مامعنى هذا المصطلح؟

[Fatima_a]

السلام عليكم
عُدنا من جديد مع مصطلح آخر

ما معنى self focussing؟
بحثت عنه بالعربي لم أجد شيئاً

بالمناسبة..ياليت يتغير عنوان الموضوع إلى "مامعنى هذا المصطلح؟"
والله يعطيكم العافية

[رجب مصطفى]

المتذبذب الضوئي ومعامل الكسب (Gain) في الليزر

http://hazemsakeek.com/vb/showthread.php?t=20905

[Fatima_a]

جزيل الشكر لكم أستاذ رجب
يبدوا أن الموضوع قيم جداً، لذا احتاج أن أُعيد قرآءته بشكل أعمق
أكرر شكري لكم

وبإنتظار جواب لسؤالي الأخير
دعائكم

[رجب مصطفى]

Self-focusing

Definition: focusing of a beam in a transparent medium, caused by the beam itself through a nonlinear process in the medium

Due to a Kerr lens, an intense optical pulse propagating in a nonlinear medium can experience self-focusing (or self-focussing): the beam diameter is decreased compared with that of a weak pulse. The physical mechanism is based on a Kerr nonlinearity with positive χ(3). In this situation, the higher optical intensities on the beam axis, as compared with the wings of the spatial intensity distribution, cause an effectively increased refractive index for the inner part of the beam. This modified refractive index distribution then acts like a focusing lens.

A related effect, occurring in the case of a negative χ(3) nonlinearity, is self-defocusing, where the Kerr lens has a reduced refractive index on the beam axis.

Self-focusing can also be caused by other effects, such as thermal lensing.

Consequences of Self-focusing

As the decrease in the beam radius further increases the strength of the Kerr lens, there may be total collapse of the beam: as the beam radius is reduced, the optical intensities become even higher, further increasing the self-focusing effect. This mechanism can lead to very high optical intensities which can easily destroy the optical medium (optical damage). Such a run-away effect can occur when the optical power is above the critical power

where n2 is the nonlinear index, λ the vacuum wavelength, and n the refractive index. Remarkably, the critical power does not depend on the original beam area. (A larger beam generates a weaker Kerr lens, but it is also more sensitive to lensing.) It is assumed, however, that the medium is long enough to allow the beam collapse. For silica (as used e.g. in silica fibers), the self-focusing limit in terms of peak power is roughly 4 MW in the 1-μm wavelength region. No method is known for increasing the self-focusing limit of optical fibers beyond that value.

A beam with a power exactly at the self-focusing limit could theoretically exhibit self-trapping [1], where the beam profile stays constant over a longer distance, because divergence is exactly compensated by the nonlinear focusing effect. That state, however, is unstable; small deviations from that state would grow rapidly.

For optical powers far above the self-focusing limit, filamentation can occur, where the beam breaks up into several beams with smaller powers. The resulting beam pattern can be random, but in some cases it has a fairly regular structure.

Different dynamics (transient self-focusing [2]) occur for ultrashort pulses when the pulse duration is not much longer than the characteristic time scale of the nonlinearity [2].

The reduction in beam radius for high intensities (for peak powers below the self-focusing limit) can be used for Kerr lens mode locking of a laser, when it leads to a better overlap of laser and pump beam, or to reduced losses at some aperture. In both cases, it generates a kind of artificial saturable absorber.

Another application of self-focusing is the measurement of the magnitude of the Kerr nonlinearity (→ z-scan measurements).

Note that cascaded χ(2) nonlinearities can also lead to an effective χ(3) nonlinearity, resulting either in self-focusing or (for negative effective χ(3)) self-defocusing.

[مرجانه]

شكر لك استاذي الفاضل على المعلومة

[الاستاذ مناف دحروج]

رائع
تحياتي
لكم
احب هذا المنتدي واجلس عليه اكثر من ما ادرس

[Fatima_a]

أكرر شكري وإمتناني لكم أستاذ رجب..

الكثير من الكتب الأنجليزية تشرح هذه المصطلحات، لكن المزعج في الأمر هو عدم إحتواء الكتب العربية على هذه المعلومات التي هي أساس تعلم فيزياء الليزر، وإن كانت هناك كتابات صغيرة لبعض الأساتذة على الانترنت؛ إلا إنها لا تُشفي الغليل.


سؤالي التالي..
مامعنى filamentation هو مثال على تأثير كير Kerr effect ؟
لم أجد له وجود في الكتب العربية، ولم أفهم معناه من الكتب الأنجليزية.
حبذا لو شرح بسيط وإن كان في سطر واحد بشكل مجمل

دعائكم