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余文志 0811122121 理 学 院 2008 级光信 1 班 成纯富 2012 年 4 月 20 日

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1.5 Experimental Setup Due to the many concepts and variations involved in performing the experiments in this project and also because of their introductory nature, Project 1 will very likely be the most time consuming project in this kit. This project may require as much as 9 hours to complete. We recommend that you perform the experiments in two or more laboratory sessions. For example, power and astigmatic distance characteristics may be examined in the first session and the last two experiments (frequency and amplitude characteristics) may be performed in the second session. A Note of Caution All of the above comments refer to single-mode operation of the laser which is a very fragile device with respect to reflections and operating point. One must ensure that before performing measurements the laser is indeed operating single-mode. This can be realized if a single, broad fringe pattern is obtained or equivalently a good sinusoidal output is obtained from the Michelson interferometer as the path imbalance is scanned. If this is not the case, the laser is probably operating multimode and its current should be adjusted. If single-mode operation cannot be achieved by adjusting the current, then reflections may be driving the laser multimode, in which case the setup should be adjusted to minimize reflections. If still not operating single-mode, the laser diode may have been damaged and may need to be replaced. Warning The lasers provided in this project kit emit invisible radiation that can damage the human eye. It is essential that you avoid direct eye exposure to the laser beam. We recommend the use of protective eyewear designed for use at the laser wavelength of 780 nm. Read the Safety sections in the Laser Diode Driver Operating Manual and in the laser diode section of Component Handling and Assembly (Appendix A) before proceeding. 1.5.1 Semiconductor Diode Laser Power Characteristics 1. Assemble the laser mount assembly (LMA-I) and connect the laser to its power supply. We will first collimate the light beam. Connect the laser beam to a video monitor and image the laser beam on a white sheet of paper held about two to ten centimeters from the laser assembly. Slowly increase the drive current to the laser and observe the spot on the white card. The threshold drive current rating of the laser is


supplied with each laser. Increase the current to about 10-20 mA over the threshold value. With the infrared imager or infrared sensor card, observe the spot on the card and adjust the collimator lens position in the laser assembly LMA-I to obtain a bright spot on the card. Move the card to about 30 to 60 centimeters from the lens and adjust the lens position relative to the laser to obtain a spot where size does not vary strongly with the position of the white card. When the spot size remains roughly constant as the card is moved closer or further from the laser, the output can be considered collimated. Alternatively, the laser beam may be collimated by focusing it at a distance as far away as possible. Protect fellow co-workers from accidental exposure to the laser beam. 2. Place an 818-SL detector on a post mount (assembly M818) and adjust its position so that its active area is in the center of the beam. There should be adequate optical power falling on the detector to get a strong signal. Connect the photodetector to the power meter (815). Reduce the background lighting (room lights) so that the signal being detected is only from the laser. Reduce the drive current to a few milliamperes below threshold and, again, check to see that room light is not the dominant signal at the detector by blocking the laser light. 3. Increase the current and record the output of the detector as a function of laser drive current. You should obtain a curve similar to Figure 1.2. If desired, the diode temperature may also be varied to observe the effects of temperature on threshold current. When examining laser diode temperature characteristics, the laser diode driver should be operated in the constant current mode as a safeguard against excessive currents that damage the diode laser. Note that as the diode temperature is reduced, the threshold decreases. Start all measurements with the diode current off to prevent damage to the laser by preventing drive currents too high above threshold. To prevent destruction of the laser, do not exceed the stated maximum drive current of the laser.


1.5.2 Astigmatic Distance Characteristics The laser diode astigmatic distance is determined as follows. A lens is used to focus the laser beam at a convenient distance. A razor blade is, then, incrementally moved across the beam to obtain data for total optical power passing the razor edge vs. the razor blade position. A plot of this data produces an integrated power profile of the laser beam (Figure 1.9a) which through differentiation exposes the actual power profile (Figure 1.9b) which, in turn, permits determination of the beam diameter (W). A beam diameter profile is obtained by measuring the beam diameter while varying the laser position. Figure 1.9c illustrates the two beam diameter profiles of interest: one for razor edge travel in the direction perpendicular to the laser diode junction plane and the other for travel in the direction parallel to the junction plane. The astigmatic distance for a laser diode is the displacement between the minima of these two profiles. This method is known as the knife edge technique.

