Discussion

7. Answer the questions to Text 11B:

1) Who was the first to predict acousto-optic interaction? What does the term mean? What devices are based on this in-teraction? 2) Describe the operation of the acousto-optic Q-switch. Use Figure 2 and the expression for Q factor. 3) What types of laser systems commonly use acousto-optic Q-switches? 4) What is the most commonly used acousto-optic medium? What applications require the use of other materials? Why?

8. Compare the configuration and operation of the rotating element Q-switch (Text 11A, Figure 1) with those of the acousto-optic Q-switch (Text 11B, Figure 2).

9. Render the text given below in English.

Коэффициент добротности Q определяется потерями лазерно-го резонатора; чем меньше потери, тем выше значение q. Рассмотрим лазерный резонатор, в котором перед одним из зеркал помещен затвор. При условии постоянной накачки активной среды и при закрытом затворе инверсная населенность активной среды в резонаторе продолжает увеличиваться и достигает максимального зна-чения. Если затвор закрыть, то величина инверсной населенности будет соответствовать начальному, гораздо выше порогового, значению; энергия, запасенная резонатором, будет выделяться в виде короткого импульса света с высоким максимальным значением интен-сивности. Поскольку при открывании затвора значение Q возрастает от очень малой величины до очень большой, эта методика получения короткого интенсивного импульса света получила название мгновенной модуляции добротности.

10. Read the following text and answer the questions:

1) What is the sensitivity of the system described? 2) What types of laser are used in the system? Why? 3) List possible applications of the fiber strain sensor.

TEXT 11C FO STRAIN SENSOR TO WATCH FOR QUAKES

A fiberoptic strain sensor being developed at the Los Alamos National Laboratory will watch for small strain in the earth’s crust, which may precede earthquakes. The system сan detect strains as small as 10^-10, which can develop over a period of years.

To achieve that sensitivity Los Alamos geophysicist Fred Homuth turned to an interferometric sensor based on two paral-lel singlemode fibers installed without cabling in a hole in the ground. One is isolated from the influence of the surroun-ding rock; the second is cemented to the rock so it experiences strains in the surrounding rock. Strain-induced alterations (изменения) in the effective path length of the second fiber are detected by comparing its output with that of the referen-ce fiber interferometrically and continually monitoring the output with a computer.

The sensing system will use about 200 meters of a special singlemode fiber with wavelength of 1095 nm. That special fi-ber is needed because Homuth is using helium-neon laser sour-ces emitting at 1152 nm. Those lasers were selected for their high stability and the low fiber losses at that wavelength.

Homuth noted that the fiber strain sensor could find ot-her applications including detecting underground nuclear ex-plosions, monitoring ground stability near waste storage and nuclear reactor sites, and monitoring stability of structures such as pipelines built on permafrost.

1500 п.зн.

SUPPLEMENTARY READING TASKS

Q-SWITCHING: THE ELECTRO-OPTIC ALTERNATIVE

Electro-optic Q-switching are used with virtually all high-power pulsed solid-state lasers and some infrared gas lasers that operate in the Q-switched or cavity-dumped modes. First applied in the early 1960s, electro-optic Q-switches have evol-ved into a variety of configurations and several crystal materials.

Typically, electro-optic Q-switching can be attained in a laser cavity having sufficient gain to sustain oscillation with the Q-switch in the optical path when the switch is in the “open” condition. Conversely, since a minute amount of optical leakage through the Q-switch almost always occurs when the switch is in the "off" condition, the "off"-state gain must be low enough to prevent the buildup of oscillations. Leakage, with subsequent pre-lasing, can be a problem with heavily pumped Nd:YAG lasers. The most common arrangement of components for Q-switching is the quarterwave configuration, wherein the voltage applied to the electro-optic crystal is just enough to induce quarterwave retardation in polarization at the laser wavelength.

The sequence of events is straightforward. First, quarter-wave voltage is applied to the Q-switch. After voltage is esta-blished, the flashlamp is fired and the laser rod is pumped to store energy. While quarterwave voltage is applied to the Q-switch, the roundtrip gain in the cavity is low; the cavity is in a lossy state. That is, the cavity quality factor, Q, is low and no oscillation is taking place. Next, while energy builds in the laser rod, the rod begins to emit spontaneously. This emission passes through the polarizer and becomes linearly polarized. Then, the beam passes through the Q-switch and be-comes circularly polarized (quarterwave retardation corresponds to circular polarization). After being reflected at the 100-per-cent reflective mirror, the beam again passes through the Q-switch and undergoes another quartarwave retardation. The re-sulting linearly polarized beam is polarized at 90° to the ori-ginal polarization. The new direction of polarization is blocked and the beam is deflected out of the laser cavity through a side escape window on the polarizer. This prevents the optical feedback necessary со build up oscillations in the cavity. Laser emission is thereby inhibited.

While the cavity is in a lossy state, the flashlamp conti-nues to рumр the laser rod until a level described as maximum po-pulation inversion is reached. At this level, additional flash-loop pumping cannst increase the amount of energy stored in the rod and only contributes to raising the temperature of the rod. Just after the population inversion is maximized and before the flashlamp is turned off, the voltage across the Q-switch is switched to zero (in about 5 to 10 ns). Now, the beam passes through the crystal without retardation, oscillations build up in the cavity, and a Q-switched pulse in generated.

