Propagation of coherently combined flattened laser beam array in turbulent atmosphere

The analytical expression for the propagation of a flattened laser beam array in a turbulent atmosphere is derived based on extended Huygens–Fresnel principle. The influence of beam order and turbulent atmosphere on beam quality is studied. It is revealed that the beam quality of a coherently combined laser beam array with higher order is better than its lower order counterpart when propagating in free-space, weak and medium turbulence (i.e. C_n²<10^?13 m^?2/3). The beam quality of higher order beam arrays degrades faster as the intensity of turbulence gets stronger. In the case of propagating in strong turbulence, the beam order has no influence on coherently combined beam quality.     

Near-diffraction-limited segmented broad area diode laser based on off-axis spectral beam combining

The beam quality of a 500-μm-wide broad area diode laser with five active segments has been improved beyond the beam quality of the individual segments. The principle of this new laser system is based on off-axis feedback in combination with spectral beam combining. By using a double-feedback scheme we are able to improve the beam quality of the laser by a factor of 23 from M²=55 for the free-running diode laser to M²=2.4 for the laser with feedback at a drive current of 2.2 A. The improved M² value is a factor of 3.4 below M²=8.2 for a single free-running segment. This is the first time that the beam quality of a segmented broad area diode laser has been improved beyond the beam quality of the individual segments. PACS 42.55.Px; 42.60.Da; 42.60.Jf     

Study on the Beam Quality of Uncoupled Laser Diode Arrays

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Study on the Beam Quality of Uncoupled Laser Diode Arrays

The beam quality of uncoupled laser diode array is studied theoretically and experimentally. By calculating the second order moments of the beam emitted from the laser diode array, the dependence of the M2-factor of the laser diode array on the M2-factor of the single emitter, the ratio of the emitting region to the non-emitting space, and the number of emitters, has been deduced. From the measurement of the beam propagation the M2-factor of a laser diode bar is experimentally determined. The measured M2-factor of the laser diode bar agrees with the theoretical prediction.     

Characterization of laser processing of single‐crystal natural diamonds using micro‐Raman spectroscopic investigations

The application of lasers for processing diamond has revolutionized the diamond industry and its applications in microelectronics, microelectromechanical system (MEMS) and microoptoelectromechanical system (MOEMS) technologies. The process quality can be evaluated using spectroscopic techniques. In the present investigation, four different types of Q‐switched solid‐state lasers (with different beam parameters), namely, a lamp‐pumped Nd:YAG laser operating at 1064 nm, a lamp‐pumped Nd:YAG laser operating at second harmonically generated 532 nm, a diode‐pumped Nd:YVO₄ laser operating at 1064 nm and a diode‐pumped Nd:YAG laser operating at 1064 nm, have been employed for the processing of a single‐crystal, gem‐quality, natural diamond. The main objective behind the selection of these lasers with different beam parameters was to study the effect of wavelength, pulse width, pulse energy, peak power and beam quality factor (M² factor) on various aspects of processing (such as microcracking, material loss and cut surface quality) and their relative merits and demerits. The overall weight loss of the diamond and formation of microcracks during processing have been studied for the above four cases. The characteristics of the graphite formed during processing, elemental analysis, surface morphology of the cut surface and process dynamics have been studied using micro‐Raman spectroscopy and scanning electron microscopy (SEM). We observed that laser cutting of single‐crystal diamonds used for industrial applications can be accomplished without microcracking or surface distortion using Q‐switched Nd:YAG lasers. This allows direct processing without extensive postgrinding and polishing stages. Very efficient diamond processing is possible using diode‐pumped lasers, which results in the lowest possible breakage rate, good accuracy, good surface finish and low weight loss. From the micro‐Raman and SEM studies, it is concluded that the surface quality obtained is superior when diode‐pumped Nd:YVO₄ laser is used, owing to its extremely high peak power. The maximum graphite content is observed while processing with lamp‐pumped Nd:YAG laser at 532 nm. An overall comparison of all the laser sources leads to the conclusion that diode‐pumped Nd:YAG laser is a superior option for the efficient processing of natural diamond crystals. Copyright © 2006 John Wiley & Sons, Ltd.     

An 8-W diode side pumped solid-state 593-nm laser

We report a double z-type folded plane-plane symmetrical cavity diode side pumped solid state yellow-orange laser at 593 nm by using intracavity sum-frequency mixing. By carefully designing the cavity and employing several techniques to increase sum-frequency efficiency, a Q-switched yellow-orange laser source, with an average output power of 8 W, a beam quality factor M ² = 4.86, and a repetition rate of 8 kHz is developed. In this paper, we first use 1338 and 1064 nm emissions of Nd:YAG crystal to generate 593 nm yellow-orange laser beam by intracavity sum-frequency mixing (SFM).     

