A conventional Fizeau wedge (FW) is built of two flat reflecting surfaces inclined at a small angle. They form a gap with linearly increasing thickness normally to the wedge ridge. Such FWs with apex angles of 5–100 microradians and 5– 1000 micrometers thickness find application in optical metrology, spectroscopy and laser spectral control. The apex angle, the reflection coefficients and the refractive index in the gap form a unique interference pattern on both FW’s sides. To benefit from a large free spectral range of thin wedges and high spectral resolution of thick wedges, recently we proposed a stack of two FWs with matched parameters. Matching provides the same change of the resonant wavelength at the same lateral displacement of both wedges. The aim of this study is to develop a technique for calculation of the resultant transmission of the stack of two matched wedges at plane wave illumination that is based on determination of the number and optical path differences of the rays leaving the stack at a given point on its rear surface. Only rays with non-negligible contribution as an amplitude are taken into account. Numerical simulation and experimental verification are provided.
The optical interferential wedge or Fizeau wedge (FW) is a useful optical element with various applications in optical metrology, spectroscopy and laser technique. Various FW applications require knowledge of its response to illumination by a laser beam with an arbitrary wavefront. Recently, we applied the plane wave expansion method to study transmission and reflection of an air-gap FW under illumination with a Gaussian beam. The approach is based on the angular spectrum of the beam and the known FW response to illumination with a plane wave. In this study, we adapt this approach for the more general and more frequently encountered case of a FW with a non-air gap. We developed an approximate algorithm, which is applicable at small incidence angles to wedges with refractive indices different from 1 and illuminating beams with arbitrary amplitude and phase distributions. Comparison to the experiment is also provided.
The focus of the work is on development and implementation of competitive optical elements based on tunable interference wedged structures. Such a structure can be a single interference wedge (two reflecting surfaces separated by a gap with increasing thickness) or a composition of two or more superimposed wedged layers with adjusted parameters. We used these structures to build a new wavelength division multiplexing (WDM) element and realized coupling of these elements with a fiber optical system as an issue of essential interest for optical communications. Under illumination with a multi-wavelengths beam, the composed WDM structure in the fiber system provides precisely controllable wavelength selection (resolution better than 0.01 nm) within the range of more than 15 nm and with controlled continuously variable transitivity from 1-3 to 80 %. The non-transmitted power with the other non-selected and completely reflected light is directed to the next output (theoretical loss of the system ~ 5 %). The WDM-structure works at completely independent spectral selection of each output/input without any influence between the tuning of the channels.
Based on our experience in the field of the interferential wedge, we present a new competitive application of such optical elements and structures built from them with theoretical description and experimental verification. We have demonstrated that a conventional interferential wedge or a structure of two wedges in compact layered implementation can be used as an attractive simple light power splitting element for a spatially and spectrally narrow light beam (basically a laser beam). The element can provide: 1) precisely and variably controlled ratio of the reflected and transmitted power by simple sliding in its plane of the list-like wedged structure with an apex angle of ~10-5 rad; 2) division practically without energy losses; 3) power ratio control without causing change of propagation direction of the reflected and the transmitted beams that is of essential interest for applications in optical schemes with complex geometry of the beams propagation; no optical properties variation of the reflective layers (mirrors) during the sliding; 4) working at beam power densities of MW/cm2 - GW /cm2 when the optical element is built from optical materials (layers) with high light damage resistivity.
Interference Wedged Structure (IWS) is an optical element with useful properties for optical metrology, spectral analysis and optical communications. We have introduced in the paper a new perspective element of this type – Composite Tunable Interference Wedged Structure (CTIWS). The CTIWS is list-like sequence of superimposed wedged layers each with reflecting surfaces. For conveniently chosen apex angles and thicknesses of the layers, the CTIWS can assure high spectral selectivity to 0.01nm within a spectral range of 10 nm and more at smooth tunability by simple sliding of the structure along the wedge arm (a few cm). We have developed simple physical description of the IWS and CTIWS by adapting Fabry-Perot theory. We show that for the most important practical cases the results are similar to the obtained by more complex exact analytical description. The theoretical predictions are confirmed by experimental results. On the base of IWS and CTIWS combined in a suitable architecture, we have introduced and studied a new lossless Wavelength Division Multiplexing (WDM) element with independent tuning of each output/input. We considered the WDM implementation for the case of fiber optical systems used in optical communications.
