المرجع الالكتروني للمعلوماتية
المرجع الألكتروني للمعلوماتية

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RESONATOR    STABILITY  
  
1515   03:17 مساءاً   date: 17-3-2016
Author : Mark Csele
Book or Source : FUNDAMENTALS OF LIGHT SOURCES AND LASERS
Page and Part : p178


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Date: 22-1-2021 1205
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RESONATOR    STABILITY

      The most general form of a laser cavity, is one consisting of two concave mirrors with coincident foci in the center of the laser medium. This is contrary, perhaps, to popular opinion, which is usually that two plane mirrors are used, although that arrangement, too, is stable (although only marginally) and is employed in some lasers. In the confocal arrangement, a true Gaussian beam traverses between the concave mirrors, and it can be found in which the curvature of the wave front of the intracavity beam approaching the mirrors matches the curvature of the cavity mirrors themselves. This ensures that the beam reflects perfectly back on itself and is completely trapped within the cavity the definition of stability. Any ray within the cavity can retrace itself exactly after one round trip through the cavity. If one mirror is partially transmitting, the beam that passes through the mirror (which becomes the output beam) continues to diverge.

         We now examine the parameters behind the design of resonators in order that one may be designed to confine radiation properly in the laser. The vast majority of lasers use stable resonator configurations, those in which light between the mirrors

Figure 1.1. Parameters for resonator stability.

is trapped completely and the beam is reflected back on itself within the cavity, keeping the power density large, to encourage stimulated emissions. To determine stability (mathematically) we introduce resonator g parameters, one representing each mirror, which define the beam path relative to the entire cavity. Knowing the distance between the cavity mirrors as well as the radius of curvature for a mirror, the expression for a g parameter is

 (1.1)

where L is the distance between the cavity mirrors and r is the radius of curvature. These parameters are defined in Figure 1.1, which shows a generalized laser cavity with concave mirrors. Stability, then, is defined as the condition where

  (1.2)

where g1 and g2 are the g parameters for each mirror. In the case of a plane mirror, the radius is infinity, so the corresponding g parameter is 1.

Example 1.1 Resonator Stability Consider a laser cavity consisting of two spherical mirrors resembling that of Figure 1.1. The distance between the two mirrors is 1 m and each mirror has a radius of 60 cm. Using equation (1.1), the g parameter for each mirror is calculated to be 1-1/0.6 = 20.67, so that the product of the g parameters is 0.44. This resonator arrangement is hence found to be stable.

      Consider a similar second cavity where each mirror has a radius of 45 cm. The g parameter of each mirror is now calculated to be -1.22, so that the product of the g parameters is 1.49. Since this value is greater than 1, the configuration is not stable. Various cavity arrangements are examined in the next section.

      It must also be noted that some cavity arrangements utilize the lasing volume better than others. A confocal cavity, for example, uses only a fraction of the gain medium, whereas a cavity consisting of two plane mirrors can use much more of the gain medium, presumably allowing higher efficiency and power output. A cavity consisting of two plane mirrors, however, is difficult to align (it presents marginal stability, as the product of the g parameters is exactly 1), so most practical laser cavities consist of one or more concave or spherical mirrors, as we examine in the next section. It is also possible to use intracavity beam expanders (consisting of two or more lenses) to improve utilization of the entire volume of the lasing medium (this is sometimes done in YAG lasers to ensure use of the entire rod).




هو مجموعة نظريات فيزيائية ظهرت في القرن العشرين، الهدف منها تفسير عدة ظواهر تختص بالجسيمات والذرة ، وقد قامت هذه النظريات بدمج الخاصية الموجية بالخاصية الجسيمية، مكونة ما يعرف بازدواجية الموجة والجسيم. ونظرا لأهميّة الكم في بناء ميكانيكا الكم ، يعود سبب تسميتها ، وهو ما يعرف بأنه مصطلح فيزيائي ، استخدم لوصف الكمية الأصغر من الطاقة التي يمكن أن يتم تبادلها فيما بين الجسيمات.



جاءت تسمية كلمة ليزر LASER من الأحرف الأولى لفكرة عمل الليزر والمتمثلة في الجملة التالية: Light Amplification by Stimulated Emission of Radiation وتعني تضخيم الضوء Light Amplification بواسطة الانبعاث المحفز Stimulated Emission للإشعاع الكهرومغناطيسي.Radiation وقد تنبأ بوجود الليزر العالم البرت انشتاين في 1917 حيث وضع الأساس النظري لعملية الانبعاث المحفز .stimulated emission



الفيزياء النووية هي أحد أقسام علم الفيزياء الذي يهتم بدراسة نواة الذرة التي تحوي البروتونات والنيوترونات والترابط فيما بينهما, بالإضافة إلى تفسير وتصنيف خصائص النواة.يظن الكثير أن الفيزياء النووية ظهرت مع بداية الفيزياء الحديثة ولكن في الحقيقة أنها ظهرت منذ اكتشاف الذرة و لكنها بدأت تتضح أكثر مع بداية ظهور عصر الفيزياء الحديثة. أصبحت الفيزياء النووية في هذه الأيام ضرورة من ضروريات العالم المتطور.