Showing papers in "Optical and Quantum Electronics in 1990"

Silica waveguides on silicon and their application to integrated-optic components

Abstract: A marriage of optical fibre fabrication technology and LSI microfabrication technology gave birth to fibre-matched silica waveguides on silicon: thick glass layers of high-silica-content glass are deposited on silicon by flame hydrolysis, a method originally developed for fibre preform fabrication. Silica channel waveguides are then formed by photolithographic pattern definition processes followed by reactive ion etching. This ‘high silica (HiS) technology’ offers the possibility of integrating a number of passive functions on a single silicon chip, as well as the possibility of the hybrid integration of both active and passive devices on silicon. This paper reviews the NTT HiS technology and its application to integrated-optic components such as optical beam splitters, optical switches, wavelength-division multi/demultiplexers and optical frequency-division multi/demultiplexers. The clear and simple waveguide structures produced by the HiS technology make it possible to design and fabricate these components with high precision and excellent reproducibility.

All-optical waveguide switching

Abstract: We present a tutorial review of all-optical switching in fibre and integrated optics waveguides. These switching devices require non-linear refractive index changes in single or coupled waveguides, and involve either the low-power guided modes of the structure or soliton-type waves guided, emitted or captured by waveguides. We discuss the physical principles involved in these all-optical switching schemes, material requirements, recent experiments and limitations. A representative rather than comprehensive list of references is provided.

Non-linear population processes of Er3+ laser levels in chromium-doped garnet crystals

Abstract: In garnets doped with chromium and rare-earth ions, the processes of energy transfer, including upconversion, were investigated. Their influence on laser properties is established.

Ground-state depleted solid-state lasers: principles, characteristics and scaling

Abstract: A novel class of rare-earth-doped solid-state lasers is described. The ground-state depleted laser is pumped by an intense (more than tens of kW cm−2) narrow-band (less than a few nm) laser source and is characterized by: (1) an unusually low laser ion doping density (5 to 10×1018ion cm−3), (2) an unusually large fractional excited population inversion density (4 to 8×1018 ion cm−3, or >75%), (3) a gain element that is optically thick at the pump wavelength and (4) a gain element that has a substantially uniform gain distribution due to a bleaching of the pump transition at the pump intensity utilized. These features enable efficient room-temperature operation of rare-earth-ion laser transitions terminating on the ground manifold. The relationships between laser parameters (cross-sections, saturation fluences and fluxes, bleaching wave velocities, etc.) are given and laser performance scaling relationships are presented and discussed.

Efficient non-linear optical fibres and their applications

Abstract: There seem to be three possible approaches to achieve efficient non-linear optical effects in fibres: the increase of optical intensity in fibres, the control of group velocity dispersion in fibres and the use of efficient non-linear optical materials for fibres. From this viewpoint, high-numerical-aperture single-mode fibres with high optical intensity, single-mode fibres with extremely small dispersion at the operating wavelength and LiNbO3 single-crystal fibres with efficient non-linear effects, are now being investigated. This paper describes the fabrication of these optical fibres, and their non-linear optical applications.

Non-linear crystals for tunable coherent sources

Abstract: High-efficiency non-linear crystals suitable for the generation of coherent sources ranging from the deep ultraviolet to mid-infrared (0.16 to 5 µm) are explored. The key parameters for frequency conversion techniques (second-harmonic-generation, sumfrequency mixing, difference-frequency mixing, optical parametric oscillation (OPO) and stimulated Raman scattering) including tunability, beam quality, beam walk-off and the effective figure-of-merit are analysed. Materials selection rules for non-linear crystals (for wide transparency, high damage threshold, stability, low absorption and large figure-of-merit) are presented. Techniques using non-critical phase-matching, quasi-phase-matching, waveguided doubling and self-frequency doubling are explored. Features and applications of the new non-linear crystals ofβ-barium borate, lithium triborate (LBO), potassium titanyl phosphate (KTP), KNbO3 and MgO:LiNbO3 are presented and compared with other crystals. In addition to a critical review, this paper also presents some new results for the analysis of frequency-conversion efficiency, experimental data of OPO in KTP, LBO for frequency conversion using temperature-tuned non-critical phase-matching and green lasers from the diode-pumped self-frequency-doubling crystal of neodymium yttrium aluminium borate.

Spectroscopic and laser properties of Er3+-doped monoclinic BaY2F8 single crystals

Abstract: The luminescence and absorption properties of Ba(Y1−x Er x )2F8 (x=0.001, 0.01, 0.05, 0.1, 0.2 and 0.3) and the Er3+-ion decay kinetics of luminescent transitions from three initial laser states, the4S3/2,4F.9/2 and4I11/2 manifolds, were measured. The crystal-field splitting schemes for allJ-manifolds which are involved in (J →J′)-luminescence transitions and stimulated emission parameters of Er3+ -ions in BaY2F8 were determined. A comparison of laser powers and efficiencies of BaY2F8 and Y3Al5O12, Lu3Al5O12 and LiYF4 single crystals doped with Er3+-ions shows the similar performance of these materials.