1. Assemble the components shown in Figure 1.8 with the collimator lens (LC), in the rotational stage assembly (RSA-I), placed roughly 1 centimeter away from the laser. The beam should travel along the optic axis of the lens. This is the same lens used in collimating the laser in the previous setup. The approximate placement of all the components are shown in the figure. Make sure that the plane of the diode junction (xz plane in Figure 1.1) is parallel with the table surface. 2. Due to the asymmetric divergence of the light, the cross-section of the beam leaving the laser and, further, past the spherical lens is elliptical. The beam, thus, has two distinct focal points, one in the plane parallel and the other in the plane perpendicular to the laser diode junction. There is a point between the two focal points where the beam cross-section is circular. With the infrared imager and a white card, roughly determine the position where the beam cross-section is circular.


Figure 1.9 – Procedure for finding astigmatic distance. 3. Adjust the laser diode to lens distance such that the razor blades are located in the xy plane where the beam cross-section is circular. 4. Move the laser diode away from the lens until minimum beam waist is reached at the plane of razor blades. Now, move the laser diode about 200 ?m further away from the lens. 5. Move razor blade 1 in the x direction across the beam through the beam spread θx and record the x position and detected intensity at each increment (≤100 ?m increments). The expected output is shown in Figure 1.9. The derivative of this curve yields the intensity profile of the beam in the x direction from which the beam diameter is determined. 6. Repeat with razor blade 2 for θy in the y direction. 7. Move the laser closer to the lens in increments (≤50 ?m) through a total of at least than 500? Repeat Steps 5 and 6 at each z increment, recording the z position. m. 8. Using the collected data, determine the beam intensity profiles in the x and y directions as a function of the lens position z. This is done by differentiating each data set with respect to position. Then, calculate the beam diameter and plot as a function of z. The difference in z for the minimum in θx and θy is the astigmatic distance of the laser diode. Use of computer software, especially in differentiating the data, is highly


recommended. If the laser junction is not parallel to the table surface, then for each measurement above, you will obtain an admixture of the two beam divergences and the measurement will become imprecise. If the laser is oriented at 45°to the surface of the table, the astigmatic distance will be zero. Different laser structures will have different angular beam divergences and, thus, different astigmatic distances. If you have access to several different laser types (gain guided, index guided), it may be instructive to characterize their astigmatic distances. 1.5.3 Frequency Characteristics of Diode Lasers In order to study frequency characteristics of a diode laser, we will employ a Michelson interferometer to convert frequency variations into intensity variations. An experimental setup for examining frequency and, also, amplitude characteristics of a laser source is illustrated in Figure 1.10. 1. In this experiment, it is very possible that light may be coupled back into the laser, thereby, destabilizing it. An optical isolator, therefore, will be required to minimize feedback into the laser. A simple isolator will be constructed using a polarizing beam splitter cube and a quarterwave plate. We orient the quarterwave plate such that the linearly polarized light from the polarizer is incident at 45°to the principal axes of the quarterwave plate so that light emerging from the quarterwave plate is circularly polarized. Reflections change left-circular polarized light into right-circular or vice versa so that reflected light returning through the quarterwave plate will be linearly polarized and 90°rotated with respect to the polarizer transmission axis. The polarizer, then, greatly attenuates the return beam. In assembling the isolator, make sure that the laser junction (xz plane in Figure 1.1) is parallel to the surface of the table (the notch on the laser diode case points upward) and the beam is collimated by the lens. The laser beam should be parallel to the surface of the optical table. Set the polarizer and quarterwave (λ/4) plate in place. Place a mirror after the λ/4 plate and rotate the λ/4 plate so that maximum rejected signal is obtained from the rejection port of the polarizing beam splitter cube as shown in Figure 1.11. When this signal is maximized, the feedback to the laser should be at a minimum. 2. Construct the Michelson interferometer as shown in Figure 1.12. Place the beam steering assembly (BSA-II) on the optical table and use the reflected beam from the mirror to adjust the quarterwave plate orientation. Set the cube mount (CM) on the optical breadboard, place a double sided piece of adhesive tape on the mount, and put


the nonpolarizing beam splitter cube (05BC16NP.6) on the adhesive tape. Next, place the other beam steering assembly (BSA-I) and the detector mount (M818BB) in location and adjust the mirrors so that the beams reflected from the two mirrors overlap at the detector.


