利用可调激光二极管来进行光谱调谐.doc

利用可调激光二极管进行光谱调节年级：2011专业：光学工程学号：G111201008姓名：宦君工作单位：理波光电科技（无锡）有限公司摘要：Frequency-modulation spectroscopy is a powerful tool that can achieve high sensitivities with a relatively simple experimental setup. Tunable diode lasers, in particular, can make the setup even simpler because they provide a narrow, tunable output that can be easily modulated. As new diode laser wavelengths become available the field of TDLAS (tunable diode laser absorption spectroscopy) will continue to grow rapidly, particularly when combined with nonlinear optical techniques that allow conversion of available wavelengths to the blue,20 UV,21, 22 or far infrared.8调频光谱是一种强大的工具,可以用一个相对简单的实验装置获得高敏感性。可调谐二极管激光、特别是,可以使它的步骤更为简单,因为它们可以提供一个狭小的、可调的输出,并且可以很容易地调制。随着新型二极管激光波长变得可用TDLAS的领域(可调谐二极管激光吸收光谱)将继续快速增长,尤其是当结合非线性光学技术,允许转换可用的波长蓝、20、21、22紫外线或远红外线8引言Tunable diode lasers are ideal for optical spectroscopy because of their narrow linewidths, large tuning ranges and stable outputs. Because they are more compact and rugged than traditional spectroscopic optical sources, 可调谐激光二极管是理想的用于光谱学的一种器件，因为他们有着较窄的线宽，较大的调节范围和比较稳定的输出。因为相比较传统的光谱光源有着更小的体积并且更坚固。like Ti:Sapphire lasers, dye lasers, color-center lasers, hollow-cathode lamps, and nonlinear systems (e.g. optical parametric oscillators), they have enabled spectroscopic methods to be used not only in laboratory environments but also in the real world. Applications of diode-laser spectroscopy include remote sensing, LIDAR, laser cooling and trapping of atoms,1 frequency standards,2 length standards,3 trace gas detection,4 and process monitoring.5像钛宝石激光器,染料激光器、激光、空心阴极灯色中心,和非线性系统(如光学参量振荡器),他们使光谱方法用于不仅在实验室环境的同时也在现实世界。应用程序的二极管激光光谱学包括遥感、激光雷达、激光冷却和俘获原子频率标准,1,2,3长度标准微量气体检测、4和过程监控5They can be used to monitor environmentally important species, such as methane, carbon dioxide, and water. In semiconductor manufacturing, they can be used for closed-loop control of deposition processes, including electron-beam, sputtering, molecular-beam epitaxy (MBE), and thermal evaporation, resulting in a significant increase in the yield of existing devices and making possible new and improved ones. Indeed the field has grown so large that a review of the research journals reveals that it has developed its own acronym, TDLAS, which stands for Tunable Diode Laser Absorption Spectroscopy. 它们可以用来监控环境重要的物种,如甲烷、二氧化碳和水。在半导体制造,它们可以用于闭环控制沉积过程,包括电子束、溅射、分子束外延(MBE)和热蒸发,导致产量显著增加现有的设备和制造新的和改进的可能。事实上应用领域变得如此巨大,回顾研究期刊,这表明它已经开发了自己的首字母缩略词,TDLAS,代表可调谐二极管激光吸收光谱。There is a wide variety of laser-spectroscopic techniques available to researchers.6 In this application note, we will focus on one particular technique, frequency-modulation spectroscopy, or FMS. FMS is a powerful technique that can achieve a high signal-to-noise ratio with a relatively simple experimental setup. In a typical FMS experiment, the wavelength of a continuous-wave laser is modulated at a particular frequency. As the center wavelength is scanned across the atomic transition, the wavelength modulation is converted into amplitude modulation, giving rise to a modulation in the optical absorption of a sample at the same frequency. (See Figure 1 for a typical absorption line.) 研究人员有各种各样的激光光谱技术可用来研究。6在本文中,我们将关注一个特定的技术,调频光谱学,或FMS。FMS是一个功能强大的技术,可以实现高信噪比与一个相对简单的实验装置。在一个典型的FMS实验,连续波激光的波长调制在特定频率。为中心波长扫描整个原子跃迁,波长调制转换成调幅,逐步形成一个调制的光学吸收的一个示例相同的频率。(请参见图1,一个典型的吸收线。) Figure 1: A typical absorption line for rubidium, showing transmitted intensity (IT) as a function of the laser frequency (v). 图1:一个典型的吸收线对铷,显示射线强度(它)是一个函数的激光频率(v)。