The frequency downshifting of fundamental laser output via stimulated Raman scattering has been previously demonstrated and studied mainly with flash-lamp pumped Q - switched Nd:YAG lasers [1,2]. With the advent of diode - pumped lasers it was an issue of time when the technique of Raman conversion would be applied to such lasers. Recently there appeared a number of publications in which Raman lasing is reported in diode - pumped lasers (see, for example [2-4]).
Femtosecond laser technology based on pulse stretching-compressing technique has been demonstrating tremendous achievements [1], but it is still complicated and expensive. In this report we present a new generation of simple and robust high peak power diode-pumped Raman lasers. They are capable of producing laser pulses in picosecond range with pulse repetition rate (PRR) from f=10 Hz to f=7.5 kHz. Passat Ltd. has recently developed Raman lasers operating in three PRR formats: low PRR (1 - 25 Hz), medium PRR (100 Hz - 1 kHz), and high PRR (2.5 - 7.5 kHz). Table 1 shows parameters of Raman lasers supplied with harmonic generators for each of the PRR formats specified above.
We have demonstrated that direct excitation of 3rd Stokes Raman emission in crystal can produce short (few nanosecond) eye-safe pulses. Produced beam has very high quality and the pulse energy can be as high as tens of millijoules. For pulsed diode pumped solid state lasers the demonstrated repetition rate was 250 Hz but higher repetition rates are certainly achievable. It is important that tested schemes do not have strict requirements on laser pump parameters, namely beam divergence and frequency bandwidth. The obtained results are very relevant to the development of eye-safe lasers, such as the new generation of rangefinders, target designators, and laser tracking and pin-pointing devices, as well as remote 2D and 3D imaging systems.
We have demonstrated a very simple scheme to excite new lines in the near IR range via the DFM of 2nd harmonic of Nd:YLF laser and Stokes seed, in particular obtaining 943 nm emission. Energy efficiency for wave interaction in RTP crystal was equal to 27%. Being measured, nonlinear coefficient of Double Photon Absorption in this crystal showed 6×10-4 cm/MW.
This paper is devoted to a brief analysis on the development of 3D holography for the last 40 years. We also consider how 3D holography has impacted on the modern laser designs. Definitely, in the frame of such a short paper, there is no possibility to discuss many important issues related to this subject. Therefore, the author mainly concentrated on only one type of application. This is nonlinear optical wave interactions, involving 3D holographic grating. Sometimes such interactions are referred to as dynamic holography.
A review of nonlinear optical techniques to be used in tunable far IR lidar, radar and spectrometer system is presented. As an example, a tunable IR remote sensing technology demonstrator system operating at 8-12 μm is described. It includes an OPO and OPA pumped by Nd:YAG laser as well as Raman emitter and receiver pumped by the OPA output. Relatively small variations of an OPA wavelength (1.85-2 μm)provide a broad operative range at far IR (8-12 μm). The unique feature of OPO is pumping by two phase conjugated beams. A pulse from the pump laser is transmitted through the OPO. A portion of the transmitted pulse is reflected in a phase conjugate cell, and the remaining pulse energy is semi- compressed in stimulated Brillouin scattering (SBS) cells. These phase conjugate (PC) cells are filled with highly purified and stable SiCl4 and GeCl4 (R=80%). This process reduces the OPO threshold as well as forms short and narrow linewidth pulses. Being amplified up to 140 mJ in a 2 ns pulse in the OPA, these pulses are directed to a hydrogen Raman emitter. Due to auxiliary pumps at 1064 nm and 1907 nm with total energy approximately 30 mJ providing phonon excitation, the SRS conversion of an OPA output in the Stokes wave starts from a relatively high phonon seed so a quantum efficiency of more than 50 percent at far IR is achieved. The high sensitive detector of far IR weak signals (~1000 photons)is based on a Raman-induced four wave mixing of these signals with a delayed OPA output. A subsequent readout of the Raman grating is performed using a 532 nm pulse. The Raman-shifted 683 nm pulse is proportional to that of a weak IR signal.
