انت هنا الان : شبكة جامعة بابل > موقع الكلية > نظام التعليم الالكتروني > مشاهدة المحاضرة
الكلية كلية العلوم للبنات
القسم قسم فيزياء الليزر
المرحلة 1
أستاذ المادة محمد حمزة خضير المعموري
5/12/2011 7:30:59 AM
Lasers
Introduction
LASER – Light Amplification by Stimulated Emission of Radiation
For stimulated emission to occur, medium must have some molecules in an excited state.
But consider the thermal population of the excited state at room temperature if the
wavelength of emission is 694 nm.
i 31 n
6.2 10
N
= ´ -
No molecules of the medium are in the excited state. Therefore to create stimulated
emission, a large number of molecules must be promoted to an excited state. The process of
creating a large number of molecules in the excited state is called population inversion.
Since excited state molecules are being continually stimulated to release a photon and drop to
the ground state, a laser must have a way of keeping the excited state continually populated.
Population inversion is created using a flash lamp or sometimes another laser.
Parts of a Laser
A laser has two main parts.
1.) Gain medium
- The medium that will be emitting the stimulated emission.
2.) Resonance cavity
- It is important to have a long gain medium to build intensity of light.
- Intensity of light is increased by being reflected back and forth through the gain
medium.
gain medium
mirror partial mirror
laser light
resonance cavity
flash lamp
Light comes out of resonance cavity via “leaking” through mirror.
2
Resonance Modes
Light emitted via spontaneous emission can have any direction.
Spontaneously emitted photons cause stimulated emission.
A stimulated emission photon has the same phase, frequency and direction as the photon that
stimulated it.
Light with its photons having the same phase, frequency and direction is called coherent light.
Recall that electronic transitions have a finite linewidth. (Natural broadening, etc…)
However within the resonance cavity, the photons that do not have an integral number of
wavelengths between mirrors destructively interfere with each other.
Only those photons with an integral number of wavelengths between the mirrors survive.
The wavelengths of light that transmit from the cavity are called resonance modes.
Surprise! Laser light is not really monochromatic, but …
- For 694.4 nm light, the modes are 0.016 nm apart
- The number of modes is approximately 10 – 20; therefore, laser light is effectively
monochromatic.
These resonance modes are one the reasons laser light is not infinitely coherent.
Two Operational Modes of Lasers
Continuous Wave Lasers (CW lasers)
Laser light is continuously emitted from resonance cavity.
Most applications of lasers use CW lasers.
- Raman
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
|