Q-SWITCHING: THE DYE-СELL ALTERNATIVE

Of all the Q-switching techniques, by far the least expen-sive to implement is the dye-cell Q-switch. Unlike the others, which require complex electronic drivers and synchronization circuitry, the dye-cell Q-switch only requires the mixture of an appropriate organic dye and solvent and an intracavity cell to hold that mixture.

With most materials, optical transparency does not change appreciably with optical intensity until very high intensities are reached, when bulk damage can occur. Certain organic dyes, properly called saturable absorbers, are the opposite. For saturable absorbers, optical transparency is very low at low op-tical intensities. But as the optical intensity increases to mo-dest levels (well below the very high intensity levels typical of damage thresholds in most materials), the absorption coeffi-cient of the saturable absorber begins to drop sharply, and the material becomes increasingly transparent. This effect is often celled bleaching.

If such a material is placed inside a laser cavity, it will act as a closed shutter during the early buildup of laser action, blocking the optical feedback path by shielding one of the mir-rors. But as the intensity of spontaneous emission along the op-tical axis increases (in step with reaching high levels of ener-gy storage within the laser medium), the saturable absorber will quickly become transparent. This opens the optical feedback path, laser action quickly ensues, and the excess energy stored in the laser medium is quickly depleted, as with other types of Q-switches. As a practical matter, this limits dye Q-switching mostly to pulsed solid-state lasers, because continuous wave solid-state lasers typically do not produce enough intensity to bleach a saturable absorber. In keeping with the analogy of a Q-switch as an intracavity shutter, the dye-cell Q-switch would be a self -actuated shutter, unlike the electro-optic or acousto-optic versions which need external electronic control.

DYE-CELL Q-SWITCHES: OPERATIONAL DIFFICULTIES

If the foregoing (see text 2) were all there is to dye-cell Q-switching, other forms of Q-switching would not be used except for special cases. But the properties of the relatively few dyes useful for Q-switching solid-state lasers contribute to operatio-nal difficulties.

First of all, candidate dyes must have an absorption peak coincident, or nearly so, with the laser emission wavelength. As a practical matter, dyes used to Q-switch ruby or alexandrite lasers won't Q-switch Md:YAG or Nd:glass lasers. Also, while a fair number of organic dyes are available for Q-switching ruby or alexandrite lasers, only a handful of dyes with absorption peaks near 106O nanometers have been synthesized for use with Nd:YAG lasers.

Second, most dyes useful for Q-switching solid-state lasers are susceptible to breakdown from ultraviolet light. Care in storing the dyes is essential, as is the inclusion of an ultraviolet filter in the cavity to shield the dye from the output of the flashlamp. Often, this filter is a coating incorporated in the dye cell.

Third, the simplicity inherent in not having electronic dri-vers for dye Q-switches comes at a price: timing jitter. Because the statistics of laser buildup vary from pulse to pulse, the exact number of cavity round-trips before maximum bleaching oc-curs will also vary. This has the effect of causing jitter in the timing of the output pulse relative to the start of the pump pul-se. Any required synchronization must be done relative to the output pulse, using optical delay paths.

Fourth, the most difficult to deal with, the properties of the laser output from a dye-Q-switched laser depend strongly on the relaxation properties of the specific dye used. If the re-laxation time is of the order of a cavity roundtrip time, typically a few nanoseconds, Q-switching is simple. But if the dye re-laxes more quickly, say tens of picoseconds or less, it will tend to modelock rather than Q-switch the laser output. The key to encouraging modelocking behavior with fast-relaxation dyes is to design the laser cavity with all the intracavity components at Brewster’s angle; the key to suppressing modelocking behavior (in favor of Q-switching behavior) with such dyes is to design the laser cavity with all the intracavity components at right angles to the optical axis. In other words, polarization plays a key role in the Q-switching/modelocking of fast-relaxation dyes. Several dyes capable of Q-switching Nd:YAG lasers fall into this category. Other relax slowly enough to permit the switching of Nd:YAG lasers in the nanosecond regime.

While the discussion has stressed the use of dyes аs saturable absorbers for solid-state lasers, the saturable absorption effect is not limited to those materials and wavelengths. Carbon dioxide lasers emitting at the 10.6-micrometer band can be Q-switched by certain gases exhibiting saturable absorption. Perhaps the most commonly used gas for this task is sulfur hexa-fluoride. Q-switched pulse durations typically are about 1 to 2 ns.

MODULE 12 MODELOCKING Texts: A. Compact Picosecond Nd: glass Mode-Locked Laser with Variable Cavity Length B. Pulse Shortening by a Nonlinear Mirror-Mode Locker C. Transverse and Longitudinal Mode Selection

Terminology A:

1. mode - locked laser – лазер в режиме синхронизации мод;

2. pulse-to-pulse spacing – расстояние между импульсами;

3. remote sensing – дистанционное зондирование;

4. gain depletion – затухание усиления;

5. shot-to-shot optical excitation – эффективность оптической накачки;

6. extender mirror – зеркало для увеличения оптического пути.