When long path length measurements are made, the interferometer signal will decrease or disappear if the laser coherence length is less than the two way interferometer path imbalance. If this is the case, shorten the interferometer until the signal reappears. If this does not work, then check the laser for single-mode operation by looking for the fringe pattern on a card or by scanning the piezoelectric transducer block (PZB)in BSA-II and monitoring the detector output which should be sinusoidal with PZB scan distance. If the laser does not appear to be operating single-mode, realign the isolator and/or change the laser operating point by varying the bias current. Additionally, to ensure single-mode operation, the laser should be DC biased above threshold before applying AC modulation. Overdriving the laser can also force it into multimode operation.


3. The Michelson interferometer has the property that depending on the position of the mirrors, light may strongly couple back toward the laser input port. In order to further reduce the feed-back, slightly tilt the mirrors as illustrated in Figure 1.13. If still unable to obtain single-mode operation, replace the laser diode.

4. Place a white card in front of the detector and observe the fringe pattern with the infrared imager. Slightly adjust the mirrors to obtain the best fringe pattern. Try to obtain one broad fringe. 5. Position the detector at the center of the fringe pattern so that it intercepts no more than a portion of the centered peak. 6. By applying a voltage to the piezoelectric transducer block attached to the mirror (part PZB) in one arm of the interferometer (i.e. BSA-II), maximize the output intensity. The output should be stable over a time period of a minute or so. If it is not, verify that all components are rigidly mounted. If they are, then room air currents may be destabilizing the setup. In this case, place a box (cardboard will do) over the setup to prevent air currents from disturbing the interferometer setup.


7. Place the interferometer in quadrature (point of maximum sensitivity between maximum and minimum outputs of the interferometer) by varying the voltage on the PZB. 8. The output signal of the interferometer due to frequency shifting of the laser is given by ?I∝ = 2π/c ?L ?ν where ?L is the difference in path length between the ?φ two arms of the interferometer and ?ν is the frequency sweep of the laser that is induced by applying a current modulation. Remember that in a Michelson interferometer ?L is twice the physical difference in length between the arms since light traverses this length difference in both directions. ?L values of 3-20 cm represent convenient length differences with the larger ?L yielding higher output signals. Before we apply a current modulation to the laser, note that the interferometer output signal, ?I, should be made larger than the detector or laser noise levels by proper choice of ?L and current modulation amplitude di. Also recall from Section 1.3 that when the diode current is modulated so is the laser intensity as well as its frequency. We can measure the laser intensity modulation by blocking one arm of the interferometer. This eliminates interference and enables measurement of the intensity modulation depth. We, then, subtract this value from the total interferometer output to determine the true dI/di due to frequency modulation. Apply a low frequency, small current modulation to the laser diode. Note that when the proper range is being observed 1 dv ? 10 ?5 mA ?1 v di and 1 dI ? 0.2mA ?1 I di for the amplitude change only.Recalling
c dI dI d(??) 2? ?v ~ 10 ?5 mA ?1 , ? ? ?L , di di c ?i 2??Lv di

or dI ?L ~ 2K? mA ?1 -5 di ?10 where K is a detector response constant determined by varying ?L.