GSAG and YSAG: a study on isomorphism and crystal growth

Abstract: The composition of gadolinium-scandium-aluminium (GSAG) and yttrium-scandium-aluminium (YSAG) garnet crystals grown by the Czochralski method and by floating zone melting was analysed. It is shown that Sc3+ and Al3+ ions in GSAG and Y3+, Sc3+ and Al3+ ions in YSAG can simultaneously occupy two types of sites in the lattice. The congruent melting composition of GSAG is {Gd2.88Sc0.12} [Sc1.77Al0.23] (Al3)O12. The homogeneity range of GSAG is rather narrow. In the Y2O3-Sc2O3-Al2O3 system a wide range of garnet-structured solid solutions exist. However, there is no ternary garnet that melts congruently. Distribution coefficients of components in YSAG are slightly varied from unity in the whole concentration range, and as the melt composition approaches YAG they tend to unity. Cr3+ and Nd3+ distribution coefficients in GSAG and YSAG are presented.

Organic second-order non-linear optical materials and devices

Abstract: Organic second-order nonlinear materials are currently receiving considerable attention for fabrication of frequency mixing and electro-optic devices. In this paper the key concepts in the development of organic nonlinear materials and waveguide device design are introduced. The recent advances in nonlinear waveguides for frequency doubling and electro-optic devices are reviewed.

Multisites Cr3+ in GGG and GSGG garnets

Abstract: The emission and excitation spectra of Cr3+ -doped Gd3Ga5O12 (GGG) and Gd3Sc2Ga3O12 (GSGG) are explained in the light of multisite effects. The situation is particularly complicated in the case of GSGG, where the different sites have2E energy levels near each other which overlap with the4A2 →4T2 absorption bands. The spectra obtained under selective excitations are interpreted on the multisite assumption.

Frequency-tunable semiconductor lasers

Abstract: Recent advances in frequency-tunable semiconductor lasers are reviewed. They are classified into hybrid and monolithic tunable lasers. Monolithic tunable lasers based on distributed Bragg reflector or distributed feedback laser structures are most attractive for practical applications. The device structures and the tuning characteristics are described, with emphasis on the tuning range, spectral linewidth and frequency-switching time. Recent system experiments using monolithic tunable lasers in an optical communication area are also described.

Fluoride fibres for optical transmission

Abstract: Optical fibres made from heavy metal fluorides have been under development for much of the past decade. There have been significant advances in understanding the fundamental characteristics of these materials. Progress towards achieving low optical losses and in optimizing the design of fluoride fibres for use in long transmission systems is reviewed.

Flashlamp-pumped laser performance of LiCaAIF6:Cr3+

Abstract: The results of flashlamp pumping of the LiCaAIF6:Cr3+ (Cr:LiCAF) laser crystal are reported. We have so far obtained slope efficiencies as high as 1.55% in a close-coupled, diffusely reflecting cavity. Based on the measured insertion loss of the presently available material, we predict that an efficiency of about 4% will be obtained when low-loss material becomes available. This extrapolated efficiency is comparable with the performance of a high-quality alexandrite laser rod in the same apparatus.

Upconversion-pumped population kinetics for4I13/2 and4I11/2 laser states of Er3+ ion in several host crystals

Abstract: Population kinetics of the upper4I11/2 and lower4I13/2 laser states of the Er3+ ion were studied experimentally and theoretically in (Er)BaY2F8, (Er)YLF, (Er)YSGG, (Er)CaF2 and (Er)YALO. Fluorescence from these states to the4I15/2 ground state was excited through upconversion simultaneously with the4I11/2 →4I13/2 lasing using 1.53 µm radiation from an erbium : glass laser for optical pumping. Lifetimes of both states are altered during lasing by co-operative energy transfer processes: the lifetime of the lower state τ1 is shortened and that of the upper state τ2 increased with the resultant ratio τ2/τ1>1. After lasing the lifetime ratio returns to the ‘normal’ value τ2/τ1 <1; that is, one obtained under weak ultraviolet excitation. Kinetic rate equations for the population density functions for both laser states were set up and solved by approximation in three time domains. It was assumed that only one co-operative energy transfer process operates in the laser crystals and determines the population inversion kinetics. Consistency relationships for comparison of the theoretical results with the experiment were developed. Only (Er)BaY2F8 spectral features showed close agreement with theory, resulting in a high score of 94% for the overall correlation in the consistency test, whereas all other crystals scored <50%. As a result of this high correlation, a close match between theoretical and experimental population decay curves was shown for (Er)BaY2F8. Most probably, more than one energy transfer process shapes the decay curves and determines the population inversion kinetics for the other laser crystals. (Er)YALO showed little lifetime change for the laser states, apparently due to inefficient co-operative energy transfer processes. As a result it probably lased in a self-terminating short-pulse mode.

