Diffraction-limited Beam Quality Research Articles

135 W average-power femtosecond pulses at 520 nm from a frequency-doubled fiber laser system

We present a high-average-power femtosecond laser system at 520 nm central wavelength. The laser system delivers sub-500 fs pulses with 135 W average power at a pulse repetition rate of 5.25 MHz. Excellent beam quality is provided by high power fiber amplifiers and maintained during frequency doubling, resulting in a beam quality factor of M2<1.2. To our knowledge, the system presented here is the highest average power green laser source generating femtosecond pulses with diffraction-limited beam quality.

High average power, high energy 155 μm ultra-short pulse laser beam delivery using large mode area hollow core photonic band-gap fiber

We demonstrate high average power, high energy 1.55 μm ultra-short pulse (<1 ps) laser delivery using helium-filled and argon-filled large mode area hollow core photonic band-gap fibers and compare relevant performance parameters. The ultra-short pulse laser beam-with pulse energy higher than 7 μJ and pulse train average power larger than 0.7 W-is output from a 2 m long hollow core fiber with diffraction limited beam quality. We introduce a pulse tuning mechanism of argon-filled hollow core photonic band-gap fiber. We assess the damage threshold of the hollow core photonic band-gap fiber and propose methods to further increase pulse energy and average power handling.

高功率单频线偏振掺镱全光纤放大器

We report an all-fiber high power, single frequency large-mode area (LMA) linearly polarized ytterbiumdoped fiber amplifiers (YDFA) module, which is based on the master oscillator multi-stage power amplifiers (MOPA). The maximum output power is 43.8 W at a wavelength of 1064 nm when 60-W launched pump light is coupled, with high slope efficiency of 88%, polarization extinction rate (PER) >17.2 dB and nearly diffraction-limited beam quality (M2<1.1).

Power scaling of a fiber master oscillator power amplifier system using a coherent polarization beam combination

A new coherent polarization beam combination (CPBC) architecture was employed to combine four amplified beams and produce over 25 W of coherent output with 94% efficiency and nearly diffraction-limited beam quality. The individual beams were produced by a fiber master oscillator power amplifier system and their phases were controlled and locked using servo-controlled phase modulators. The CPBC approach relies on a coherent superposition of orthogonal polarization states and allows the combination of an arbitrary number of beams with arbitrary power ratios.

Green laser photocoagulator for the treatment of diabetic retinopathy developed at Raja Ramanna Centre for Advanced Technology

An all-solid-state green laser photocoagulator at 532 nm with output power varying from 100 mW to 1 W in a step of 10 mW and exposure time varying from 50 ms to 1000 ms in a step of 10 ms is developed for the treatment of diabetic retinopathy. The output power stability is better than ±1.5% with a nearly diffraction-limited beam quality. The system includes various safety and operational features like internal power monitoring system, safety interlock, emergency switch-off, graphical LCD display with table-top touch mode portable control panel, smart delivery device selection, aiming laser beam with controllable intensity, foot switch, patient records, service mode etc. The system has successfully passed the clinical trials and is being used on patients.

Ge-codoped fibers for mitigating stimulated Brillouin scattering in high power fiber amplifiers

Rare-earth doped fibers for high power fiber amplifiers normally have a small refractive index difference between the core and inner cladding, but have a larger core diameter than conventional telecom fibers. We take advantage of this feature as well as the difference between the optical and acoustic waveguides and proposed several fiber designs to mitigate Stimulated Brillouin scattering (SBS) to produce higher output powers. The numerical modeling shows that an increase in SBS threshold by 3–6dB can be achieved using Ge-codopant while maintaining diffraction limited beam quality. The key parameters that can affect the SBS performance are specified and optimal parameters for SBS suppression are obtained. The study also highlights the need to take into account the effects of minor refractive index profile variation in evaluating SBS performance.

Chirally-coupled-core Yb-fiber laser delivering 80-fs pulses with diffraction-limited beam quality warranted by a high-dispersion mirror based compressor

We demonstrate a high-energy femtosecond laser system that incorporates two rapidly advancing technologies: chirally-coupled-core large-mode-area Yb-fiber to ensure fundamental-mode operation and high-dispersion mirrors to enable loss-free pulse compression while preserving the diffraction-limited beam quality. Mode-locking is initiated by a saturable absorber mirror and further pulse shortening is achieved by nonlinear polarization evolution. Centered at 1045 nm with 39-MHz repetition rate, the laser emits 25-nJ, positively chirped pulses with 970-mW average power. 6 bounces from double-chirped-mirrors compress these pulses down to 80 fs, close to their transform-limited duration. The loss-free compression gives rise to a diffraction-limited optical beam (M2 = 1.05).

