Multiple-prism grating laser oscillator

Multiple-prism grating laser oscillators,^[1] or MPG laser oscillators, use multiple-prism beam expansion to illuminate a diffraction grating mounted either in Littrow configuration or grazing-incidence configuration. Originally, these narrow-linewidth tunable dispersive oscillators were introduced as multiple-prism Littrow (MPL) grating oscillators,^[2] or hybrid multiple-prism near-grazing-incidence (HMPGI) grating cavities,^[3]^[4] in organic dye lasers. However, these designs were quickly adopted for other types of lasers such as gas lasers,^[5]^[6] diode lasers,^[7]^[8] and more recently fiber lasers.^[9]

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ULTRASONIC INTERFEROMETER Aim: To calculate the velocity of ultrasonic sound through different liquid mediums. Second, to calculate the adiabatic compressibility of the given liquid. Apparatus: Ultrasonic interferometer, sample liquids, high frequency generator etc. Theory: Ultrasonic interferometer is a simple device that yields accurate and consistent data, from which one can determine the velocity of ultrasonic sound in a liquid medium. Ultrasonic sound refers to sound pressure with a frequency greater than the human audible range (20Hz to 20 KHz). When an ultrasonic wave propagates through a medium, the molecules in that medium vibrate over very short distances in a direction parallel to the longitudinal wave. During this vibration, momentum is transferred among molecules. This causes the wave to pass through the medium. Ultrasonics can be produced by different methods. It includes mechanical method, magnetostriction method, piezoelectric etc. In an ultrasonic interferometer, the ultrasonic waves are produced by piezoelectric method. Ultrasonic waves of known frequency are produced by a quartz crystal that is fixed at the bottom of the ultrasonic cell. There is a movable metallic plate parallel to the quartz plate, which reflects the waves. These waves superimpose and standing waves are produced in the liquid medium. At this point, acoustic resonance occurs and this gives rise to an electrical reaction on the generator driving the quartz plate. Now the anode current of the generator becomes maximum. If we increase or decrease the distance, and the variation is exactly one half of the wavelength (?/2) or it’s multiple, the anode current again becomes maximum. If ‘d’ is the separation between successive adjacent maximum anode current, then, d = ?/2. We have, the velocity (v) of a wave that is related to its wavelength (?) by the relation, v=f?, where f is the frequency of the wave. Then, v= ?f= 2df. Adiabatic compressibility of a fluid is a measure of the relative volume change of the fluid as a response to a pressure change. For a medium with high compressibility, the velocity will be less. The adiabatic compressibility (ß) of the material of the sample can be calculated using the equation, where ? is the density of the material of the medium and v is the velocity of the sound wave through that medium. PROCEDURE: Arrange the ultrasonic interferometer and the liquid samples that are to be used. Take a liquid sample in a beaker. Unscrew the knurled cap of the interferometer. Pour the sample into the interferometer cell. Then tight the cap. Switch on the power supply. There are two knobs on the instrument- “Adj” and “Gain”. With the “Adj” knob, the position of the needle on the ammeter is adjusted. The knob “Gain” is used to increase the sensitivity of the instrument. When we move the micrometer the anode current begins to increase. Again move the micrometer. Then at a particular distance, the anode current becomes minimum. Note down the anode reading and micrometer reading. Repeat the procedure till we get at least ten maxima and minima. Now draw a graph between the micrometer distance and anode current. We will obtain a sine wave. The difference between the points of successive maxima will be d=?/2. Using the equation, calculate the velocity of the wave through the medium as, v= ?f= 2df. Knowing the density of the medium, the adiabatic compressibility can be calculated using the equation Repeat the experiment using different samples. Applications: Sonographic instruments are widely used in medical field. In medical sonography or Ultrasonography, an ultrasound-based diagonostic medical imaging technique is used to visualize internal organs and study their size and structure. Ultrasonic interferometry is a new possible technique for high accuracy linear and angular measurements. Some other common applications of Ultrasonic are ultrasonic Range finding, which is also called as SONAR, Ultrasonic testing which is used to find any flaws in materials and to measure the thickness of objects, Ultrasonic cleaners, ultrasonic humidifier, ultrasonic welding and so on. Ultrasonic laser interferometers are used in various areas including metrology.

Excitation

Multiple-prism grating laser oscillators can be excited either electrically, as in the case of gas lasers and semiconductor lasers,^[11] or optically, as in the case of crystalline lasers and organic dye lasers.^[1] In the case of optical excitation it is often necessary to match the polarization of the excitation laser to the polarization preference of the multiple-prism grating oscillator.^[1] This can be done using a polarization rotator thus improving the laser conversion efficiency.^[11]

Linewidth performance

The multiple-prism dispersion theory is applied to design these beam expanders either in additive configuration, thus adding or subtracting their dispersion to the dispersion of the grating, or in compensating configuration (yielding zero dispersion at a design wavelength) thus allowing the diffraction grating to control the tuning characteristics of the laser cavity.^[11] Under those conditions, that is, zero dispersion from the multiple-prism beam expander, the single-pass laser linewidth is given by^[1]^[11]

