Section: 10 | Characteristics of Laser Sources |

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CHARACTERISTICS OF LASER SOURCES

William F. Krupke

Light amplification by stimulated emission of radiation was first demonstrated by Maiman in 1960, the result of a population inversion produced between energy levels of chromium ions in a ruby crystal when irradiated with a xenon flashlamp. Since then population inversions and coherent emission have been generated in literally thousands of substances (neutral and ionized gases, liquids, and solids) using a variety of incoherent excitation techniques (optical pumping, electrical discharges, gas-dynamic flow, electron-beams, chemical reactions, nuclear decay).

The extrema of laser output parameters that have been demonstrated to date and the laser media used are summarized in Table 1. Note that the extreme power and energy parameters listed in this table were attained with laser systems rather than with simple laser oscillators.

Laser sources are commonly classified in terms of the state-of- matter of the active medium: gas, liquid, and solid. Each of these classes is further subdivided into one or more types as shown in Table 2. A well-known representative example of each type of laser is also given in Table 2 together with its nominal operation wavelength and the methods by which it is pumped.

The various lasers together cover a wide spectral range from the far ultraviolet to the far infrared. The particular wavelength of emission (usually a narrow line) is presented for some six dozen lasers in Figures 1A and 1B.

By suitably designing the excitation source and/or by controlling the laser resonator structure, laser systems can provide continuous or pulsed radiation as shown in Table 3.

Besides the method of excitation and the temporal behavior of a laser, there are many other parameters that characterize its operation and efficiency, as shown in Tables 4 and 5.

Although many lasers only emit in one or more narrow spectral “lines,” an increasing number of lasers can be tuned by changing the composition or the pressure of the medium, or by varying the wavelength of the pump bands. The spectral regions in which these tunable lasers operate are presented in Figure 2.

Reference

Krupke, W. F., in Handbook of Laser Science and Technology, Vol. I, Weber, M. J., Ed., CRC Press, Boca Raton, FL, 1986.

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TABLE 1. Extrema of Output Parameters of Laser Devices or Systems

Parameter	Value	Laser medium
Peak power	1 × 10¹⁴ W (collimated)	Nd:glass
Peak power density	10¹⁸ W/cm² (focused)	Nd:glass
Pulse energy	>10⁵ J	CO₂, Nd:glass
Average power	10⁵ W	CO₂
Pulse duration	3 × 10^-15 s continuous wave (cw)	Rh6G dye; various gases, liquids, solids
Wavelength	60 nm ↔ 385 µm	Many required
Efficiency (nonlaser pumped)	70%	CO
Beam quality	Diffraction limited	Various gases, liquids, solids
Spectral linewidth	20 Hz (for 10^-1s)	Neon-helium
Spatial coherence	10 m	Ruby

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TABLE 2. Classes, Types, and Representative Examples of Laser Sources

Type (characteristic)	Representative example	Nominal operating wavelength (nm)	Method(s) of excitation
Gas
Atom, neutral (electronic transition)	Neon-Helium (Ne-He)	633	Glow discharge
Atom, ionic (electronic transition)	Argon (Ar⁺)	488	Arc discharge
Molecule, neutral (electronic transition)	Krypton fluoride (KrF)	248	Glow discharge; e-beam
Molecule, neutral (vibrational transition)	Carbon dioxide (CO₂)	10600	Glow discharge; gasdynamic flow
Molecule, neutral (rotational transition)	Methyl fluoride (CH₃F)	496000	Laser pumping
Molecule, ionic (electronic transition)	Nitrogen ion (N₂⁺)	420	E-beam
Liquid
Organic solvent (dye-chromophore)	Rhodamine dye (Rh6G)	580–610	Flashlamp; laser pumping
Organic solvent (rare earth chelate)	Europium:TTF	612	Flashlamp
Inorganic solvent (trivalent rare earth ion)	Neodymium:POCl₄	1060	Flashlamp
Solid
Insulator, crystal (impurity)	Neodymium:YAG	1064	Flashlamp, arc lamp
Insulator, crystal (stoichiometric)	Neodymium:UP(NdP₅O₁₄)	1052	Flashlamp
Insulator, crystal (color center)	F₂^–:LiF	1120	Laser pumping
Insulator, amorphous (impurity)	Neodymium:glass	1061	Flashlamp
Semiconductor (p-n junction)	GaAs	820	Injection current
Semiconductor (electron-hole plasma)	GaAs	890	E- beam, laser pumping

figure 1a

FIGURE 1A. Wavelengths of lasers operating in the 120 to 1200 nm spectral region.

figure 1b

FIGURE 1B. Wavelength of lasers operating in the 1300 to 12,000 nm spectral region.

