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Lernmaterialien für Laser Physics an der TU Ilmenau

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TESTE DEIN WISSEN

ways to generate short pulses:

q switch

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TESTE DEIN WISSEN

--> variable attenuator

The quality factor (Q) is the ratio of the energy stored in a cavity to its energy loss per cycle.


The main purpose of Q‐switching is to rapidly change the feedback of a laser cavity from minimum to maximum. The Q‐switch blocks all feedback between the laser mirrors until the excitation mechanism has pumped the maximum energy into the active medium. Then it opens and allows maximum feedback to occur, which results in a very high peak power pulse.  

Q‐switching also involves storing energy in the laser gain medium but not by modulating the pump source. Here, the laser pumping process is allowed to build up a population inversion far in excess of the typical threshold value by ensuring that the cavity losses are large, which prevents lasing. Inhibiting the optical feedback is accomplished by adding a loss in the laser cavity. After a large inversion has been achieved, the cavity feedback is switched back on. The laser then experiences gain that greatly exceeds losses, and the stored energy is released as a short and intense light pulse.


Devices used for Q‐switching must be able to rapidly modulate the cavity Q to generate short pulses and are grouped into two categories: 

 Active devices require an external operation to induce modulation which include acousto‐optical transducers, electro‐optical transducers, and rotating mirrors/prisms

Passive devices switch automatically based on the non‐linear optical response of the element being used, e.g., saturable absorption in organic dyes or semiconductors.

Lösung ausblenden
TESTE DEIN WISSEN

ways to generate short pulses:

cavity dumping

Lösung anzeigen
TESTE DEIN WISSEN

Cavity dumping stores energy in the photons within the resonator. The losses within the resonator are kept low for some time by keeping the cavity mirror transmittances negligible, effectively trapping the photons in the cavity and allowing an intense pulse to build up. This pulse is extracted by switching an intra‐cavity element after one round trip and “dumping” the pulse out of the cavity


Two advantages of cavity dumping: 1. Cavity dumping allows for very high pulse repetition rates, e.g., several MHz, while maintaining pulse durations of a few ns. 2. Cavity dumping can be combined with other pulse generation techniques in order to allow for extraction of higher pulse energies than would typically be available using other techniques.

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TESTE DEIN WISSEN

direct pumping

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• Optical pumping: solid‐state and organic dye lasers. Absorption of the pumping light within gain medium. 

•  Particle pumping: gas and semiconductor lasers

Disadvantages: 

• Might be no efficient direct route from ground state 0 to laser state u. 

• Might be a better route from 0 to l.

• Might be no good source of available pumping flux: insufficient intensity for optical pumping or insufficient electron density for electrical pumping.

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TESTE DEIN WISSEN

indirect pumping

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Intermediate level q: always much more closely located in energy to the upper laser level u than to the initial level 0.


Advantages: 

• τq>> τu , Nq>>Nu , a reservoir of population, q-u transfer is much easier than 0-u transfer. 

• Pumping probability (cross section) : 0-q>>0-u, much less pumping requirements. 

• q-u transfer can be quite selective. 

• q can belong to a different species. Energy transfer to the laser level by a process from one species to another.

q‐level capability: a very broad width and accepting pumping flux over a broad range of energies.

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Transfer from below

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Gas lasers, Level q: long lifetime, accumulating energy, serving as a storage state.

Initial high voltage ionizes the gas to conduct current. Electrons transfer energy to Ar atoms, ionize them and raise the ions to a group of high energy levels. 

• Different processes populate the metastable 4p level 

• Electrons collision with Ar+ ions in ground state 

• Collision with ions in metastable state 

• Radiative transitions from higher states 

• Transitions can occur between multiple pairs of upper and lower lasing levels, producing multiple laser wavelengths. 

• Most important wavelengths: 488 nm and 514 nm. 

• Ar+ ions quickly drop from lower laser level to ground state of the ion by emitting a UV light at 740 Å

• Conditions of population inversion satisfied between 4p and 4s levels  

Lösung ausblenden
TESTE DEIN WISSEN

Transfer from across

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The conservation of energy and momentum allow efficient transfer from one excited state to another state, but only if those two states have equal or nearly equal energy

Lösung ausblenden
TESTE DEIN WISSEN

Transfer from above

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• Most widely used excitation process: especially effective in producing population inversion.