Narrow-band demodulation techniques, such as phase-sensitive detection using a lock-in amplifier, then allow the absorption information to be realized at DC. Because the signal has been moved to a high frequency via modulation, FMS avoids the typical limitations of absorption measurements such as laser-intensity fluctuations, which peak at DC and fall off roughly as 1/f, hence the name 1/f noise. Using this technique, absorption sensitivities can reach the part per million (ppm) level. For example, H2S has been detected at the ppm level in air,7 absorption of yttrium has been measured at the ppm level,5 and methane has been detected with a precision of 1ppb.8窄带解调技术,如相敏检波使用锁定放大器,然后允许吸收信息意识到在直流。因为信号已经搬到一个高频率通过调制、FMS避免了典型的局限性吸收测量如激光强度的波动,它最高直流和脱落大约为1 / f,因此得名1 / f噪声。使用这种技术,吸收敏感性达到百分率(ppm)水平。例如,H2S已经测到ppm水平在空气中,7吸收钇一直按ppm水平,5和甲烷已经被检测出一个精密的1 ppb 8As we will discuss later in this application note, further enhancements of the signal can be achieved by taking advantage of experimental geometries that cancel particular noise sources or undesirable aspects of the signal. Finally, if a reasonable level of attention is given to the optics and electronics, systematic errors can be suppressed and high accuracy can be achieved with FMS.9 This is important, for instance, when trying to determine the exact center of an absorption line. For many applications the center must be found to better than 0.1% of the linewidth. 正如我们将在本文后面讨论到,进一步增强的信号可以通过利用实验几何图形,取消特定的噪声来源或不良方面的信号。最后,如果一个合理水平的关注给了光学和电子产品、系统误差可以抑制和高精确度可以达到与FMS。9这是重要的,例如,当试图确定确切的中心的吸收线。对于许多应用程序必须发现中心比0.1%的线宽。FMS is particularly well suited to diode lasers such as the New Focus TLB-7000 Series, TLB-6000 Series and TLB-6300 Series external-cavity diode lasers because of their simple, yet powerful modulation capabilities. The main methods for achieving wavelength modulation (shown with the maximum modulation rates for the TLB-6900 Series lasers) include mechanically “dithering” the mirror (1.5 to 3.5 kHz), modulating the injection current into the diode (100 MHz), and using external phase modulators (10 GHz). FMS尤其适合二极管激光如新福克斯tlb - 7000系列、6000系列和tlb tlb - - 6300系列外腔二极管激光器由于其简单而有力的调制功能。主要的方法来实现波长调制(如图所示的最大调制利率tlb - 6900系列激光)包括机械“抖动”镜像(1.5至3.5千赫),调节注入电流进入二极管(100 MHz),并使用外部阶段调节器(10 GHz)。FMS can be broken down into two regimes: wavelength modulation (WM) and frequency modulation (FM). In the case of WM, the modulation depth is very large, generating a large number of sidebands, but the modulation frequency is low (100 MHz). We will discuss FM in this application note, but the physics of WM and FM are nearly identical.10 The methods discussed here have been used for atomic spectroscopy of Cs, Rb, K, Li, Ne, Pb, Tl, Y, Sr, and Ca. In addition, there are also numerous examples of diode lasers being used in molecular spectroscopy utilizing O2, I2, CO2, NO2, H2O, H2S, CH3, CH4 C2H2, CH3CH2OH, and others. Many applications, such as interferometry and frequency stabilization of lasers,11 benefit from virtually identical techniques. The narrow band transmission of a Fabry-Perot interferometer is quite similar to a narrow absorption line12 (Figure 2), so much of the discussion in this applications note is directly relevant to such a setup. FMS可以分解成两个调制:波长调制(WM)和频率调制(FM)。对于WM、调制深度是非常大的,产生大量的sidebands,但调制频率很低( 100 MHz)。在本文中我们将讨论FM,但物理学的WM和FM几乎都是相同的。这里讨论的方法10已经使用了原子光谱学的Cs,Rb、钾、锂、氖、铅、钛,Y,Sr和Ca。此外,也有大量的例子证明二极管激光器被用于分子光谱学利用O2、I2二氧化碳、NO2、水、H2S,CH3,CH4 C2H2,酒精,和其他人。许多应用程序,比如干涉法和频率稳定的激光,11受益于几乎相同的技术。这个窄带传输的法布里-珀罗干涉仪非常类似于一个狭窄的吸收line12(图2),讨论了太多的在这个应用程序注意直接关系到这样一个设置。