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TABLE 3. Temporal Characteristics of Lasers and Laser Systems

Form	Technique	Pulse width range(s)
Continuous wave	Excitation is continuous; resonator Q is held constant at some moderate value	∞
Pulsed	Excitation is pulsed; resonator Q is held constant at some moderate value	10^-8 – 10^-3
Q-Switched	Excitation is continuous or pulsed; resonator Q is switched from a very low value to a moderate value	10^-8 – 10^-6
Cavity dumped	Excitation is continuous or pulsed; resonator Q is switched from a very high value to a low value	10^-7 – 10^-5
Mode locked	Excitation is continuous or pulsed; phase or loss of the resonator modes is modulated at a rate related to the resonator transit time	10^-12 – 10^-9

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TABLE 4. Properties and Performance of Some Continuous Wave (CW) Lasers

Parameter	Unit	Ne-He (gas)	Ar⁺ (gas)	CO₂ (gas)	Rhodamine 6G dye (liquid)	Nd:YAG (solid)	GaAs (solid)
Continued on next page...
Excitation method		DC discharge	DC discharge	DC discharge	Ar⁺ laser pump	Krypton arc lamp	DC injection
Gain medium composition		Neon:helium	Argon	CO₂:N₂:He	Rh 6G:H₂O	Nd:YAG	p:n:GaAs
Gain medium density	Torr	0.1:1.0	0.4	0.4:0.8:5.0
	ions/cm³				2(18):2(22)	1.5(20):2(22)	2(19):3(18):3(22)
Wavelength	nm	633	488	10600	590	1064	810
Laser cross-section	cm^-2	3(-13)	1.6(-12)	1.5(-16)	1.8(-16)	7(-19)	~6(-15)
Radiative lifetime (upper level)	s	~1(-7)	7.5(-9)	4(-3)	6.5(-9)	2.6(-4)	~1(-9)
Decay lifetime (upper level)	s	~1(-7)	~5.0(-9)	~4(-3)	6.0(-9)	2.3(-4)	~1(-9)
Gain bandwidth	nm	2(-3)	5(-3)	1.6(-2)	80	0.5	10
Type, gain saturation		Inhomogeneous	Inhomogeneous	Homogeneous	Homogeneous	Homogeneous	Homogeneous
Homogeneous saturation flux	W cm^-2			~20	3(5)	2.3(3)	~2(4)
Decay lifetime (lower level)	s	~1(-8)	~4(-10)	~5(-6)	<1(-12)	<1(-7)	<1(-12)
Inversion density	cm^-3	~1(9)	2(10)	2(15)	2(16)	6(16)	1(16)
Small signal gain coefficient	cm^-1	~1(-3)	~3(-2)	1(-2)	4	5(-2)	40
Pump power density	W cm^-3	3	900	0.15	1(6)	150	7(7)
Output power density	W cm^-3	2.6(-3)	~1	2(-2)	3(5)	95	5(6)
Laser size (diameter:length)	cm:cm	0.5:100	0.3:100	5.0:600	1(-3):0.3	0.6:10	5(-4):7(-3);2(-2)^a
Excitation current/voltage	A/V	3(-2):2(3)	30:300	0.1:1.5(4)		90:125	1.0/1.7
Excitation current density	A cm^-2	0.15	600	6(-3)		140	4.5(3)

^a Junction thickness:width:length.
^b Pressure dependent.

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CHARACTERISTICS OF LASER SOURCES

Reference

TABLE 1. Extrema of Output Parameters of Laser Devices or Systems

TABLE 2. Classes, Types, and Representative Examples of Laser Sources

TABLE 3. Temporal Characteristics of Lasers and Laser Systems

TABLE 4. Properties and Performance of Some Continuous Wave (CW) Lasers

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