We have studied a structure composed from two wavelength tunable wedged interferometers and proved its potential for providing an increased free spectral range in comparison to a conventional interferential wedge. The interferential wedges with optical thickness from several micrometers to several hundred micrometers and apex angles of the order of tens microradians have been used. We have computed transmission for a monochromatic light beam with arbitrary wavefront using the angular spectrum approach for interferential wedges with different optical thicknesses and apex angles. We have conducted experiments with several stacks formed from different two wedges. Thus, we have confirmed selection of a single transmission resonance within the impact area of a large diameter beam instead of occurrence of multiple transmission peaks observed for a single interferential wedge. The stack enables wavelength tuning for a small diameter multi-wavelength beam in an increased spectral range at keeping high spectral resolution if the structure is formed by a thin and a thick interferential wedges. The experiments have shown that the selected transmission peak is narrowed spatially and spectrally at the expense of lower transmission, which is 50-60% of transmission provided by the used interferential wedges.
By using the developed by us approaches and instrumentation, we have obtained and presented series of systematized
data, which are important for the use of the laser light in infrared (IR) spectral region. The obtained data include: 1) reflectivity of the human tooth dentin; 2) the spatial intensity distribution in the cross-section of the light beam penetrating the tooth’s dentin; 3) the absorbed and the diffused parts of the laser light that have been determined separately through combination of optical and calorimetric techniques. The last result is the most important because it permits to calculate the dentin absorption and scattering coefficients. The study is performed for the laser light at two
easily generated wavelengths – 1.06 μm and 1.36 μm, emitted by the Nd:YAG laser that is well known, commercially
available, economical and widely used in many laboratories and medical institutions. The study is made on the basis of
fresh in-vitro teeth samples from the persons of Bulgaria, Sofia region.
On the base of our previous experience in the dual-color lasers we present our actual development - experimental realization, theory of a flash-lamp pumped Nd3+:YAG laser, that produces output at two desired (tunable) lines in the spectral range of 1μm -1.44 μm in free lasing and Q-switching operation. The laser operation avoids the wave’s competition effect for the two generations operating in two different parts of the active volume of an active single crystal and two closely disposed prism-selected resonators. Also, as we have shown, varying the partial laser volumes and the losses at each resonator in combination, it is possible to produce the generation at the two desired wavelengths with near equal output energy, peak power simultaneously or at desired time interval. We have found and treated some peculiarity of the formation of the generation, such as the actually used parts of the partial volumes.
We present two novel applications of optical properties of the interference wedge (compact realization of the Fizeau
Interferometer) for the case of illumination with a small diameter laser beam. The first one is a new and competitive
wavelength division multiplexing structure. The main advantages of the device are the possibility for separate frequency
tuning of each input-output channel as well as its compactness. The made computer simulation proves that the structure
can work with very short laser pulse duration (~ 0.1 ns), which corresponds to pulse repetition rate of order of ten GHz
and more, thus fulfilling the requirements of modern digital communication systems. The theoretical analysis and the
experimental check with laboratory model of a free-optical communication system show a sufficient resonant narrowline
transmission up to 75% with the linewidth of emission from 0.05 nm to few nm. The second proposal is a device
which allows for distant laser measurement of small linear translation of a rigid object for distances from a few meters
up to hundred meters.
We present two original, all optical techniques, to produce a narrowline laser light, fixed at the frequency of a chosen
reference atomic absorption transition. The first type of systems is an essential improvement of our method 3,4 for laser
spectral locking using a control by two frequency scanned, competitive injections with disturbed power ratio by the
absorption at the reference line. The new development eliminates the narrowing limiting problem, related with the fixed
laser longitudinal mode structure. We have proposed an original new technique for continuously tunable single mode
laser operation in combination with synchronously and equal continuous tuning of the modes of the amplifier. By
adapting the laser differential rate equations, the system is analyzed theoretically in details and is shown its feasibility.
