调Q运转外腔面发射激光器 
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Objective Optically-pumped vertical-external-cavity surface-emitting lasers (OP-VECSELs) combine advantages of vertical-cavity surface-emitting lasers (VCSELs) and solid-state disk lasers. In addition, OP-VECSELs can produce high output power and good beam quality simultaneously. VECSELs are of interest in many fields due to the tailorability and tunability of emitting wavelengths. In addition, pulsed VECSELs with high-energy and high peak power are of considerable demand in applications such as frequency conversion, fluorescence excitation, and laser medicine. Q-switched VECSEL has been reported once, but it has not been investigated specifically and extensively, and there is no published experimental or theoretical work on a Q-switched VECSEL so far. This study introduced passively Q-switched VECSELs with a Cr 4+∶YAG crystal and a semiconductor saturable absorb mirror (SESAM), respectively. Based on the time characteristics of the quantum wells in the active region of the VECSELs and the time behaviors of the saturable absorbers (the Cr 4+∶YAG crystal and the SESAM), the experimental results were analyzed, and the formation mechanisms of the microsecond pulses were proposed.
Methods The gain chip used in the VECSELs is epitaxially grown on a GaAs substrate in reverse sequences as following: an etch stop layer of AlGaAs with high Al composition, a protective layer of GaAs, an AlGaAs layer with a high barrier, an active region comprising 12 InGaAs/GaAsP quantum wells (designed to meet a target laser wavelength of 980 nm), and a distributed Bragg reflector (DBR, which is composed of 30 pairs alternate AlGaAs layers with high and low Al composition). When the grown wafer is split into small chips with 4 mm×4 mm dimension, the epitaxial end face of the chips are sequentially metalized with titanium-platinum-aurum; then, the chips are bonded to a copper heatsink, and the substrate is removed using a chemical etching. The passively Q-switched VECSEL with Cr 4+∶YAG crystal uses a linear cavity, and the Q-switching crystal is placed to the gain chip as near as possible during the experiment, while the passively Q-switched VECSEL with SESAM uses a folded cavity, and the length of the arm containing the gain chip is longer than that of the arm with SESAM to produce a tighter focusing of the light on SESAM, to satisfy the need of SESAM saturation and start the Q-switching process.
Results and Discussions When a Cr 4+∶YAG crystal is placed into the resonant cavity, a stable pulse train is obtained. Under room temperature and 4.5 W pump power, the pulse width and period are 10 and 38 μs, respectively, corresponding to a repetition rate of 26.3 kHz (Fig. 5). As SESAM is inserted into the folded cavity, a steady pulse train is produced, and the pulse width and period are 8 and 38 μs, respectibvely (Fig. 6), corresponding to a repetition rate of 26.3 kHz (the same as that in the Cr 4+∶YAG Q-switched VECSEL) under room temperature and 4.5 W pump power. The maximum average output power of the SESAM Q-switched VECSEL is 33 mW, the repetition rate is 58.1 kHz, and the single-pulse energy is 0.57 μJ when the pump power is 7.2 W (Fig. 8). The relationship between pulse periods and pump powers of the Q-switched VECSELs is different from a typical Q-switched solid-state laser. In a passively Q-switched solid-state laser, the pulse period decreases reciprocally with the increase in pumping power; however, for the passively Q-switched VECSELs, the pulse period decreases approximately exponentially (Fig. 7), and we believe this is due to the short life of the nanosecond magnitude of carriers in the active region of the VECSELs.
Conclusions We have demonstrated passively Q-switched VECSELs with Cr 4+∶YAG crystal and SESAM, respectively. The pulse widths of the Cr 4+∶YAG and SESAM Q-switched VECSEL are 10 and 8 μs, respectively, with the same repetition rate of 26.3 kHz when the pump power is 4.5 W. A maximum average output power of 33 mW is obtained from the SESAM Q-switched VECSEL with 7.2 W pump power, and the pulse repetition rate is 58.1 kHz, corresponding to a single-pulse energy of 0.57 μJ. With the increase in pump power, the pulse period decreases approximately exponentially in the Q-switched VECSELs instead of reciprocally in a typical Q-switched solid-state laser. The time characteristic of quantum wells in active region, i.e., the short life time of nanosecond magnitude of carriers, is the reason the pulse duration is of microsecond magnitude. Since the VECSELs can produce high output power and good beam quality simultaneously, these compact and wavelength tailorable passively Q-switched VECSELs have potential application in many fields when the average output power is improved and the pulse peak power is upgraded.