Femtosecond pulse generation and amplification in Yb-doped fibre oscillator-amplifier system

In recent times ytterbium (Yb) doped fibre-based mode-locked master oscillator and power amplifier have attracted a great deal of interest because of their inherent advantages like flexibility, reliability, compactness, high power handling capability and diffraction limited output beam quality as compared to the solid-state counterpart. But, to successfully develope of high-power femtosecond oscillator-amplifier system based on Ybdoped fibre, an appropriate choice of the mode-locking regime and the amplifier geometry are required. Development of an all-fibre integrated high-power Yb-doped fibre oscillator-amplifier system in which the advantages of a fibre-based system can be fully exploited remained a challenge as it requires the careful optimization of dispersion, nonlinearity, gain and ASE contribution.

Improving the modulation efficiency of high-power distributed Bragg reflector tapered diode lasers

Different measures to improve the modulation efficiency of a distributed Bragg reflector tapered diode laser emitting at 1060 nm were investigated. Due to the 6-mm long cavity, the device reached an output power of 10 W with a nearly diffraction-limited beam quality. The input currents to the ridge-waveguide (RW) and tapered gain-region sections can be independently controlled. This allows a low-current modulation of the optical output power in the Watt range. Under optimized quasi-static conditions the power could be modulated between 0.2 and 3.1 W (4.8 W) by a variation of the RW current between 0 and 50 mA (350 mA). Due to the integrated 6th order surface Bragg grating the emission wavelength remains within the spectral range of 80 pm.

Coherent Polarization Beam Combination

A new, scalable coherent beam combination architecture has been developed and experimentally verified. Coherent Polarization Beam Combination (CPBC) relies on the combination of multiple beams through a coherent superposition of orthogonal polarization states. By actively controlling the phases and polarizations of individual beams, the creation of new polarization states at polarization beam combiners enables the coherent combination of any number of beams with arbitrary power ratios. A four-beam CPBC was experimentally demonstrated at low laser power level, with 96% combining efficiency, nearly diffraction-limited beam quality, and variable polarization of the output beam. The CPBC method will enable power scaling of master oscillator power amplifier laser systems.

Fiber lasers and amplifiers: an ultrafast performance evolution

The first rare-earth-doped fiber lasers were operated in the early sixties and produced a few milliwatts at a wavelength around 1 mum. For the next several decades, fiber lasers were little more than a low-power laboratory curiosity. Recently, however, fiber lasers are entering the realm of kilowatt powers in continuous as well as in pulse operation with diffraction-limited beam quality. In this article we review this power evolution.

Kilowatt-level stimulated-Brillouin-scattering-threshold monolithic transform-limited 100 ns pulsed fiber laser at 1530 nm

We demonstrate a high-stimulated-Brillouin-scattering-threshold monolithic pulsed fiber laser in a master oscillator power amplifier configuration that can operate over the C band. In the power amplifier stage, we used a newly developed single-mode, polarization maintaining, and highly Er/Yb codoped phosphate fiber with a core diameter of 25 microm. A single-frequency actively Q-switched fiber laser was used to generate pulses in the hundreds of nanoseconds at 1530 nm. We have achieved peak power of 1.2 kW for 105 ns pulses at a repetition rate of 8 kHz, corresponding to a pulse energy of 0.126 mJ, with transform-limited linewidth and diffraction-limited beam quality.

Narrow Linewidth Tm3+-Doped Large Core Fiber Laser Based on a Femtosecond Written Fiber Bragg Grating

A diode laser (LD) clad-pumped narrow linewidth all-fiber Tm3+-doped fiber laser is reported with a maximal output power of 27 W at 1.947μm. By successively splicing an LD pigtail fiber, a single-mode Tm3+-doped fiber, and a multi-mode Tm3+-doped fiber, the fiber laser has 70 pm narrow linewidth output, and a high slope efficiency of nearly 47.5% with respect to the launched pump power. The high reflectivity fiber Bragg gratings, which are directly written into the single-mode Tm3+-doped fiber core by the 800 nm femtosecond pulsed laser, act as the high reflectivity coupler. The output laser has diffraction-limited beam quality with a factor M2 of 1.29, when the output laser power is nearly 27 W.

Experimental demonstration of spatially coherent beam combining using optical parametric amplification

We experimentally demonstrated coherent beam combining using optical parametric amplification with a nonlinear crystal pumped by random-phased multiple-beam array of the second harmonic of a Nd:YAG laser at 10-Hz repetition rate. In the proof-of-principle experiment, the phase jump between two pump beams was precisely controlled by a motorized actuator. For the demonstration of multiple-beam combining a random phase plate was used to create random-phased beamlets as a pump pulse. Far-field patterns of the pump, the signal, and the idler indicated that the spatially coherent signal beams were obtained on both cases. This approach allows scaling of the intensity of optical parametric chirped pulse amplification up to the exa-watt level while maintaining diffraction-limited beam quality.