9. With the interferometer and detection system properly adjusted, vary the drive frequency of the laser and obtain the frequency response of the laser (Figure 1.4 or 1.10a).You will need to record two sets of data: (i) the modulation depth of the interferometer output as a function of frequency, and (ii) the laser intensity modulation depth. The difference of the two sets of collected data will provide an estimate of the actual dI/di due to frequency modulation. Also note that if the current modulation is sufficiently small and the path mismatch sufficiently large, the laser intensity modulation may be negligible. You may need to actively keep the interferometer in quadrature by adjusting the PZB voltage. Make any necessary function generator amplitude adjustments to keep the current modulation depth of the laser constant as you vary the frequency. This is because the function generator/driver combination may not have a flat frequency response. The effect of this is that the current modulation depth di is not constant and varies with frequency. So to avoid unnecessary calculations, monitor the current modulation depth by connecting the LASER MONITOR connector on the laser diode driver system to an oscilloscope and keep the modulation depth constant by adjusting the amplitude of the applied sinusoidal wave as a function of frequency. Record the frequency for your laser at which the thermal contribution to dν/di begins to become negligible and dν/di drops off (see Section 1.3). 10. Keeping the above equations in mind, we will, now, measure the FM chirp characteristics of the laser. At a constant current modulation frequency (choose a modulation frequency where dν/di varies rapidly, i.e. where the slope of your graph from Step 9, which should be similar to Figure 1.10a, is maximum), vary the current modulation depth di for different laser bias levels and derive a curve such as the one in Figure1.10b.The output dν should not vary significantly except around threshold and at high currents. Caution

Do not exceed the specified drive currents/output power ratings of the diode or it may be damaged. 11. The phase noise characteristic behavior (Section1.4) as a function of interferometer path length imbalance ?L may be determined by inducing phase noise through application of laser current modulation. Make sure that the interferometer is in quadrature. Set the laser diode current above threshold, apply a small current modulation, and fix the modulation frequency at a desired value. Convenient frequencies may include 50 Hz, 2 kHz, and 50 kHz (see Reference 1.5). Monitor the detector output with a spectrum analyzer or an oscilloscope and record the peak-to-peak output intensity at interferometer quadrature. You may accomplish this by manually sweeping the PZB voltage to cause a minimum of π/2 phase shift, recording the maximum peak-to-peak intensity as a function of path length imbalance. It is important to ensure that instrument noise is below the signal levels expected and it is assumed that single-mode operation of the laser is maintained. Curves similar to Figure 1.10c should be obtained. 1.5.4 Amplitude Characteristics of Diode Lasers The measurements of the intensity characteristics are taken by placing the detector before the interferometer as in Figure 1.10 or by blocking one mirror in the interferometer. Again, the laser must be operated single-moded with minimum feedback or the noise level and functionality will drastically change. The relative intensity noise (RIN) is defined as 20log(dI/I) where dI is the RMS intensity fluctuations so that for dI~10-4 , the RIN is -80 dB. Normally, these measurements are made with a spectrum analyzer and a 1 Hz bandwidth. When making RIN measurements, electronic and photodetector shot noise must be below the RIN levels. (OPTIONAL) You may determine the shot noise using an incoherent source (e.g. lamp) with an intensity level similar to that of the laser. The resultant frequency spectrum of noise with the light source excited gives a measure of the shot noise level which should be adjusted to be at least 10 dB greater than electronic noise levels. The measured shot noise should be checked with Equation 0.47. 1. Vary the laser drive current from below threshold through and above the threshold and record the laser output power and intensity noise at a desired frequency using a spectrum analyzer. When you calculate the RIN, assuming that shot and electronic noises are below the RIN level, a plot similar to that presented in Figure 1.10d should


be obtained. In most cases, for single-mode operation, the noise peaks at threshold. The shape of the noise curve may vary if the laser is modulated, if it becomes multi-modal, or if the side-mode suppression on a nominally single-mode laser is not adequate (< 20 dB). 2. It is instructive to operate the laser with modulation signals of varying depth and/or degrading the isolator performance by rotating the λ/4 plate to increase feedback to the laser. This will illustrate noise properties for various feedback conditions which are important to subsequent sensor and communication experiments. RINs of less than -150dB and -120dB are required for television broadcast signals and sensitive interferometric sensors, respectively. 3. The intensity noise of diode lasers has a 1/f characteristic (performance is degraded as the frequency is lowered). With the laser above threshold and the photodetector connected to a spectrum analyzer, determine the RIN as a function of modulation frequency. The response shown in Figure 1.10e should be obtained where the noise becomes white (flat with frequency) starting somewhere between 100 kHz and 1 MHz for typical lasers. NOTE: The Michelson interferometer setup used in this project will again be used in Project3. It may, therefore, save time to proceed directly to Project3 before completing characterization of diode lasers in Project2.