微透镜阵列误差对半导体激光匀化性能的影响

为了减小微透镜阵列误差对匀化光斑的影响,深入研究微透镜阵列光束匀化系统中微透镜阵列相对位置误差对光束匀化性能的影响,设计了一种微透镜阵列光束匀化系统。依据相对位置误差类型的不同,将双列微透镜阵列间六个自由度变化导致的误差分为距离误差、偏移误差以及转动误差进行分析,并对每种误差对光束匀化性能的影响进行了研究。采用6板条半导体激光器堆栈对上述匀化系统进行实验验证,实现了均匀性为90.75%的光斑,并对系统影响光斑性能的原因进行了分析。     

Multi-wavelength,high spatial brightness operation of a phase-locked stripe-array diode laser

Stable continuous wave multi-wavelength operation of a stripe-array diode laser with an externalcavity spectral beam combining geometry is presented. In this setup each emitter of the stripe-array is forced to operate at a different wavelength, which leads to a decoupling between the usually phase-locked emitters. With a reflective diffraction grating with a period of 300 lines per mm, 33 equidistant laser lines around a center wavelength of 978 nm were realized, spanning a spectral range of 26 nm. With this novel approach near-diffraction limited emission with a beam quality of M ² < 1.2 and an output power of 450 mW was achieved. This laser light source can be used for applications requiring low temporal but high spatial coherence.     

1064 nm Nd:YAG laser intracavity pumped at 946 nm

We present for the first time a Nd:YAG laser emitting at 1064 nm intracavity pumped by a 946 nm diode-pumped Nd:YAG laser. A 885 nm laser diode is used to pump the first Nd:YAG crystal emitting at 946 nm, and the second Nd:YAG laser emitting at 1064 nm intracavity pumped at 946 nm. We achieved an output power of 7.97 W at 1064 nm for an absorbed pump power at 946 nm of 9.55 W, corresponding to an optical efficiency of 83.4%. The beam quality M² quality factor is about 1.1 at the maximum output power.     

Amplified luminescence in InGaAs/AlGaAs laser diode arrays at high pump levels

The development of amplified luminescence fluxes in powerful InGaAs/AlGaAs laser diode arrays (lasing wavelength 940–960 nm) has been studied experimentally and theoretically at pump levels above the threshold value. Flux density values for amplified luminescence propagation along (1.88⋅10⁹ W/m²) and across (1.21⋅10⁹ W/m²) the laser diode array cavity axis have been evaluated for the threshold pump level at room temperature (293 K). The contribution of the recombination rate induced by the amplified luminescence to the threshold current generation of the laser diode array reaches 7%. It has been shown that the amplified luminescence flux density is increased by 49% as the pump level rises from one to three threshold values.     

Wavefront reconstruction of high power laser diode array

A new method for the wavefront measurement of high power laser diode array with large dynamic ranges is presented. The wavefront sensor has a large dynamic range of −π/2 to π/2 and high precision on the measured wavefront slopes (±1′). According to the measurements of the slopes and intensity of the high power laser diode array, complex amplitudes can be obtained and the wavefront can be reconstructed. The reconstructed wavefront of high power laser diode array can be used in designing a laser beam shaping system in future work.     

1.15 kW连续波全固态Nd:YAG激光器

研制了五组双线二极管列阵侧面泵浦Nd: YAG棒的高效率、高功率激光头。将一块90° 石英旋光片插入两个这样的激光头中间并置入热近非稳对称平平腔，产生了1157 W高光束质量（M2 ~ 39）1064 nm连续波输出，据我们所知，这是侧面泵浦双棒Nd: YAG激光器产生的最高功率。     

All-solid-state CW Nd:GdVO<Subscript>4</Subscript>-LBO red laser under direct 912 nm pumping

We report a red laser at 670.5 nm generation by intracavity frequency doubling of a continuous wave (CW) laser operation of a 1341 nm Nd:GdVO₄ laser under in-band diode pumping at 912 nm. An LBO crystal, cut for critical type I phase matching is used for second harmonic generation of the laser. At an incident pump power of 8.9 W, as high as 347 mW of CW output power at 670.5 nm is achieved. The fluctuation of the red output power was better than 3.7% in the given 30 min, and the beam quality factor M ² is 1.65.     

Thermal- and Spectral-Characteristics of High-Power Quasi-Continuous Wave 940-nm InGaAs Diode Laser Arrays

Experimental results of the thermal and spectral characteristics of a monolithic stack of high power quasicontinuous wave 940-nm InGaAs linear laser diode arrays have been evaluated. Thermal resistance as the most important thermal parameter characterizing a high-power laser diode package was obtained using the temperature rise measured directly by a thermo-camera. A new simple and convenient technique to measure a spectral transition of the emission from laser diode arrays is proposed. Spectral chirping due to the transient thermal power dissipated during the laser pulse was observed as a time-evolution of the spectral profile; it gave a comprehensible image of the chirping behavior. Comparing the temperature rise in the diode junction with the thermal simulation, it was determined that the thermal shift of central wavelength dλ/dT was 0.21 nm/°C. Detailed performances were identified for pumping a Yb³⁺ doped crystalline laser, and it was verified that the laser diode arrays were satisfactory to meet pumping source requirements for coupling to Yb³⁺ absorption linewidth.     