1 引言
调Q技术常用在固体激光器中,获得时间宽度在纳秒量级、重复频率在千赫兹量级、峰值功率在兆瓦量级的高能量脉冲,以满足科研及工业加工等领域的需要[1-2]。半导体激光器中,载流子的寿命在纳秒量级,远低于固体激光介质微秒至毫秒量级的上能级寿命,因而不能直接获得高能量的脉冲。但从另一方面看,正因为载流子的寿命很短,故在半导体激光器中,调Q脉冲可以实现高达吉赫兹的重复频率,在高速通信、高速光电采样检测等方面有着重要的应用。
不同于固体激光器中常用的电光调Q、声光调Q等主动调Q方法,以及插入可饱和吸收体等被动调Q手段,半导体激光器中的调Q一般是采用所谓的双节结构,即激光器中包含了功能不同的两节,其中一节是增益区,另一节是吸收区,或者说调制区,中间的波导部分将两节相互连接。这种调Q运转的半导体激光器可以产生时间宽度在亚纳秒量级、重复频率达吉赫兹的稳定脉冲[3-6]。如果要进一步在半导体激光器中获得皮秒及飞秒量级的超短脉冲,则需要用到另外两种在半导体激光器中产生超短脉冲的技术:增益开关技术[7]和激光锁模技术[8]。半导体激光器本身具备结构简单紧凑、性能可靠、波长设计灵活、调谐方便等诸多优点,如果再与主振荡功率放大器(MOPA)结合,调Q半导体激光器功率不足的缺点便可得到补偿,其应用范围会更加广泛。
光泵浦垂直外腔面发射激光器(OP-VECSELs)结合了垂直腔面发射激光器(VCSELs)的增益结构和固体薄片激光器的几何结构,能够同时获得高功率和高光束质量[9-12],已被广泛用于非线性频率变换[13,14]、超短脉冲产生[15,16]、太赫兹时域光谱仪[17]、生命科学[18]和医疗[19]等领域。
关于VECSEL中的调Q现象,曾有报道[20],但还没有相关工作对其进行比较全面深入的实验研究和分析讨论。本文分别用Cr4+∶YAG晶体和半导体可饱和吸收镜(SESAM)作为可饱和吸收介质,在VECSEL中实现稳定的被动调Q,并根据实验结果,结合所用增益芯片的量子结构,以及Cr4+∶YAG晶体和SESAM各自的时间特性,对VECSEL中被动调Q脉冲的形成过程进行分析讨论,试图获得VECSEL这一特定种类的半导体激光器中与被动调Q过程相关的较为清晰的物理图像。
2 实验装置
所用VECSEL增益芯片采用逆向生长的结构,在GaAs基质上首先生长高Al组分的AlGaAs刻蚀阻挡层,然后是芯片的保护层GaAs层,接下来是用于阻挡载流子的AlGaAs高势垒层,之后是多量子阱有源区。有源区包括12个InGaAs/GaAsP量子阱,InGaAs中In的含量满足设计发光波长980 nm。GaAsP既作为应变补偿层,也作为量子阱的势垒层和泵浦光的吸收层,因此,其中P的含量必须能够胜任对应变的补偿,且不会影响对泵浦光的吸收。有源区之上是30对高Al组分和低Al组分AlGaAs交替构成的分布式布拉格反射镜(DBR),中心波长980 nm,反射带宽100 nm。外延结构的最后用抗氧化的GaAs层来结束。在激光器运行过程中,增益芯片的前端界面与底部DBR之间会构成一个微腔,激光在此微腔中将形成驻波场。为了提升芯片的增益,芯片中所有外延层的厚度,以及多量子阱有源区,都必须精细地设计与生长,确保每个量子阱都置于激光驻波场的波峰处,形成所谓的谐振周期增益结构[21]。
生长好的外延片被划分为4 mm×4 mm的小块芯片,在外延结束端面分别用钛、铂、金对其进行金属化,然后键合到铜热沉上,再用化学腐蚀方法把芯片的基质层刻蚀掉。
实验所用被动调Q晶体Cr4+∶YAG的厚度为1.6 mm,该晶体在1000 nm附近存在明显的吸收,在工作波长λ=980 nm处的吸收率约为4.5%。晶体的两个通光面均对980 nm波长镀增透膜,其在980 nm处的反射率小于0.185%。
SESAM的主要技术参数如下:量子阱的吸收波长为980 nm,DBR的高反射率(R>96%)带介于910~990 nm之间。吸收镜的调制深度ΔR=1.2%,饱和吸收系数A0=2.0%,非饱和损耗Ans=0.8%,饱和通量Φsat=120 μJ/cm2,弛豫时间τ=500 fs。
Cr4+∶YAG晶体被动调Q的VECSEL实验采用简单的直腔结构,如
SESAM被动调Q的VECSEL实验采用如
图 1. 被动调Q的VECSELs实验示意图。(a)Cr4+∶YAG晶体被动调Q的VECSEL;(b)SESAM被动调Q的VECSEL
Fig. 1. Schematic of passively Q-switched VECSELs. (a) Cr4+∶YAG passively Q-switched VECSEL; (b) SESAM passively Q-switched VECSEL
3 结果与分析
3.1 连续运转VECSEL
在如
图 2. 室温下,1.2 W泵浦功率时增益芯片的荧光谱,以及5.6 W泵浦功率时VECSEL的激光光谱
Fig. 2. Photoluminescence of the gain chip under 1.2 W pump power and laser spectrum of VECSEL under 5.6 W pump power at room temperature