1.5 实验装置 由于在这个项目中执行这个实验时涉及到许多新的概念和变化, 也因为它们 是初始性的工作, 项目1可能是这个实验单元中最耗时的项目。 这一项目可能需 要9小时才能完成。因此我们建议用两个或两个以上的实验课时来做此实验。例 如, 功率和发散特性可以在第一次实验中研究, 而后两个特性 (频率和振幅特性) 可以在第二次试验中研究。 注意事项 以上所有的评论说明, 就反射和工作点调节而言激光器单纵模输出装置是十 分脆弱的。首先必须保证在测量之前,激光器确实运行于单模态。如果获得一个 单一的, 清晰的干涉图样或者进行差分扫描时从迈克逊干涉仪获得一个好的正弦 曲线,则可以认为其工作于单模态。如果不是这种情形,激光器可能工作于多模 态,此时应调整它的电流。如果不能通过调整电流获得单模输出,而反射却可能 控制激光模式,在这种情况下,应调整设备,以尽量减少反射。如果仍然不能获 得单模输出,激光二极管可能已经坏了,需要更换。 警告 在这个项目装置中所提供的激光器发出的不可见光会损害人的眼睛。 所以必 须注意避免眼睛直接暴露于激光光束中。我们建议使用专门针对780nm的激光护 目镜。 在操作前请仔细阅读二极管激光器操作手册中的安全操作章节和有关激光 二极管组成和处理的章节(附录A) 。 1.5.1 半导体激光器的功率特性 1. 装配激光器(LMA-I)并为激光器供电。首先我们将准直光束。接着将激光光 束输入到一个监视器中并使激光光束成像在距激光器约两到十厘米的白屏上。 慢 慢的增加激光器驱动电流并观察白屏上的光斑。每个激光器都有各自的阈值电 流。把电流增加到阈值以上10-20 mA左右。 通过红外成像设备或红外感应卡片, 观察卡片上的光斑并且调整准直透镜在 激光装置LMA-I中的位置以在卡片上获得一个明亮的光斑。将卡片移至距透镜 30-60cm左右同时调整激光器与透镜的相对位置以便获得一个大小不随白屏的位 置改变而强烈变化的光斑。 当白屏到激光器的距离被调近或拉远时,而光斑大小 依然保持不变,可认为此时输出的光束被准直了。另外,可通过将激光光束聚焦 到无穷远来对其进行准直。注意不要使同伴暴露于激光中。 2. 将一个818SL型号的探测器放置在装置(M818组件)后,并调整它的位置以 使其有效面积处于光束中央。 并保证有足够的光强照在探测器上以产生一个较强


的信号。 同时将光电探测器连接到功率计 (815) 实验时应减少背景照明 房 上。 ( 间的灯光) 以使所探测的信号仅来源于激光。 将驱动电流减小到阈值以下数毫安, 并再次检查确使房间光与激光相比不是探测器中占优势的信号。 3. 增加电流并画出探测器的输出与激光器的驱动电流的关系曲线。你应该得到 一个与图1.2类似的曲线。如果需要,也可以通过改变二极管温度以观察温度对 阈值电流的影响。 在研究激光二极管温度特性时,应保证激光二极管工作在恒定 电流模式以防止过大的电流损害激光二极管。注意,当二极管温度降低时,阈值 下降。 所有的操作都应当在二极管断电的情况下进行以防止驱动电流高于阈值而 损坏激光器。 为了防止破坏激光器的结构,应保证工作电流不超过激光器的最大 驱动电流。