• Pump energy can occur over a wide range of excitation energies: pumping band can have a wide range of wavelength of tens of nanometers in visible spectral region. 

• Population in level q preferentially decays to level u as opposed to level l.

• Energy moves to level u from level q “automatically” or without additional stimulus of any kind at a very fast rate because of the thermalization process, where the downward rate is controlled by thermal equilibrium to production Boltzmann distribution.

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Laser - Def.

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Light Amplification by Stimulated Emission of Radiation 

A laser is a device that amplifies light and produces a high directional, high-intensity beam, that often has a very pure frequency or wavelength. 


 • Size: from one tenth of the diameter of a human hair, to the size of a very large building 

• Power: 10-9 to 1020 W 

• Wavelength: from microwave (106 -109 Hz) to the soft-Xray spectral regions (1011 – 1017 Hz)

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Spontaneous Emission

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In the spontaneous emission process, the atom decays from level 2 to level 1 through the emission of a photon.

Excited atoms can emit photons spontaneously, i.e., without external cause. The radiation emitted spontaneously is incoherent and the emission occur s into all spatial directions.

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Stimulated Emission

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In the stimulated emission process, the incident photon stimulates the level 2 to level 1 transition and we then have two photons (the stimulating plus the stimulated one).


The energy difference E2 -E1 is delivered in the form of a wave (e.m.) that adds to the incident one.


An excited two level atomic system transfers by a stimulated emission process its excitation energy to the light field. The strength of stimulated emission is proportional to N 2. 

Einstein: Radiation created by the stimulated emission has the same frequency, direction, polarization and phase as the stimulating radiation.

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Absorption

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In the absorption process, the incident photon is simply absorbed to produce the level 1 to level 2 transition.

The energy difference E2 -E1 required by the atom to undergo the transition is obtained from the energy of the incident wave.


In an absorption process, a photon is converted to excitation energy of a two-level atomic system by a 1→2 transition. The strength of absorption is proportional to N1.

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How to calculate Einstein coefficients through experiments?

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1. Measuring the spontaneous life time by a luminescence experiment 

--> calculate the Einstein coefficient of spontaneous emission, and then to calculate the Einstein coefficient of absorption and stimulated emission according to the Einstein relations. 

2. Measuring the absorption coefficient

--> to calculate the Einstein coefficient of spontaneous emission

Lösung ausblenden
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Q:

ways to generate short pulses:

q switch

A:

--> variable attenuator

The quality factor (Q) is the ratio of the energy stored in a cavity to its energy loss per cycle.


The main purpose of Q‐switching is to rapidly change the feedback of a laser cavity from minimum to maximum. The Q‐switch blocks all feedback between the laser mirrors until the excitation mechanism has pumped the maximum energy into the active medium. Then it opens and allows maximum feedback to occur, which results in a very high peak power pulse.  

Q‐switching also involves storing energy in the laser gain medium but not by modulating the pump source. Here, the laser pumping process is allowed to build up a population inversion far in excess of the typical threshold value by ensuring that the cavity losses are large, which prevents lasing. Inhibiting the optical feedback is accomplished by adding a loss in the laser cavity. After a large inversion has been achieved, the cavity feedback is switched back on. The laser then experiences gain that greatly exceeds losses, and the stored energy is released as a short and intense light pulse.


Devices used for Q‐switching must be able to rapidly modulate the cavity Q to generate short pulses and are grouped into two categories: 

 Active devices require an external operation to induce modulation which include acousto‐optical transducers, electro‐optical transducers, and rotating mirrors/prisms

Passive devices switch automatically based on the non‐linear optical response of the element being used, e.g., saturable absorption in organic dyes or semiconductors.

Q:

ways to generate short pulses:

cavity dumping

A:

Cavity dumping stores energy in the photons within the resonator. The losses within the resonator are kept low for some time by keeping the cavity mirror transmittances negligible, effectively trapping the photons in the cavity and allowing an intense pulse to build up. This pulse is extracted by switching an intra‐cavity element after one round trip and “dumping” the pulse out of the cavity


Two advantages of cavity dumping: 1. Cavity dumping allows for very high pulse repetition rates, e.g., several MHz, while maintaining pulse durations of a few ns. 2. Cavity dumping can be combined with other pulse generation techniques in order to allow for extraction of higher pulse energies than would typically be available using other techniques.