Recent advances in the development of discharge‐pumped oxygen‐iodine lasers

AbstractChemical oxygen‐iodine lasers are unique in their ability to generate high‐power beams with near diffraction limited beam quality. The operating wavelength, 1.315 µm is readily transmitted by the atmosphere and compatible with fiber optics beam delivery systems. However, applications of the laser are severely limited by logistical problems associated with the complex chemistry used to power the device. Electrical or microwave discharge excitation of oxygen‐iodine lasers offers an attractive alternative that eliminates the chemical power generation problems and has the possibility of closed‐cycle operation. A discharge oxygen‐iodine laser was first demonstrated in 2005. Since that time the power of the device has been improved by a factor of 400 and much has been learned concerning the physics and chemistry of the discharge driven system. Although our current understanding of the chemical kinetics is incomplete, parametric studies of laser performance show considerable promise for further scaling. This article reviews the basic principles of the discharge oxygen iodine laser, summarizes the most recent advances, and outlines some of the unresolved questions regarding the production and removal of excited species in the gas flow.

High power narrow linewidth single-frequency line-polarized fiber amplifier based on master-oscillator power amplifier technology

High power single-frequency (SF) lasers are of great significance in science, industry, and defense applications such as gravitational wave detection, free apace optical communication, range finding, lidar, parametric wavelength conversion in resonant cavities. We report on a high power single-frequency linearly polarized radiation with a nearly diffraction-limited beam quality by use of a two-stage fiber master-oscillator power amplifier system. The master oscillator is an SF distribution feedback fiber laser with polarization maintaining (PM) and single mode (SM) fiber pigtailed output power of about 10 mW at 1064 nm, and its line-width is of 20 kHz. This light power is amplified to about 1 W by the first stage of all-fiber SM/PM amplifier and then seeded at the second stage of high power main amplifier with large mode area (LMA) ytterbium-doped fiber (YDF). The pumping diode laser of the main amplifier is fiber-coupled (the core diameter is 400 μm and corresponding numerical aperture is 022) with a max emitting power as high as 190 W at 976 nm. By employing the co-pump configuration with a space coupling efficiency of about 85%, we demonstrate that the power output from the LMA/PM YDF (core diameter is 20 μm, inner cladding diameter is 400 μm) reaches 128 W and its corresponding overall slope-efficiency is 85% of the launched pump power. The beam profile is diffraction-limited and linearly polarized with a polarization extinction ratio over 12 dB. There is observed neither amplified spontaneous emission nor stimulated Brillouin scattering phenomenon, which means that the substitution of the pump source with the more powerful one can further enhance power output if LMA YDF with a larger core size, shorter fiber length and higher doping efficiency is used.

Effect of Polarization States on the Laser Performance of Passively $Q$-switched Yb:YAG/Cr,Ca:YAG Microchip Lasers

The polarization states of passively <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -switched microchip lasers were investigated by adopting different combinations of Yb:YAG, Cr,Ca:YAG crystals and ceramics. The results show that except for the random polarization oscillation of Yb:YAG/Cr,Ca:YAG all-ceramics combination, other three combinations including crystals exhibit linearly polarization. Highly efficient, nearly diffraction-limited beam quality, high peak power, sub-nanosecond passively <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -switched lasers were obtained for all combinations of Yb:YAG, Cr,Ca:YAG ceramics and crystals. The best laser performance was obtained with Yb:YAG/Cr,Ca:YAG crystals combination. The effect of polarization states on the laser performance was also addressed.

High power quantum cascade lasers

We report the most recent state-of-art quantum cascade laser results at wavelengths around 4.8 and 10 μm. At 4.8 μm, a room temperature wall plug efficiency (WPE) of 22 and 15.5% are obtained in pulsed mode and continuous wave (cw) mode, respectively. Room temperature cw output power reaches 3.4 W. The same laser design is able to reach a WPE of 36% at 120 K in pulsed mode. At 10 μm, room temperature average power of 2.2 W and cw power of 0.62 W are obtained. We also explore lasers utilizing the photonic crystal distributed feedback mechanism, and we demonstrate up to 12 W peak power operation at three different wavelengths around 4.7 μm with a waveguide width of 100 μm and diffraction limited beam quality.

Compact second-harmonic generation laser module with 1 W optical output power at 490 nm

We demonstrate continues-wave 1 W at 490 nm on a 2.5 cm(3) micro-optical bench using single-path second-harmonic generation with a periodically poled MgO:LiNbO(3) bulk crystal. The pump laser is a distributed Bragg reflector tapered diode laser having a single-frequency spectrum and a pump power of 9.5 W. Based on that 1 W blue light could be achieved resulting in an optical conversion efficiency of 11%. Furthermore, the module has an output power stability of better than 2% and the blue laser beam shows an nearly diffraction limited beam quality of M(2)(sigma) = 1.2 in vertical and M(2)(sigma) = 2 in lateral direction.

Solid-state lasers: status and perspectives

Diode-pumped solid-state lasers are attractive sources of coherent radiation for various scientific and technological applications. In the past years these lasers have emerged as attractive power scalable laser concept for continuous wave and pulse operation. However, the different fields of applications increasingly demand more powerful, efficient, and rugged lasers with diffraction limited beam quality in a wide spectral range. Hence, at present a new generation of solidstate lasers is in the focus of the developments. Thin disc and fiber lasers with improved thermo-optical parameters have now entered the regime of multi-kilowatt of average