The results are in agreement with previous our experiments. The essential advantage, except simplicity of realization, is
that the laser line can be of order of magnitude and more narrowed than the absorption linewidth. The second system is
based of the laser amplifier arrangement with a gain knock-down from the competitive frequency scanned pulse, except
at the wavelength of the desired absorption reference line. The essential advantages of the last system are that the
problem of fixing laser mode presence is naturally avoided. The theoretical modeling and the numerical investigations
show the peculiarity and advantages of the system proposed. The developed approaches are of interest for applications in
spectroscopy, in DIAL monitoring of the atmospheric pollutants, in isotope separation system and potentially - for
creation of simple, all optical, frequency standards for optical communications. Also, the continuously tunable single
mode laser (and the combination with the simultaneously tunable amplifier) presents itself the interest for many practical
applications in spectroscopy, metrology, and holography. We compare the action and the advantages of the two systems
proposed.
Recently, using an aopposite injection we have introduced a novel laser injection-locking approach for high gain (~104-108) and linear amplification of modulated low power (~μW-mW) laser radiation. In this work we report the investigation of the noise characteristics of such amplifier at the example of high repetition rate pulse pumped linear dye lasers. The background emission of the laser at the laser modes outside the signal and counter injection wavelengths is considered as a main source of the noise. By modeling the laser action with adapted system of differential equations and numerical analysis, solving this system for the cavity modes, we have obtained the total noise and its spectral distribution. The results show that for suitably chosen conditions the intracavity noise is very low or acceptable (~10-2 with respect to the signal output). The essential dependencies of noise characteristics from the system parameters are carried out.
We report a simple technique for generation of tunable subnanosecond pulses (0.1-0.3 ns) based on single spike selection from the relaxation oscillations in a dye laser by use of an active mirror. The selection is in original two-wavelength cavity with competitive generations at two wavelengths, the second of which is forced in appropriate moment by the quickly included amplification of the active mirror to suppress the initially started lasing in the first channel. To produce such switching we form the pump pulse by temporal division of a 15-50 ns standard pump laser pulse using a Pockel's cell with a 1-2 ns switching time and an optical delay line. We have shown that in addition, such formation increases essentially the selected spike power. The proposed technique widens the choice of suitable lasers in which the single spike selection technique may be applied to include dye lasers excited by a Q-switched solid-state laser or a Cu-vapor laser. The other advantages in comparison with the given in the literature single-spike selection techniques is the improved reproducibility of the selection for high (~100-200%) pump pulse power fluctuations, shorter pulse duration and pulse shape improvement. Both theoretical consideration and experimental verification are carried out.
We demonstrate and investigate (theory, experiment) a peculiar mode of cw Yb3+:doped crystal laser operation when two emissions, at two independently tunable wavelengths, are simultaneously produced. Both emissions are generated from a single pumped volume and take place either in a single beam or in spatially separated beams. The laser employs original two-channel cavities that use a Passively Self-Injection Locking (PSIL) control to reduce the intracavity losses or a very simple for practical realization two near collinear channels cavity. A two-wavelength, single, longitudinal mode generation is also obtained. The results reported are based on the example of Yb3+:GGG laser and Yb3+:YAH laser. The lasers operate in the range 1023-1033 nm and 1030-1040 nm, respectively with a total output power of ~0.4 W.
We have developed and investigated a simple solution of tunable laser with strongly decreased background emission (max pick-to-pick power ratio ≤ 10-8) by generation at additional wavelengths, except the selected. The cavity scheme permits to obtain easily laser output only at the selected wavelength, using a simple system with interference wedges. The spectral position and the losses at both generations are easily and independently controlled to needed optimal values. The theoretical treatment based on the rate equation system, shows that for the appropriate chosen conditions the technique developed permits to reduce by approximately two orders of magnitude the intensity of the background emission about the selected line (pick power to be less than 10-8). The investigation is on the example of pulsed dye lasers. The results are applicable for other tunable lasers with homogeneously broadened active medium such as Ti:sapphire, diode, etc. The lasers of considered type (with high spectral purity) are of essential interest for application in biomedical investigations of tissue luminescence spectrum, in Raman spectroscopic investigation, in isotope separation.