\Delta \lambda \approx \Delta \theta \left(M{\partial \theta  \over \partial \lambda }\right)^{-1}

where $\Delta \theta$ is the beam divergence and M is the beam magnification provided by the beam expander that multiplies the angular dispersion provided by the diffraction grating. In the case of multiple-prism beam expanders this factor can be as high as 100–200.^[1]^[11]

When the dispersion of the multiple-prism expander is not equal to zero, then the single-pass linewidth is given by^[1]^[11]

\Delta \lambda \approx \Delta \theta \left(M{\partial \theta  \over \partial \lambda }+{\partial \phi _{2,m} \over \partial \lambda }\right)^{-1}

where the first differential refers to the angular dispersion from the grating and the second differential refers to the overall dispersion from the multiple-prism beam expander.^[1]^[11]

Optimized solid-state multiple-prism grating laser oscillators have been shown, by Duarte, to generate pulsed single-longitudinal-mode emission limited only by Heisenberg's uncertainty principle.^[12] The laser linewidth in these experiments is reported as $\Delta \nu$ ≈ 350 MHz (or $\Delta \lambda$ ≈ 0.0004 nm at 590 nm) in pulses ~ 3 ns wide, at power levels in the kW regime.^[12]

Applications

Applications of these tunable narrow-linewidth lasers include:

Coherent anti-Stokes Raman spectroscopy and combustion diagnostics^[13]
LIDAR^[14]
Laser spectroscopy^[15]^[16]
Atomic vapor laser isotope separation^[17]^[18]

References

^ ^a ^b ^c ^d ^e ^f ^g F. J. Duarte, Narrow-linewidth pulsed dye laser oscillators, in Dye Laser Principles (Academic, New York, 1990) Chapter 4.
^ F. J. Duarte and J. A. Piper, A double-prism beam expander for pulsed dye lasers, Opt. Commun. 35, 100-104 (1980).
^ F. J. Duarte and J. A. Piper, A prism preexpanded grazing incidence pulsed dye laser, Appl. Opt. 20, 2113-2116 (1981).
^ F. J. Duarte and J. A. Piper, Narrow linewidth high prf copper laser-pumped dye-laser oscillators, Appl. Opt. 23, 1391-1394 (1984).
^ F. J. Duarte, Multiple-prism Littrow and grazing incidence pulsed CO₂ lasers, Appl. Opt. 24, 1244-1245 (1985).
^ R. C. Sze and D. G. Harris, Tunable excimer lasers, in Tunable Lasers Handbook, F. J. Duarte (Ed.) (Academic, New York, 1995) Chapter 3.
^ P. Zorabedian, Characteristics of a grating-external-cavity semiconductor laser containing intracavity prism beam expanders, J. Lightwave Tech. 10, 330–335 (1992).
^ P. Zorabedian, Tunable external cavity semiconductor lasers, in Tunable Lasers Handbook, F. J. Duarte (Ed.) (Academic, New York, 1995) Chapter 8.
^ T. M. Shay and F. J. Duarte, in Tunable Laser Applications, 2nd Ed., F. J. Duarte (Ed.) (CRC, New York, 2009) Chapter 9.
^ F. J. Duarte, T. S. Taylor, A. Costela, I. Garcia-Moreno, and R. Sastre, Long-pulse narrow-linewidth disperse solid-state dye laser oscillator, Appl. Opt. 37, 3987–3989 (1998).
^ ^a ^b ^c ^d ^e ^f ^g F. J. Duarte, Tunable Laser Optics, 2nd Ed. (CRC, New York, 2015).
^ ^a ^b F. J. Duarte, Multiple-prism grating solid-state dye laser oscillator: optimized architecture, Appl. Opt. 38, 6347-6349 (1999).
^ R. J. Hall and A. C. Eckbreth, Coherent anti-Stokes Raman spectroscopy: applications to combustion diagnostics, in Laser Applications (Academic, New York, 1984) pp. 213-309.
^ W. B. Grant, Lidar for atmospheric and hydrospheric studies, in Tunable Laser Applications, 1st Ed. (Marcel-Dekker, New York, 1995) Chapter 7.
^ W. Demtröder, Laserspektroscopie: Grundlagen und Techniken, 5th Ed. (Springer, Berlin, 2007).
^ W. Demtröder, Laser Spectroscopy: Basic Principles, 4th Ed. (Springer, Berlin, 2008).
^ S. Singh, K. Dasgupta, S. Kumar, K. G. Manohar, L. G. Nair, U. K. Chatterjee, High-power high-repetition-rate capper-vapor-pumped dye laser, Opt. Eng. 33, 1894-1904 (1994).
^ A. Sugiyama, T. Nakayama, M. Kato, Y. Maruyama, T. Arisawa, Characteristics of a pressure-tuned single-mode dye laser oscillator pumped by a copper vapor oscillator, Opt. Eng. 35, 1093-1097 (1996).

External links

Diagrams of MPG laser oscillators

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This page was last edited on 15 October 2022, at 05:16

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