泵浦匀化对Innoslab激光放大器光束质量的改善

针对结构紧凑的部分端面泵浦混合腔板条激光放大器,研究了泵浦匀化对其输出激光光束质量的改善作用。基于ZEMAX仿真软件,设计并对比分析了直接成像和波导匀化后再成像两种泵浦耦合方式对激光二极管阵列慢轴（晶体宽度）方向光强分布均匀性的影响,发现波导匀化后泵浦均匀性得到明显提升且激光二极管阵列发光坏点对泵浦成像的影响得以弱化。进一步实验验证了泵浦匀化对Innoslab激光放大器性能的改善作用:波导匀化后慢轴方向泵浦均匀度从90.6%提高到95.4%,进而使3通放大后输出激光光束质量因子M2从2.41提升到1.55,并且有效抑制了多通放大腔内的自激振荡。     

Diode both ends pumped two crystals Tm:YLF laser

In this paper, we report two crystals Tm:YLF laser pumped by a fiber coupled diode laser with total power of 180 W at 792 nm. When the incident power reached up to 117.0 and 37.3 W of the emission wave-length at 1.91 μm was achieved. Through linear fit, the corresponding slope efficiency of 38.5%, optical-to-optical conversion efficiency of 31.9% and the beam quality factor of M ² ∼ 1.1 were obtained, indicating nearly diffraction limited beam propagation. To our best knowledge, this is the best beam quality among the reported paper results under the similar experiments.     

Narrow-wavelength-spread spectral combining laser with a reflector for a double pass with a single grating

We proposed a novel wavelength-spread compression technique for spectral beam combining of a diode laser array. A reflector, which is parallel to the grating, is introduced to achieve a double pass with a single grating.This facilitated the reduction of the wavelength spread by half and doubled the number of combined elements in the gain range of the diode laser. We achieved a power of 26.1 W under continuous wave operation using a19 element single bar with a wavelength spread of 6.3 nm, which is nearly half of the original wavelength spread of 14.2 nm, demonstrating the double-compressed spectrum capability of this structure. The spectral beam combining efficiency was 63.7%. The grating efficiency and reflector reflectance were both over 95%; hence, the efficiency loss of the double-pass grating with a reflector is acceptable. In contrast to double-grating methods,the proposed method introduces a reflector that efficiently uses the single grating and shows significant potential for a more efficient spectral beam combining of diode laser arrays.     

三种半导体阵列光谱组束结构的输出特性

基于光谱光束组合技术,利用光栅的衍射和外腔的反馈,并通过加入光束整形系统,将标准的半导体激光阵列的发光单元锁定在窄线宽的不同波长上,以近似平行光束沿组合方向输出,以实现半导体激光阵列输出光束质量的改善和线宽的压窄。实验中采用发光单元宽度100μm,周期500μm,由19个单元构成的标准阵列,分别对快、慢轴准直后光谱组束、光束整形后光谱组束和线宽压窄外腔组束进行了实验验证,实现了组合光束与单个发光单元近似的光束质量,同时得到了较窄的线宽输出,并对实验结果进行了分析。     

高功率、高效率808nm半导体激光器阵列

通过设计高效率808 nm非对称宽波导外延结构,减少P型波导层和包层的自由载流子光吸收,实现腔内光吸收损耗为0.63 cm~(-1).制备的808 nm半导体激光器阵列在室温25?C下,实现驱动电流135 A,工作电压1.76 V,连续输出功率大于150 W,斜率效率高达1.25 W/A,中心波长809.3 nm,器件最高电光转换效率为65.5%,这是目前国内报道的808 nm半导体激光器阵列的最高电光转换效率,达到国际同类器件最好水平.     

Design of an Optical Fiber Device for Pumping a Double-Clad Fiber Laser with a Laser Diode Bar

Pumping of a rare-earth-doped double-clad fiber laser with a GaAIAs laser diode bar requires imaging the near field output of a 1 cm long one-dimensional linear array of tens of multi-mode laser diode sources onto a two-dimensional oblong cross section approximately matching the fiber’s first cladding shape. This work describes the design and preliminary implementation of a device (here called a high brightness geometric transformer) that achieves this transformation with minimum brightness loss. This is done by imaging the laser diode bar near field using a fast cylindrical micro-lens onto a linear array of N (≈10) soft-glass, thin-clad rectangular fibers; in turn, the fibers’ output ends are arranged to form a stack that matches the required first cladding shape. For a typical 20W CW, laser diode bar with brightness of 25 mW·μm²·sr^-1, the geometric transformer output brightness is 0.6 mW·μm^-2·sr⁻¹, i.e., there is a ·40 intermediate loss of brightness. If the output of the geometric transformer is used to pump a Nd-doped double-clad fiber laser, an overall brightness gain of ·340 can be achieved.Presented at the International Commission of Optics Topical Meeting, Kyoto, 1994.