泵浦功率增加到5.6 W时,测得的激光波长为971 nm,光谱线宽1.1 nm。量子阱发光波长的红移效果非常明显,说明芯片有源区内温度上升较快,也就是芯片的热效应问题较为严重,这部分与我们使用高反镜作为输出镜有关。
室温下,使用曲率半径为100 mm的980 nm镀膜高反镜(激光波长的透过率约为0.1%),所得的连续运转VECSEL的输出功率与泵浦功率的关系曲线如
图 3. 室温下,连续运转VECSEL的输出功率随泵浦功率的变化关系
Fig. 3. Relationship between output power of continuously running VECSEL and pump power at room temperature
图 4. 输出功率为924 mW时,激光束的M2因子(插图为光强分布的三维图)
Fig. 4. M2 factor of laser beam when output power is 924 mW (inset is three-dimensional diagram of light intensity distribution)
3.2 Cr4+∶YAG被动调Q的VECSEL
搭建如
图 5. 室温下,泵浦功率为4.5 W时,Cr4+∶YAG被动调Q的VECSEL输出的脉冲波形
Fig. 5. Pulse waveform of Cr4+∶YAG passively Q-switched VECSEL with 4.5 W pump power at room temperature
虽然脉冲宽度在μs量级的被动调Q固体激光器[23]和光纤激光器[24]都曾有过报道,但
在典型固体激光器中,吸收晶体Cr4+∶YAG的弛豫时间为3~4 μs[25-26],而激光介质的上能级寿命在数百μs乃至ms量级,比吸收体的弛豫时间高出2~3个数量级。激光介质中反转粒子数的积累较为容易,并获得较大的初态与终态反转粒子数比值。在上能级反转粒子数超过振荡阈值,脉冲开始建立之后,吸收体被迅速漂泊,从而产生ns量级的短脉冲输出。根据相关动力学分析,固体激光器中形成稳定调Q脉冲的重复频率在kHz量级[27],调Q脉冲的周期在数十至数百μs量级。
而VECSEL中的情况则大不一样。InGaAs量子阱中载流子的寿命在ns量级[28],导带中载流子的积累过程非常缓慢,且所获得的初态载流子与终态载流子数目的比值也不高,因而载流子的释放过程同样缓慢,与之对应的则是吸收体缓慢的被漂泊的过程,综合起来使得调Q脉冲的上升及下降沿都被延长到μs的尺度,从而形成如
3.3 SESAM被动调Q的VECSEL
使用SESAM作为可饱和吸收体,搭建如
图 6. 室温下,泵浦功率为4.5 W时,SESAM被动调Q的VECSEL输出的脉冲波形
Fig. 6. Pulse waveform of the SESAM passively Q-switched VECSEL with 4.5 W pump power and room temperature
SESAM被动调Q的VECSEL输出脉冲的周期随泵浦功率的变化关系如
图 7. SESAM被动调Q的VECSEL输出脉冲的周期随泵浦功率的变化关系曲线
Fig. 7. Relationship between pulse periods of the SESAM passively Q-switched VECSEL and pump power
图 8. 室温下,SESAM被动调Q的VECSEL输出功率随泵浦功率的变化关系
Fig. 8. Output power varies with pump power of the SESAM passively Q-switched VECSEL at room temperature
4 结论
分别用Cr4+∶YAG晶体和SESAM作为吸收介质,在VECSEL中获得了稳定的调Q脉冲输出,脉冲的宽度和脉冲间隔都在μs量级。Cr4+∶YAG晶体被动调Q脉冲的宽度为10 μs,脉冲重复频率为26.3 kHz。在相同脉冲重复频率下,SESAM调Q脉冲的宽度为8 μs。泵浦功率为7.2 W时,获得SESAM被动调Q的最大输出功率为33 mW,重复频率为58.1 kHz,激光单脉冲能量为0.57 μJ。VECSEL被动调Q的输出脉冲特性主要取决于增益介质中InGaAs量子阱的载流子寿命,而受可饱和吸收体弛豫时间的影响很小。随泵浦功率的增大,调Q脉冲的重复频率近乎以指数规律趋于饱和。鉴于VECSEL能同时获得较高的输出功率和良好的光束质量,若能提高调Q运转VECSEL的输出功率,增大脉冲的能量和峰值功率,则这种结构简单紧凑、发射波长覆盖范围广、调谐方便的VECSEL脉冲激光,在非线性频率变换、荧光激发、生命科学及激光医疗等领域将存在广泛的应用。
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