Q:

direct pumping

A:

• Optical pumping: solid‐state and organic dye lasers. Absorption of the pumping light within gain medium. 

•  Particle pumping: gas and semiconductor lasers

Disadvantages: 

• Might be no efficient direct route from ground state 0 to laser state u. 

• Might be a better route from 0 to l.

• Might be no good source of available pumping flux: insufficient intensity for optical pumping or insufficient electron density for electrical pumping.

Q:

indirect pumping

A:

Intermediate level q: always much more closely located in energy to the upper laser level u than to the initial level 0.


Advantages: 

• τq>> τu , Nq>>Nu , a reservoir of population, q-u transfer is much easier than 0-u transfer. 

• Pumping probability (cross section) : 0-q>>0-u, much less pumping requirements. 

• q-u transfer can be quite selective. 

• q can belong to a different species. Energy transfer to the laser level by a process from one species to another.

q‐level capability: a very broad width and accepting pumping flux over a broad range of energies.

Q:

Transfer from below

A:

Gas lasers, Level q: long lifetime, accumulating energy, serving as a storage state.

Initial high voltage ionizes the gas to conduct current. Electrons transfer energy to Ar atoms, ionize them and raise the ions to a group of high energy levels. 

• Different processes populate the metastable 4p level 

• Electrons collision with Ar+ ions in ground state 

• Collision with ions in metastable state 

• Radiative transitions from higher states 

• Transitions can occur between multiple pairs of upper and lower lasing levels, producing multiple laser wavelengths. 

• Most important wavelengths: 488 nm and 514 nm. 

• Ar+ ions quickly drop from lower laser level to ground state of the ion by emitting a UV light at 740 Å

• Conditions of population inversion satisfied between 4p and 4s levels  

Mehr Karteikarten anzeigen
Q:

Transfer from across

A:

The conservation of energy and momentum allow efficient transfer from one excited state to another state, but only if those two states have equal or nearly equal energy

Q:

Transfer from above

A:

• Most widely used excitation process: especially effective in producing population inversion.

• Pump energy can occur over a wide range of excitation energies: pumping band can have a wide range of wavelength of tens of nanometers in visible spectral region. 

• Population in level q preferentially decays to level u as opposed to level l.

• Energy moves to level u from level q “automatically” or without additional stimulus of any kind at a very fast rate because of the thermalization process, where the downward rate is controlled by thermal equilibrium to production Boltzmann distribution.

Q:

Laser - Def.

A:

Light Amplification by Stimulated Emission of Radiation 

A laser is a device that amplifies light and produces a high directional, high-intensity beam, that often has a very pure frequency or wavelength. 


 • Size: from one tenth of the diameter of a human hair, to the size of a very large building 

• Power: 10-9 to 1020 W 

• Wavelength: from microwave (106 -109 Hz) to the soft-Xray spectral regions (1011 – 1017 Hz)

Q:

Spontaneous Emission

A:

In the spontaneous emission process, the atom decays from level 2 to level 1 through the emission of a photon.

Excited atoms can emit photons spontaneously, i.e., without external cause. The radiation emitted spontaneously is incoherent and the emission occur s into all spatial directions.

Q:

Stimulated Emission

A:

In the stimulated emission process, the incident photon stimulates the level 2 to level 1 transition and we then have two photons (the stimulating plus the stimulated one).


The energy difference E2 -E1 is delivered in the form of a wave (e.m.) that adds to the incident one.


An excited two level atomic system transfers by a stimulated emission process its excitation energy to the light field. The strength of stimulated emission is proportional to N 2. 

Einstein: Radiation created by the stimulated emission has the same frequency, direction, polarization and phase as the stimulating radiation.

Q:

Absorption

A:

In the absorption process, the incident photon is simply absorbed to produce the level 1 to level 2 transition.

The energy difference E2 -E1 required by the atom to undergo the transition is obtained from the energy of the incident wave.


In an absorption process, a photon is converted to excitation energy of a two-level atomic system by a 1→2 transition. The strength of absorption is proportional to N1.

Q:

How to calculate Einstein coefficients through experiments?

A:

1. Measuring the spontaneous life time by a luminescence experiment 

--> calculate the Einstein coefficient of spontaneous emission, and then to calculate the Einstein coefficient of absorption and stimulated emission according to the Einstein relations. 

2. Measuring the absorption coefficient

--> to calculate the Einstein coefficient of spontaneous emission

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