Dr. Lee Wing Kee
Chinese University of Hong Kong

Here we briefly describe how to construct a laser.

A schematic diagram of a typical laser cavity is shown in Fig. 1.

(Fig. 1)

•M1, M2 ¡V mirrors, intensity reflectivity of M1 ~ 99.5% or higher.
•M2 is also called output coupler (OC), intensity reflectivity typically 98% —> 90%.

•Characteristics:

(1) M1, M2 very flat .
(2) M1, M2 are aligned such that they are "very" parallel. (see exercise).

•Remarks:
(1) M1, M2 - multi-layer dielectric coating mirrors.
(2) Mirror in your bedroom, reflectivity?
(3) Sometimes concave mirrors are used. (Why?)

(Fig. 2)

•Represents an atom with an excited electron in level A. Light inside the cavity has gain (i.e. number of photons increases in time) if there is population inversion and the reflectivity of OC is high enough. Several round trips after photon p1 is emitted, this process results in a lot of photons (see exercise).

Remark: Stimulated emissions shown in Fig. 3will be lost through the walls of cavity.

(Fig. 3)

	•"Single" frequency (or wavelength), also known as monochromatic. •Coherent: photons are "vibrating" in phase. •Beam profile: Gaussian, i.e.	(Fig. 4)
(Fig. 5)

light intensity as a function of z is , where is a constant, and is the width of the profile.   Another way to visualize this property is depicted in Fig. 11 showing the electric field of light wave as a function of z.

(Fig. 6)

Remark: Laser beam intensity could have other distributions, an example is depicted in Fig. 12. Again the electric field of the light wave (at a certain instant) is shown as a function of z.

(Fig. 7)

• Small beam divergence

Typical value of is .

Example:

Gain means that signal got amplified (the "A" in laser). Substance, inside the laser cavity, that achieves population inversion and thus leads to laser action. It could be:
• gas
• liquid
• solid.

Lasers can be classified according the following different schemes.

(A) By active media
•
• Dye laser - active medium: dye molecules in liquid solvent (sometimes in solids also).
• Solid state laser - crystal, or glass, doped with impurities, e.g. ruby laser, Ti:sapphire laser, semiconductor laser.

(B) By mode of operation
• CW
• pulsed (see exercise)

(C ) By pumping and laser levels • 3-level laser • 4-level laser

(Fig. 8)

In this Section, we give examples of specific lasers and briefly describe their characteristics.

VI.1 He-Ne laser

(Fig. 9)

Remark: Recently, orange, yellow, and green outputs of wavelengths 612.0, 594.0, and 543.5 nm, respectively, are also available, typical output power.

VI.2 Pulsed laser [Transverse Excited Atmospheric (TEA) laser]

(Fig. 10)

• Laser cavity filled with gas at atmospheric pressure.
• Spark plugs create charges to facilitate discharge between the two flat electrodes.
• The electrodes have to be very smooth - avoid local discharge.
• Pulse width ~1 ns = sec.
• Wavelength , that is far in the infrared.

Remark: similarly, a pulsed laser (quite common) can be constructed. A laser lases at 337.1 nm (in the utra-violet spectrum).

Exercise: How long does it take for an electron to travel a distance of ~1 cm under 20,000 V?

VI.3 Dye laser

(Fig. 11)

• Pump sources - usually blue, green, violet, ultraviolet lasers.
• Dichroic mirror - allow ~90% of pumping laser light to pass through. But its reflectivity for the lasing wavelength (mostly yellow or read) is ~ 99.5%.
• Dye continuously flowing - avoid heating up.
• Tunable ¡V insert, into the cavity, a plate (or wedge) that is transparent, changes cavity property slightly, and thus changes the laser wavelength.
• Disadvantage: dye degradation upon prolonged irradiation by the intense pumping light, has to be changed ~ every 2 weeks, which is very laborious.
• Short pulse generation, ~ sec. pulse width can be achieved (see exercise). Remark: For the generation of this type of short pulses, the dye cell has to be replaced by a dye jet.

(Fig. 12)

The reason for using a dye jet is as follows:
A short pulse becomes boarder after passing a piece of thick (~ cm) material (say glass, water, etc).

VI.4 Ti:sapphire laser

Ti:sapphire means sapphire crystal doped with impurities. A Ti:sapphire laser is a dye laser with the dye cell replaced by a piece of Ti:sapphire.

(Fig. 13)

The reason for choosing sapphire is that its thermal conductivity is good compared to other non-metallic solids.
• Advantage - active medium suffers no degradation.
• Can be pumped ¡§harder¡¨, and hence higher output. By ¡§harder¡¨ we mean that the crystal is pumped by a higher blue-green laser power.
• Tuning range ~700 —> 1000 nm.

VI.5 Ruby laser (Earliest solid-state laser)

(Fig. 14 Schematic of a ruby laser. Capacitor bank is a cabinet containing many capacitors in parallel. The resultant capacitance could be as large as a Farad.)

• The first laser invented (in 1960).
• Pump source: flash lamp.
• Classic 3-level laser.
• Very low repetition rate ~1 pulse/min. Since its repetition rate is so slow, nobody wants to use this type of laser these days.
• Laser wavelength 694.3 nm, pulse width ~ sec.

VI.6 Semiconductor laser

(Fig. 15)

• Found in CD players.
• Recent years 20 W output at ~800 nm wavelength are available.
• Blue-green semiconductor lasers appear in recent years. The reason that people are interested in blue-green semiconductor lasers is clear. In a CD operated by blue-green would store about 4-times information as that (of the same size) operated by a red laser.

Question: where are the cavity mirrors?

VI.7 Excimer laser

(Fig. 16 RAB -- distance between the two atomic nuclei.B* represents an atom in an excited state.)

1. In their ground states, atoms A, B repel one another.
2. A, B* can form compound, but de-excite and dissociate quickly, and thus achieves a population inversion.

Examples

Excitation methods:

• Electron beam
• Discharge - similar to that of a TEA laser

Points to consider for successful operation:

• F, Cl, etc., are extremely corrosive, so cavity should be corrosion resisting. Teflon (a polymer) is commonly utilized for structural construction.
• Windows - transparent to UV.
• Active medium should be extremely pure. Otherwise, if the laser cavity is contaminated by a small amount of impurities, the laser cannot laser anymore.

Applications of Excimer lasers:

• Etching
• Medical, e.g. corneal sculpting
• Laser deposition of thin films

VI.8 X-ray laser
In recent year, there is intense interest in x-ray lasers. However, it is not easy of make a x-ray laser. One of the key problems is that there is no mirror that can reflect x-ray efficiently.

Note: Brewster's Angle (Window)

(Fig. 17 and are refractive indices of media 1 and 2, respectively. Double-headed arrows indicate direction of the electric field (E) of the optical radiation. ¡E indicates that E is pointing into the paper.)

(Fig. 18)

Reasons for using Brewster windows in a laser cavity are:
• Low loss in each trip through the window.
• Output laser light is polarized, which is convenient for many optical applications.