This application note is broken into two main parts. The first part explains essential elements of the physics behind FMS and certain details about the signals that are obtained. Then a simple FMS setup utilizing intensity noise cancellation is described and analyzed. The second part concerns itself with FM “saturation” spectroscopy. Saturation spectroscopy uses a setup similar to the first part, but gives much narrower signals that are free from Doppler broadening (typically the dominant source of line broadening). A short discussion of the physics of saturation spectroscopy is included. This note concludes with a reality checka brief discussion of the most common experimental difficulties encountered. 本文将被分解为两个主要部分。第一部分解释了基本要素的物理现象FMS和某些细节信号获得的。然后一个简单的FMS设置利用强度噪音消除的论述和分析。第二部分关注的FM“饱和”光谱学。饱和光谱学使用一个设置类似于第一部分,但给更窄的信号,是免费的从多普勒展宽(通常的主要来源谱线增宽)。简单讨论了物理的饱和光谱学包含。这个注意结尾是一个现实检查一个简短的讨论中最常见的实验时总是遇到麻烦。The Physics of FM Spectroscopy物理学的FM光谱学Figure 2: The intensity versus frequency of a laser beam transmitted through a glass cell containing an atomic or molecular vapor (top) and of a laser beam transmitted through a Fabry-Perot interferometer (bottom). 图2:强度和频率的激光光束传输通过玻璃细胞包含一个原子或分子蒸气(顶部)和激光光束的传输通过法布里-珀罗干涉仪(底部)。Figure 2 shows a laser beam transmitted through a gas cell containing a resonantly absorbing atomic or molecular vapor and a laser beam transmitted through a Fabry-Perot interferometer. For the interferometer, the transmitted intensity (measured with a photodiode) has a narrow, peaked response centered around some frequency (or wavelength), while the absorption will display a narrow dip in the response around the central absorption frequency. Either situation could be analyzed here, but for the sake of clarity lets look at the details of the absorption. 图2显示了一个激光光束传输通过气体细胞包含一个共鸣地吸收分子或原子蒸汽和一束激光传输通过法布里-珀罗干涉仪。对于干涉仪,传播强度(测量光电二极管)有一个狭窄的,达到峰值响应围绕一些频率(或波长),而吸收将显示一个狭窄的蘸响应围绕着中央吸收频率。这两种情况下可以分析在这里,但是为了清楚让我们看看细节的吸收。The central aspect of FMS is the modulation of the laser frequency and its effect on the intensity of light transmitted by the gas cell. Consider the situation where the frequency at which the laser is modulated is fairly low and the amplitude of that modulation is small. In this case the central laser frequency can be thought of as periodically increasing and decreasing by a small amount. Such a situation is shown in Figure 3a where the laser linewidth is assumed to be much smaller than the width of the absorption. If the laser frequency is in the vicinity of an absorption line then the frequency modulation causes the absorption to modulate synchronously. In this way, the laser frequency modulation is mapped onto the lasers transmitted intensity. Another way to state this is that the frequency modulation on the laser has been converted into an amplitude modulation by the absorption: FM has become AM. This change allows the photodiode to detect the modulation, since the photodiode cannot detect frequency changes. Figure 3a also demonstrates the phase relationship between the lasers FM and the absorptions AM. A final point concerning Figure 3a is that the conversion of FM to AM at a particular frequency depends on the slope (or derivative) of the absorption at that frequency. This relationship between FMS and the slope will be formalized on the next page. 中央的方面是调制的FMS激光频率及其作用强度的光通过气体细胞。考虑下面这种情况:这个频率的激光调制是相当低的,振幅调制的小。在这种情况下,中央激光频率可以被认为是定期增加和减少由少量。这一局面是图3所示一个假定的激光线宽远小于宽度的吸收。如果激光频率在附近的一个吸收线那么频率调制使吸收调节同步。通过这种方式,激光频率调制是映射到激光射线强度。另一种状态,频率调制激光已被改造成一个振幅调制的吸收:FM已经成为我。这种变化使得光电二极管检测调制,因为光电二极管不能检测频率的变化。图3 a还演示了之间的相位关系的激光的调频和吸收的点。