Lasers, Laser implementation

Properties of laser light • Large spectral range: UV to IR covered • Extremely paraxial: light is close to

optical axis, e.g. HeNe 633 nm, d=1 mm 1 mrad beam divergence

• Monochromatic: bandwidth of single mode lasers typically in low MHz range, the width of 1 Hz was achieved

9 15/ 10 10f f − −∆ = −Source: U.S. Navy

maser UV-, X-ray laser

Coherence: - Coherence time (= propagation time of the laser light to become incoherent): τcoh = 1/(2π∆ν) = 160ns for ∆ν = 1MHz - Coherence length (= propagation time of the laser light to become incoherent): Lcoh = cτcoh = 50m - Note that when the resonator determines the line width, the coherence time is equal to the photon lifetime

He-Ne laser:

Noble gas ion laser Ar+ ion laser

Kr+ ion laser

Mixed gas lasers

Nitrogen laser

Hydrogen laser

Excimer laser [mixure of inert gas (e.g. noble gas) and reactive gas (e.g. F2), – pumped e.g. by electron collisions, the lasing states are excited dimers – usually for UV range and very high power applications

Carbon monoxide and dioxide laser (IR range)

Iodine laser

Gas lasers

Energy levels , energy transfer and lasing

External excitation creates population inversion

Thermal equilibrium 3

2

expS

P

N EN kT

∆ = −

We need population inversion to create appreciable probability for stimulated emission

Question: where do stimulated photons go after emission? 1) They can escape 2) The can go into one of the resonant modes of FP cavity, where

they stay for very long time

Important properties of photons (and other boson particle) – they “like” to stay together. Once we have N photons in a cavity mode, the ration of probabilities of a photon to be go into this mode and escape the cavity is Pmode/Pout=N. We have a system with positive feedback -> the mode gets more and more populated.

Types of laser cavities

Not used – beam is unstable

In He-Ne laser Ne atoms have typical FWHM about 1400 MHz. It is set by the time scale of spontaneous emission and Doppler broadening . For wave length 633 nm and laser cavity 0.5 m we have free spectral range about 300 MHz. So this laser will have about 5 populated modes. Typical width of each mode is 1 MHz (Finess 300). I shorter lasers (L<0.15 m) only one mode will be populated.

Solid state lasers:

Semiconductor laser

Vibronic lasers (e.g. Titanium-sapphire (TiSa) laser)

Due to the extend of the topic: Only qualitative description

If you want to know it exactly, refer to: N. W. Ashcroft and N. D. Mermin, Solid State Physics, Saunders College (1975).

Titanium-sapphire laser (TiSa-laser):

- So called “vibronic” laser: Cr or Ti are in a solid matrix in very low concentration <0.1%

- The doped Ti atoms substitute Al in Sapphire (Al2O3) which is used as host matrix.

- Tunable in the range from 650 to 1100 nanometers

- Usually pumped with another laser with a wavelength of 514 to 532 nm

configuration coordinate (usually displacement)

ener

gy

fast relaxation

pum

ping

lasin

g

⇒ TiSa is excellent pulsed Laser

- Invented by Peter Moulton (MIT Lincoln Lab) in 1982, commercially available since 1988

4.4.3. Ti:Sapphire Laser Oscillation of Ti:Sapphire laser was first realized in 1982 by P.F.Moulton. Titanium sapphire (Ti:Al2O3 or Ti:Sapphire) crystal is synthesized by Ti3+ doping on sapphire crystal, which is also used as a base material for ruby laser (Cr:Al2O3 laser). Al2O3 is appropriate as a base material of high-repetition and high power laser since it has a good thermal conductivity. Instead of dye laser, Ti:Sapphire laser is widely used for a variety of research fields of femtosecond spectroscopy, nonlinear optics, white light generation, and terahertz generation. Figure 4.4.5 shows an absorption spectrum and a fluorescence spectrum of Ti:Sapphire laser. Because the fluorescence spectrum of Al2O3 is very broad, Ti:Sapphire laser is tunable in the range of 660-1100 nm by an insertion of a wavelength selection device in the resonator. By mode-locking, the pulse duration of 5.5 fs has been realized. Nowadays, studies for a shorter duration is proceeded. For a typical Ti:Sapphire laser, the pulse duration is ~100 fs, the repetition rate is 70~80 MHz, and the average power is ~2.5 W.

Some possible optical setups for the Ti:sapphire laser

Bulk-type mode-locking laser A typical configuration of mode-locking titanium sapphire laser is Kerr-lens mode-locking (KLM) or self-mode-locking, both of which employs nonlinear optical effects. As well as typical solid-state lasers, laser light is oscillated and amplified via input of the pumping light in the resonator. The intense part of amplified laser light converges via the Kerr-lens effect induced in the medium (Intense light changes a refractive index of medium, then the medium turns to be like a lens). The intense part of laser light is selected by a slit, subsequently only intense pulses stably exist in the resonator. This is a generation mechanism of the ultrashort pulses. A couple of prisms in the resonator are used for compensating pulse elongation by temporal Kerr-effect (self phase modulation) induced in the laser medium. Chirped mirror can be also used instead of the prisms. Recently, by using semiconductor saturable absorber mirror (SESAM) but not KLM, a mode-locked laser has been realized.

Pulse lasers

1. Q-switching, controlling F by some external means 2. Mode-locking lasers: to achieve short (femto-second) pulse duration very broad range of frequencies is needed. Nonliner optical element brings operation to a pulsed regime.

Bulk-type mode-locking laser A typical configuration of mode-locking titanium sapphire laser is Kerr-lens mode-locking (KLM) or self-mode-locking, both of which employs nonlinear optical effects. As well as typical solid-state lasers, laser light is oscillated and amplified via input of the pumping light in the resonator. The intense part of amplified laser light converges via the Kerr-lens effect induced in the medium (Intense light changes a refractive index of medium, then the medium turns to be like a lens). The intense part of laser light is selected by a slit, subsequently only intense pulses stably exist in the resonator. This is a generation mechanism of the ultrashort pulses. A couple of prisms in the resonator are used for compensating pulse elongation by temporal Kerr-effect (self phase modulation) induced in the laser medium. Chirped mirror can be also used instead of the prisms. Recently, by using semiconductor saturable absorber mirror (SESAM) but not KLM, a mode-locked laser has been realized.

Ruby laser

Lasers can have rather complicated modes