Bragg grating as the output coupler. The reflective volume Bragg grating, serving as a longitudinal selector and passive frequency stabilizer, effectively eliminates the mode hopping effect of the laser. The maximum output energy of the SLM obtained from the current experimental setup is 18.5 mJ. The maximum separation of frequencies is significantly less than the longitudinal mode separation, indicating that a stable SLM laser is achieved.

Spectral beam combining using superimposed reflective volume Bragg grating

It is an effective approach to obtain high-power laser output by spectral beam combining (SBC) technologies. In this paper, a simple SBC system based on the superimposed reflective volume Bragg grating (RVBG) is proposed to reduce the scale of the SBC system. Considering the cross-coupling between the two gratings in a volume, two structure models of the superimposed RVBG-the superimposed RVBG with the same period and the superimposed RVBG with a different period-are analyzed and compared by using the rigorous coupled wave analysis, and their applications for SBC system are discussed. Numerical results show that the superimposed RVBG is easy to be fabricated and has the potential to combine multiple lasers with one volume. Cross-coupling can be neglected for the same period structure when the slant angle divergence is larger than 0.01 rad and for the different period structure when the spectral separation is larger than 1.0 nm. The superimposed RVBG with a different period can achieve high diffraction efficiency simultaneously for each grating with a small slant angle divergence, and is convenient to combine multiple beams with small spectral separation. The effect of wavelength shift on diffraction efficiency can be ignored.

Single resonance monolithic Fabry–Perot filters formed by volume Bragg gratings and multilayer dielectric mirrors

A new class of Fabry-Perot filters produced by a multilayer dielectric mirror deposited on top of a reflecting volume Bragg grating is described. The first fabricated prototype for the 852 nm region demonstrates a 30 pm bandwidth, 90+% transmission at resonance, and a good agreement with theoretical simulation.

Channel Spectral Separation Narrowing for Spectral Beam Combining by Apodisation of the Reflecting Volume Bragg Grating

The sidelobe in diffraction efficiency of reflecting volume Bragg grating (RVBG) limits the wavelength channel spectral separation, which determines the combining power output in spectral beam combining (SBC) systems. A novel SBC system based on the apodised RVBG has been proposed to suppress the sidelobe. Several apodised RVBGs have been compared and the optimal apodised RVBG is obtained by using the chain-matrix approach. Numerical results show that the sidelobe could be suppressed excellently with Blackman apodised RVBG. In the numerical example, the minimal channel spectral separation was 1.0 nm for SBC system based on the RVBG and it decreased to 0.6 nm for the novel SBC system based on the Blackman apodised RVBG when the spectral combining efficiency of both systems achieves the same maximum value. Defence Science Journal, 2011, 61(3), pp.201-205 , DOI:http://dx.doi.org/10.14429/dsj.61.528