最后一点关于图3 A是,转换调频到是在特定的频率取决于坡(或衍生品)的吸收在那个频率。这种关系和边坡FMS将正式在下一页。Figure 3: In frequency modulation spectroscopy, as the wavelength is scanned across the atomic transition, the wavelength modulation is converted into amplitude modulation, giving rise to a modulation in the optical absorption of a sample at the same frequency. As you continue to scan across the absorption profile, as is done in Figures 3b and 3c, you can see that the amount of FM to AM conversion varies. Figure 3d is a plot of the ratio of AM to FM versus the laser frequency, taking phases into account. 图3:在频率调制光谱波长扫描整个原子跃迁,波长调制转换成调幅,逐步形成一个调制的光学吸收的一个示例相同的频率。当你继续扫描整个吸收概要信息,就像在图3 b和3 c,你可以看到数量的调频到正在转换不同。图3 d是一个阴谋的比率是到FM和激光频率,以阶段考虑在内。Qualitatively, it is useful to continue the scan across the absorption profile, as shown in Figures 3b and 3c. Near the point of maximum absorption, the conversion of FM to AM is very small, and in fact goes to zero at line center. On the other side of the absorption peak it can be seen that the FM to AM conversion is large again, but the phase relationship between the FM and AM has reversed. Finally, if the scan is continued away from the absorption, the AM goes away because of the lack of absorption and near-zero slope. In this way, one could plot the ratio of AM to FM versus the laser frequency and obtain the curve shown in Figure 3d, where the change in phase has been represented by a sign change. This curve looks suspiciously like the derivative of the absorption, and this is indeed the case. 定性地说,它是有用的继续扫描整个吸收概要,如图3 b和3 c。点附近最大吸收,转换成调频到我是非常小的,事实上在线中心变为零。在另一边的吸收峰可以看到,调频到正在转换是大了,但之间的相位关系的调频和我扭转了。最后,如果扫描是继续远离吸收,正在消失,因为缺乏吸收和接近于零的斜坡。通过这种方式,可以绘制比点到FM和激光频率和获得曲线如图3 d,那里的变化在阶段被表示为一个信号变化。这个曲线疑似的导数吸收,这的确是事实。If the laser frequency v is modulated at the frequency (sometimes called “dithering”), with modulation amplitude m, then the transmitted intensity IT through the vapor cell can be written如果激光频率v是调制的频率(有时称为“抖动”),与调制振幅米,然后传播的强度,它通过蒸汽细胞可以写IT(n)=IT(n + msint)Here we take m, G, where G is the linewidth of the absorption. Now we can expand IT as a Taylor series. 在这里我们把米, G,G是线宽的吸收。现在我们可以扩大它作为一个泰勒级数和结合条件and combine termsSo, the transmitted intensity contains a DC term, a term oscillating at , a term oscillating at 2, and so on. If phase-sensitive detection is performed at , for instance using a lock-in amplifier, the coefficient of the sint term can be extracted. In particular, since we have assumed that m is small, the coefficient of the sint term is essentially m multiplied by the first derivative of the transmitted intensity (or absorption). In an analogous way, detection at 2 reveals the second derivative, 3 the third derivative, and so on. For this reason, FMS is sometimes called “derivative” spectroscopy.所以,传播强度包含一个直流术语,一个学期,一个学期在振动摆动在2等等。如果在执行相敏检波,例如使用锁定放大器,系数的sint术语可以提取。特别是,因为我们已经假设m很小,系数的sint术语本质上是m乘以导数的传播强度(或吸收)。在一个类似的方法,检测在2揭示了二次导数,3第三导数,等等。出于这个原因,FMS有时被称为“衍生品”光谱学。A Simple FM Spectroscopy Experiment一个简单的FM光谱实验It is easy to improve the basic FMS setup described above by reducing the noise that arises from laser-intensity fluctuationstypically the largest source of noise. Basically, the experiment above is repeated, but a second laser beam is derived from the same laser which is not passed through the vapor cell (Figure 4). The beam that passes through the cell performs the FM to AM conversion, and possesses intensity fluctuations. The second beam also has laser-intensity fluctuations but no induced AM. If the absolute intensities of the two beams after the vapor cell are balanced so they have the same power (and this procedure should be performed away from the absorption feature), and one measures the difference between the two photocurrents, then the laser intensity fluctuations will exactly cancel. 