图 1.2 –驱动电流和输出功率关系曲线 1.5.2 散光距离特性 激光二极管散光距离由下列各项决定。 先用一个透镜将激光束聚焦到合适距 离。 再通过渐次移动光路中的刀片来获得刀片位于不同位置时通过刀口的总光强 数据。这样一些数据描绘了激光束的整体光强分布(如图1.9a) ,通过差异来显 示实际的光强分布(如图1.9b) ,反过来又可得出光束的直径。通过测量激光不 同位置时的光束大小可以获得光束直径与距离的关系曲线。图1.9c描绘了两个有 意义的光束直径: 一个为刀口沿垂直激光二极管的结平面方向另一个沿平行于结 平面方向。 激光二极管的散光距离即是这两个曲线极小值之间的重叠部分。这种 方法被称为刀刃技术。 1. 用准直透镜(LC)按图1.8所示装配元件,在合理的装配步骤中,应将准直 透镜放在距激光器一厘米左右的位置。且光线应该沿着透镜的光轴传播。这个透 镜与之前准直激光所用的透镜相同。所有元件摆放位置如图所示。应确保二极管 的结平面(即图1.1所示的xz平面) 平行于平台表面。 2. 由于光的不对称分布,激光输出光束的横截面经过球面透镜后变成椭圆的。 因此, 光束有两个明显的焦点, 一个在平行于激光二极管结平面的平面上另一个 在垂直于激光二极管结平面的平面上。在两焦点之间存在这样一点,在此点光束

的横截面是圆形的。 用红外成像仪和白屏,可以粗略地判定横断面光束为圆形时 的位置。

图 1.8 –测量激光二极管散光距离的实验装置

图 1.9 – 确定散光距离的步骤



上。 4. 把激光二极管移离透镜直至最小的光腰移动到刀片所处的平面。现在,再将 激光二极管移离透镜200 ?m。 5. 沿x方向移动刀片1同时记录下x坐标的位置和每个单位坐标(≤100um递增) 所测的光强。我们将得到如图1.9所示的输出曲线。这个光强分布曲线沿x方向的 导数决定了光束的直径。 6. 在y方向对刀片2重复以上操作。 7. 以一定间隔 (≤50μ m) 移动激光器使其靠近透镜, 使总的移动量不少于500 μ m。 在z方向上重复第5和6步骤,并记录z的位置。 8. 使用收集的数据,可推导出x和y方向光强与于透镜z坐标之间的函数关系。 这是通过分析每组数据与坐标间关系得出的。然后,计算光束直径并画出关于z 的函数曲线。Z坐标方向上对应于 ? x 和 ? y 最小量的差值为激光二极管的发散距 离。建议使用计算机软件处理数据。 如果激光器结面不平行平台表面,对上述每一次测量,你将会获得两束发散 光的混合,且测量将变得不准确。如果激光与平台平面成45° 发散距离将变为 , 零。 不同的激光器结构有不同的角光束发散角,因此有不同的发散距离。如果你 知道一些不同激光器的型号(附加指导,索引指导) ,它们的发散距离可能被标 出了。 1.5.3 半导体激光器的频率特性 为了研究激光二极管的频率特性, 我们将使用一个迈克尔逊干涉仪把频率变 化转换成强度变化。用于研究激光光源的频率和振幅特性的实验装置的如图1.10 所示。. 1. 在这个实验中,光束有可能耦合回激光器,从而,使其变得不稳定。因此需 要加一个光隔离器以减少反馈回激光器中的光。用一个偏振片和一个1/4波片就 可以组成一个简单的隔离器。我们使1/4波片的光轴与线偏振器的透光轴成45° 以 使从1/4波片出射的光为圆偏振光。经反射左旋圆偏振光变为右旋圆偏振光或者 相反以使反射光通过1/4波片后变成线偏振光并与偏振片光轴成90° 。之后偏振片 会极大的削弱返回的光束。 在组装隔离器时应确保激光器的结面(图1.1所示的xz平面)平行于平台表 面(二极管激光器盒盖上有刻痕的一面朝上)并且用透镜准直光束。激光光束应 平行于光学平台的表面。调整偏振器和1/4波片的位置。在1/4波片后放置一块反 射镜并旋转1/4波片以便从图1.11所示的偏振片中反射区域获得最大的反射信号. 当这信号达到最大值,返回激光器的光强将达到最小值。

图 1.10–振幅、相位、频率噪声的测量

图 1.1 –激光二极管结构及激光模式辐射场.