We present two simple and effective approaches for widening of the tuning range of the two-wavelength generation in two-channel laser cavity. The first is based on the use of combination of interference wedge and linear neutral filter and the second on the use of active mirror. This widening is combined with near equal intensities in both emissions. The use of the active mirror assures a real simultaneous two-wavelength operation. We consider a dye laser.
We present new solutions of a diode-pumped, cw Yb:YAG laser that is of interest for applications in spectroscopy and metrology. The proposed schemes permit producing of the emission in a single pumped volume at two independently tunable wavelengths, including single-mode regime. A passively self-injection (PSIL) controlled operation that ensures low threshold and high output is presented. The peculiarities of the two-wavelength operation of the considered laser are carried out. The laser operates in the range 1028 - 1035 nmw with a total output power of 0.4 - 0.7 W.
We propose a simple interferometer-type-device that permits to produce a variable length (1 - 15 ns) rectangular laser pulses, starting from a long (approximately 10 - 50 ns) conventional pulse that is emitted by Q-switched Nd3+-doped laser. The system uses an one time switched electro-optical pulse division at the input laser pulse and cutting all-optically a rectangular part of one of the formed partial pulses by appropriate bleaching of interferometer (Fabry-Perot, Fizeau) which gap is an Cr4+:YAG plate. The precisely controlled bleaching is produced in reliable manner using the second partial pulses and an optical delay line. The length of the obtained rectangular pulse can be precisely varied between 1 and 15 ns. The technique proposed can be applied also for the other lasers.
We describe and analyze an original technique for all-optical and high gain (approximately 103 - 106) amplification of a periodically modulated (PM) intensity low-power (approximately (mu) W) laser radiation. The amplification is based on the injection locking control in linear laser schemes with homogeneously broadened active medium. The main problem of the injection locking amplifiers is to obtain a linear response to the PM injection. We propose to solve this problem by injecting a second laser beam with a constant power and a slightly different wavelength or orthogonal polarization compared to the injected PM beam.
We show, that the unbalanced bi-directional passive self- injection permits to obtain a pulsed unidirectional operation at the reference atomic absorption line in a ring Ar3+ pumped Ti3+:Al2O3 laser with a repetition rate of order of 103 - 105 Hz. The model used for the description of this type of laser is based on the adaptation of the rate equations systems. By computer simulation, the dependence for the repetition rate from the laser parameters and the switching time is obtained.
We report an original approach for tunable subnanosecond pulse (200 - 500 ps) generation in a ring dye laser. It is based on limitation of generation to a single `spike' from the starting transient process using for excitation a quasi- rectangular pulse with duration less or comparable with the time interval between the spikes. The pumping pulse (3 - 7 ns) is formed both by electrooptical division of a standard nanosecond (15 - 50 ns) pulse emitted by a Q-modulated laser in an original scheme and by overlapping of the obtained partial pulses which are fed bi-directionally into the active medium of the ring laser.
We demonstrate how a pulsed two-wavelength tunable Ti:Al2O3 laser can operate at near maximum efficiency, obtained in an optimized non-selective cavity. To achieve this efficiency we solve the intrinsic of the Passive Self- Injection Locking (PSIL) method problem of non-controlled free lasing at high pump level by combining the PSIL control with an appropriate dual-pulse pumping. Laser efficiency is increased and the background emission is limited to less than 1 percent in a region ten times larger compared with the standard PSIL control. From analysis we have found optimum conditions and show the advantages of this combined technique. The proposed approach is compared with spectral control of a Ti:Al2O3 laser, realized by injection from a coupled two-wavelength dye laser.
A novel approach for narrow-line selection and wideband tuning of lasers using a self-selection interferometer is applied to obtain a high spectral purity emission. A comparison on the base of the background emission level is made between a proposed and a conventional cavity scheme with same selector. The results of theoretical investigation and experiments show a decreasing of the background emission in more than two orders of magnitude for the new realized scheme. Such kind of lasers are of special interest for application in Raman spectroscopy and isotope separation systems.
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