它很容易提高FMS上面描述的基本设置降低噪音,来自激光强度波动通常的最大来源噪声。基本上,这个实验重复以上,但第二束激光源自相同的激光而不是通过蒸汽细胞(图4)。光束通过细胞执行调频到正在转换,具有强度波动。第二束激光强度也有波动,但没有诱导点。如果绝对强度的两束在蒸汽细胞是平衡的,所以他们有同样的权力(和这个过程应该执行远离吸收特性),和一个措施两者的区别photocurrents,那么激光强度波动将完全取消。Such a differential measurement is easily accomplished using the New Focus Model 20X7 auto-balanced photoreceiver. The Model 20X7s patented auto-balancing circuit uses a low-frequency feedback loop to automatically maintain DC balance between the signal and reference arms. In effect the circuit behaves as a variable-gain beamsplitter, so you wont have to manually balance the power in the two beams. This circuit, in conjunction with the subtraction node, cancels common-mode laser noise with greater than 50-dB rejection at frequencies less than 125 kHz. Combining this auto-balancing detector with phase-sensitive detection (a lock-in amplifier) allows you to obtain the derivative of the absorption signal free from intensity noise and with other noise sources suppressed due to the modulation.这样一个微分测量轻松完成使用新的焦点模型20 x7汽车平衡光接收器。该模型20 x7的专利自动平衡电路使用低频反馈回路的自动维护直流之间的平衡信号和参考武器。实际上,电路就作为一个可变增益分光镜,所以不需要手动平衡力量在两束。这个电路,结合减法节点,取消共模激光器噪声与大于50分贝拒绝在频率低于125 kHz。结合这汽车平衡探测器与相敏检波(一个锁定放大器)允许您获得的导数吸收信号强度噪声和自由与其他噪声来源抑制由于调制。You can perform this experiment very easily with the New Focus TLB-7000 or TLB-6900 laser. This lasers external-cavity design creates a narrow-linewidth output which can be scanned across by many tens of GHz across an absorption feature. A BNC input to a piezo-electric transducer allows you to modulate the laser frequency by up to 3.5 kHz. All New Focus tunable lasers are also guaranteed to be mode-hop free over their entire tuning ranges and can be built to almost any desired wavelength range covered by existing semiconductor diode lasers.您可以执行这个实验很轻易与新福克斯tlb - 7000或tlb - 6900激光。这种激光的外腔设计创建了一个窄线宽的输出,可以扫描在许多数以GHz整个吸收特性。一个BNC输入到一个压电换能器允许您调整激光频率高达3.5千赫。所有的新焦点可调谐激光器也保证模式跳自由整个调谐范围和建立成能够几乎任何想要的波长范围覆盖现有的半导体二极管激光器。The Physics of Saturation Spectroscopy物理学的饱和光谱学As in typical FM spectroscopy, FM saturation spectroscopy measures the intensity of a laser beam passed through a gas cell. The important difference is that, in saturation spectroscopy, a strong laser beam (typically at the same frequency) is sent through the vapor cell counter-propagating (or with a very small angle) to and overlapped with the original beam passing through the cell (Figure 5). This results in significantly narrowing the widths of absorption lines, as will be shown. 在典型的FM光谱学,FM饱和光谱学措施的强度激光束通过气体细胞。最重要的区别是,在饱和光谱学,强大的激光光束(通常是在相同的频率)被送到蒸汽细胞计数传播(或者一个很小的角度)和重叠与原梁通过细胞(图5)。这导致显著缩小宽度的吸收线,将显示。Saturation spectroscopy essentially eliminates the Doppler broadening of absorption lines, which is usually the largest source of line broadening. For many applications, it is very important to realize a narrow absorption width near the “natural” (or quantum-limited) width so that line center can be determined with high accuracy. Once the Doppler broadening is removed, other interesting effectssuch as the hyperfine structurebecome resolved. It also allows the study of much smaller sources of line broadening, which will reveal more subtle physics (collisional shifts, time-of-flight broadening, magnetic-field effects, photon-recoil effects, and pressure broadening are just a few).Doppler broadening arises because atoms or molecules are in