图 1.11 –光隔离器调整 2. 构建如图1.12所示的迈克尔逊干涉仪。在光学平台上放置光学臂组件(BSA II) 并利用反射镜反射的光束调整1/4波片的方向。 将方形底座安放在光学平台上, 把双面胶带粘在底座上,并把非极化滤光片(05BC16NP.6) 粘在胶带另一面。接 着调整另一个光学臂组件(BSA-I)和探测器(M818 BB)的位置并调整反射镜以 使两个反射镜反射的光束在探测器上重叠。 当长距离测量时如果激光相干长度小于两个干涉路径的光程差, 干涉光信号 将减弱甚至消失。如果是这种情形,缩短干涉仪测量距离直到信号再出现。如果 不起作用,可通过在卡片上寻找干涉图样或扫描BSA-II中压电转换器(PZB)在 来检查激光器是否为单模输出并观测探测器与PZB扫描距离成正弦关系的输出。 如果激光仍不工作于单模态, 重新调整隔离器或通过改变偏置电流来改变激光器 的工作点。此外,为了保证工作于单模态,激光器在交流调谐之前应加上高于阈 值的直流偏置电流。过高的驱动电流可使激光器进入多模态工作。


图 1.12 – 研究激光二极管频率特性的实验设备 3. 因为迈克尔逊干涉仪中使用了反射镜,光可能耦合回输入端。为了进一步减 小反馈光, 可以将图1.13所示的反射镜稍微倾斜一点。 如果仍然不能得到单模态, 则须替换激光二极管。

图 1.13– 倾斜干涉仪反射镜以减小反馈 4. 在探测器前放置一个白屏并用红外成像仪观察干涉图样。轻微地调整一下反 射镜以获得最好的干涉图样。试着获得一个清晰的条纹。 5. 将探测器放在干涉图样的中央以使它仅能探测到中心峰值的一部分。 6. 通过调节干涉仪一个臂上(例如BSA-II) 附加于反射镜上的压电转换器的电 压, 以获得最大的输出强度。输出应保持稳定一分钟左右。如果不是如此,检查 一下是否所有元件都安装牢固。 如果都安装正确,可能是房间气流使装置变得不 稳定。在这种情况下,可以将一个盒子 (可用厚纸箱)罩在在装置上以防止气 流干扰干涉仪工作。 7. 通过调节PZB上电压使干涉仪正交(干涉仪输出的最小值和最大值之间敏感 度最大的点) 。 8. 干涉仪由激光器频移所决定的输出信号由式?I∝?φ = 2π/c?L?ν给出其中?L 是干涉仪两臂的差值?ν是激光器电流调谐获得的频率宽度。 注意由于光在这段距 离内双向传播,故在迈克尔逊干涉仪中?L是实际两臂长度差的两倍。因为更大 的?L与更高的输出信号有关?L取值范围在3-20 cm为较合适的长度。


在我们对激光器进行调谐前, 应通过选择合适的?L和电流振幅di的调整使干 涉仪的输出作信号?I大于探测器或者激光器的噪音水平。从1.3节中可知当调节 二极管的电流时激光器的频率和强度也得到调制。 通过阻断干涉仪中的一支我们 可以测量出激光器的强度调制。 这样可以消除干扰并使强度调制度的测量成为可 能。 然后我们从总的干涉仪输出值中减去这个值得到由频率调制所决定的dI/di 。 使用低频率,小的电流调制激光二极管。在适当范围内可得 1 dv 1 dI ? 10 ?5 mA ?1 且 ? 0.2mA ?1 又因为只有振幅变化 v di I di dI d( ?? ) 2? ?v c dI ? ? ?L ~ 10 ?5 mA ?1 或 又 , di di c ?i 2??Lv di dI ?L ~ K 2? mA ?1 其中 K 是由不同的 ? L 所决定的探测器常数。 di ?10 ?5

