Answer to Question #155633 in Optics for kaashfi

Question

In color center laser, what are the active medium, host material, and laser wavelength? Illustrate the factor, on which laser wavelength depends?

Answer 1

The active medium is a thin crystal, into which point defects have been introduced deliberately. The resulting microscopic defects in the crystals tend to absorb light, coloring the normally colorless crystals and earning the name color centers. Color centers are found in many types of normally colorless crystals. Laser action in color centers, first seen in 1965 (Fritz and Menke, 1965) is mostly in alkali halide crystals.

Color centers in alkali halide crystals can be applied as high-gain active materials in tunable solid-state lasers. When cryogenically cooled and optically pumped, these laser crystals have low threshold pump powers, relatively high output powers, and are smoothly tunable over a large fraction of their broad emission bands. Using various color center types and host lattices, the combined tuning range of color center lasers covers presently the near-infrared region from about 0.8 to 4 μm. In single-mode CW operation, color center lasers have extremely narrow spectral line-widths; in mode-locked operation, they can provide pulses with ultranarrow temporal widths. These properties make them very attractive for spectroscopic studies requiring high spectral or temporal resolution. This paper reviews the optical properties of the main laser-active color center types, the techniques used for their production, and their laser characteristics. Furthermore, a brief summary is given on the application of color center lasers.

This chapter discusses color center lasers. Certain color centers in the alkali halides can be used to make efficient, optically pumped, broadly tunable lasers for the near-infrared. Like their dye-laser counterparts, the color center lasers can be operated CW as well as in a pulsed mode. In the CW mode, pump powers at threshold are quite modest—often as low as 20 mW. In pulsed operation, outputs of the order of 10 kW have been obtained, and it is reasonable to expect this figure can be scaled up by at least an order of magnitude. Frequency definition of the color center lasers is excellent, with performance in this respect promising to equal or exceed that attained with the best dye lasers, in either CW or pulsed operation. In particular, there are no fundamental obstacles to the production of an oscillator-amplifier combination, capable of ∼100 kW output pulses, continuously tunable over the range 3000–4000 cm^–1. In this chapter, some pertinent color center physics is discussed, and processes for color center formation are elaborated. Some useful formulas are developed for the optical gain or amplification that can be attained in a four-level system. Laser cavities with a highly concentrated modal beam are also analyzed.

The lasing material can be a solid, liquid, gas or semiconductor, and can emit light in all directions. The pump source is typically electricity from a power supply, lamp or flash tube, but may also be another laser. It is very common in Princeton University laboratories to use one laser to pump another.

The excitation medium is used to excite the lasing material, causing it to emit light. The optical cavity contains mirrors at each end that reflects this light and cause it to bounce between the mirrors. As a result, the energy from the excitation medium is amplified in the form of light. Some of the light passes through the output coupler, usually a semi-transparent mirror at one end of the cavity. The resulting beam is then ready to use for any of hundreds of applications.

The laser output may be steady, as in continuous wave (CW) lasers, or pulsed. A Q-switch in the optical path is a method of providing laser pulses of extremely short time duration. The Q-switch may use a rotating prism, a pocket cell, or a shutter device to create the pulse. Q-switched lasers may produce a high-peak-power laser pulse of a few nanoseconds duration.

A continuous-wave laser has a steady power output, measured in watts (W). For pulsed lasers, the output generally refers to energy, rather than power. The radiant energy is a function of time and is measured in joules (J). Two terms are often used to when measuring or calculating exposure to laser radiation. Radiant Exposure is the radiant energy divided by the area of the surface the beam strikes. It is expressed in J/cm2. Irradiance is the radiant power striking a surface divided by the area of the surface over which the radiant power is distributed. It is expressed in W/cm2. For repetitively pulsed lasers, the pulse repetition factor (prf) and pulse width are important in evaluating biological effects.