Guerman Pasmanik, E. Shklovsky, Gennady Freidman, Vladimir Lozhkarev, Alexander Matveyev, Alexander Shilov, Ivan Yakovlev, Darrel Peterson, Judy Partin
Lidar systems developed over the last decade have demonstrated impressive results when applied to the detection of specific volatile chemicals. MOst of these systems are limited to a single wavelength or, at best, a narrow wavelength band. Exceptions are DIAL systems, CO2 lidars, and dye laser sources. Currently under development at INEEL and PASSAT Ltd. are technologies that convert Nd:YAG laser energy to the 8-11 micrometers band with an output of 20 millijoules/pulse or higher. Wavelength shifting is accomplished using a tunable optical parametric oscillator and amplifier, and stimulated Raman scattering cells as the emitter. This system can be made tunable continuously from 6-11 microns which makes this an eyesafe laser system. In addition, identical SRS cells are used as low noise, narrow band receivers that are sensitive to extremely low levels of scattered laser radiation. Use of this technology is to generate a pair of pulses at different wavelengths for DIAL applications. A description of this system will be provided along with test results.
Guerman Pasmanik, E. Shklovsky, Gennady Freidman, Vladimir Lozhkarev, Alexander Matveyev, Alexander Shilov, Ivan Yakovlev, Darrel Peterson, Judy Partin
The brief description of new lidar prototype for remote chemical monitoring and profiling in the 8 - 12 micron range is given. The lidar includes a Nd:YAG laser (1 J per pulse) source, optical parametrical oscillator (0.2 J per pulse), and four-wave Raman emitter (20 mJ output per pulse). The receiver consists of the hydrogen SRS cells, pumped by an additional OPO pulse. Sensitivity of this receiver reaches approximately 1000 photons per pixel. The applications of this lidar for remote detection of chemicals in atmosphere will also be discussed.
The possibility of using OPO and two-cascade OPO-SRS converters is considered both for the generation of probing pulses and for the amplification of radiation scattered in the atmosphere. The lidars based on such combined use of OPO and OPO-SRS converters are protected against the background radiation and ensure the probing distance in the IR a few times greater than when the conventional lidars are employed. It is shown that the use of two-cascade parametric and multicascade SRS image amplifiers will make it possible to record the spatial distribution of ultrashort radiation attenuated by a factor of 1013 divided by 1014, having an energy of order 1 mJ, for about 102 image cells.
The features of liquid purification from molecular and dispersive admixtures are studied. The analysis has revealed the processes (thermal effects, microparticles heating with a subsequent optical breakdown, stimulated Raman scattering) limiting pumping pulse energy. These effects complicate also a realization of a high quality phase conjugation at SBS. The data concerning physical properties of liquid tetrachlorides and freons are presented. The picture of a behavior of liquid under conditions of an optical breakdown is described. Some recommendations regarding a choice of nonlinear media are formulated. The two-cell scheme providing a phase conjugation of powerful short laser pulses is proposed.
Detection of different chemical species in the atmosphere is performed with a frequency- tunable middle IR laser. The design of lidar for this range is discussed. Two pulses can be used, one propagates to an object another provides receiving a reflected signal. When emitting, it is possible to start with YAG:Nd repetitively-pulsed solid-state laser radiation of 1.06 micrometers , 1 - 8 J, 10 - 100 Hz. The first pulse is further converted by OPO in 1.5 - 2.1 micrometers range and then into 4 - 14 micrometers radiation with SRS in hydrogen or other gases. To receive a weak reflected signal of 4 - 14 micrometers we propose using a hydrogen or other Raman amplifier which is pumped with the second OPO pulse (two pulses spaced by 2L/c, L being the distance from an object). A sensitivity for the method better than 10 photon per mode is achieved. For a wavelength less 4 micrometers it is possible to use only one OPO (for example by LiNbO3) as an emitter and receiver.
The basic principles of nonreciprocal optical systems with phase -conjugating mirrors design and their main properties are presented. Such system can form high quality images by means of telescope with distorting large primary mirror. The demands to quality of the mirror are hundreds of times less strict, than for the common type telescope. Several different realizations of such systems were investigated experimentally and numerically.
In present time six types or siliconorganic materials are
commonly used in microelectronic. tecImolog: polysiloxanes,
polysilozanes , polysilmethylenes , polysilphenylenes , organic
polymers with silicon getheroathoms and polysilanes [1 ] .
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