图 1.4-dv/di 和频率的关系曲线 9. 通过对干涉仪和探测系统的调整,进而改变激光器的驱动频率并获得激光器 响应频率(图1.4或1.10 a所示) 。你需要记录两组数据: (i)干涉仪输出的调制度 和频率的函数关系; (ii)激光光强的调制度。收集到的两组不同的数据将会为频 率调制度的实际值dI/di提供一个估计值。同时也注意到,如果调制电流足够小 并且路径的失配足够大,激光强度调制是可以忽略的。你需要调节PZB的电压使 干涉仪保持正交。 当你改变频率时,应注意调整函数发生器的幅度以保持激光器的电流调制度 恒定。 这是因为函数发生器可能没有平坦的频率响应特性。这样做的作用是使电 流调制度di不为常数而是一个关于频率的变量。为了避免不必要的计算,可以将 激光二极管驱动系统的监测接口连到一个示波器上, 并通过调整与频率相关的振 幅正弦波形来保持调制度恒定。当温度对dν/di的贡献开始可以忽略不计时略去 dν/di并记录下此时激光频率(参见1.3节). 10. 记住上述方程,现在,我们开始研究激光调制的线性特性。在一个恒定的 调制频率下(选择dν /di变化迅速的调制频率,也就是第9步骤与图1.10a类似的 曲线中斜率最大的地方) ,对不同的激光偏置水平改电流调制度di且得到一个如 图1.10b所示的一个曲线。输出dv’不会出现明显的变化除非在阈值附近且电流很


高。注意:不要超过额定的驱动电流/二极管的输出功率否则它可能受损。 11. 应用激光电流调制来推断相位噪声从而得到与干涉仪双臂长度差?L成函数 关系的相位噪声特性 。应确保干涉仪正交。 将二极管的电流设置在阈值以上,应用小的调制电流,并将调制频率固定在 一个期望值上。合适的频率包括50 HZ、2 KHZ和50KHZ。 (见叁考1.5) 用一个光 谱分析仪或者一个示波器监测探测器的输出并记录干涉仪正交时的峰峰值。 你可 以通过手动扫描PZB电压引起至少π/2的相位变化来完成这些,记录下与光程差 成函数关系的光强的最大峰峰值。 同时应确保装置噪音处于预期信号水平以下并 假定激光器一直工作于单模态。将获得类似图1.10 c曲线。 1.5.4 半导体激光器的振幅特性 强度特性的测量通过在如图1.10所示的干涉仪前放置探测器或者阻断干涉 仪中的一块反射镜。 再次强调, 激光器一定要在最小的反馈下工作于单模态否则 噪音水平和功能将会大幅改变。相对的噪音强度(RIN)定义为20log(dI/I)其中 dI是RMS相对强度波动如对应于dI~10-4的RIN是-80分贝。通常,用一个光谱分 析仪和 1HZ带宽来进行测量。 当进行RIN测量时,电子的和光电探测器注入噪音一定要低于RIN水平。 (选 做)你可以利用一个光强与激光器相类似的非相干光源(如灯)决定注入噪声。 激发光源噪音的合频率谱提供了测量注入定噪音水平的标准即至少比电噪声大 10dB。注入噪音的测量应用方程0.47验证。 1. 将激光器的驱动电流从低于阈值调制到高于阈值并使用光谱分析仪记录下 在期望频率下的激光输出功率和强度噪声。当计算RIN时,假定注入和电噪声在 RIN水平以下,将得到一个与图1.10d类似的曲线。大部份情形下,对于单模态工 作,噪音的峰值出现在阈值处。如果激光被调制,或变成多模,或激光器名义上 为单模输出而其边模态抑制又不足时 (<20分贝)噪音曲线的形状可能发生变化。 , 2. 改变激光器调制信号的调制度或通过旋转1/4波片增强激光器的反馈来降低 隔离器的性能对激光器的操作是有意义的。 由此可得出不同反馈条件下的噪音特 性, 这对后来的传感和通信实验十分重要。对于电视广播信号和灵敏的干涉传感 器要求其RINs分别小于-150分贝和-120分贝。 3. 二极管激光器的噪声强度有1/f 噪声特性。 (当频率降低时,性能降低)用一 个电流在阈值以上的激光器和带有频谱分析仪的光电探测器进行测量,可获得 RIN关于频率调制的函数。即可获得图1.10 e 所示的频率响应曲线,对于典型的 激光器从100 kHz到1 MHz区间某个位置开始噪声将变得平坦(对频率而言) 。 注意:在这个项目中使用的迈克尔逊干涉仪将会再次用于项目3。如果在完成项 目2中二极管激光器特性测量前直接进行项目3,将可以节约很多时间。


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