High-power solid-state lasers with unstable resonators

Abstract: The thermal lensing properties of stable and unstable resonators are compared and rules are given for the design of optimized unstable resonators containing a focusing rod. Experimental verification with a high-power Nd: YAG system proved that for unstable resonators the restricting relationship between beam quality and output power no longer holds. Careful resonator design enables high output power to be extracted with unstable resonators without destroying the output couplers.

The role of excited-state absorption in tunable solid-state lasers

Abstract: The lasing properties of tunable solid state lasers can be strongly influenced by Excited State Absorption (ESA). It is shown that the tuning behaviour of Cr:GSGG and Cr:LLGG is limited at short wavelengths due to the ESA transitions2E →2T2 (b),2T1 (b) and4T2 →4T1 (b), respectively. At long wavelengths the4T2 →4T1 (a) ESA transition narrows the tuning range largely. The reason is that both levels are Jahn-Teller split due to the centric chromium site resulting in a very broad ESA band. In Ti: YAIO3 strong ESA of the pumping wavelength into a charge transfer band occurs, which is caused by a Ti3+/Fe3+ complex centre. By this transition the pumping efficiency is diminished largely. It is believed that the lasing behaviour of Ti: YAIO3 can be improved considerably by growing of highest iron-free crystals.

Simulations and experiments of mode-locking of semiconductor lasers: pulse evolution, frequency detuning, and bias dependence

Abstract: Simulations of a mode-locked diode laser based on a travelling-wave rate-equation model have been compared with experiments. The pulse measurement technique involved a conventional intensity autocorrelator together with an internally generated second-harmonic emission measurement set-up. The latter is ideal for systematic relative measurements. For the first time, pulse evolution as a function of the number of round-trips was measured. Short optical pulses were obtained after approximately 40 round-trips. The experimental and simulated detuning range was about 1 MHz and the d.c. bias dependence was investigated.

Efficient room-temperature operation of Cr3+-sensitized, flashlamp-pumped, 2µm lasers

Abstract: Utilizing the results of Cr3+ → Tm3+ transfer efficiency studies, we have demonstrated that yttrium aluminium garnet (YAG) is the preferred host for room-temperature, flashlamp-pumped solid-state lasers operating in the 2.0 µm spectral range. We report data on two different sensitizer-activator combinations in YAG and yttrium scandium gallium garnet (YSGG) laser materials: one is doped with Cr:Tm:Ho and operates on the Ho3+ 5I7 →5I8 transition at 2.097 µm; the other is doped only with Cr:Tm, which lases on the Tm3+ 3F4 →3H6 transition at 2.014 µm. We have achieved a slope efficiency of 5.1% with the Cr:Tm:Ho:YAG laser, which is the highest slope efficiency yet reported for a room-temperature, flashlamp-pumped, 2 µm solid-state laser. We have measured thresholds as low as 38 J and output energies >1.5 J for that system. We also report the first room-temperature operation of an efficient flashlamp-pumped Cr:Tm:YAG laser at 2.014 µm. Thresholds as low as 43 J, output energies exceeding 2 J, and slope efficiencies as high as 4.5% have been achieved. This is an order of magnitude higher than the efficiency previously reported for a 2.01 µm Cr:Tm:YAG laser operated at cryogenic temperatures. These two efficient 2 µm laser systems (Cr:Tm:Ho:YAG and Cr:Tm:YAG) are discussed in terms of their potential for Q-switched operation.

Energy transfer from Cr3+ multisites to Tm3+ multisites in yttrium aluminium garnet

Abstract: Multisite energy transfer from Cr3+ to Tm3+ was studied using low-temperature high-resolution laser spectroscopy. Selective excitation routes for Tm3+ sites were demonstrated using excitation and emission spectroscopy, and were confirmed by lifetime measurements.

Induced probe-beam modulation and resonatorless optical bistability in semiconductor (CdSxSe1−x)-doped glass

Abstract: Induced probe-beam modulation in semiconductor-doped glass was observed. The dephasing between pump and probe modulation is dependent on the pump power, the modulation frequency and the selenium concentration. At room temperature, with a proper choice of these parameters, the dephasing can change from 0° (laser-induced clearing) to −180° (laser-induced darkening). A slow switching behaviour and optical bistability without external feedback are also reported.

Optical isolators using Bi-substituted rare-earth iron garnet films

Abstract: Using Bi-substituted rare-earth iron garnet film, (GdBi)3 (FeAlGa)5O12 or (YbTbBi)3 Fe5O12, grown by liquid phase epitaxy technique, we have fabricated a compact, low cost and high performance optical isolator for near-infrared wavelength region, in comparison with conventional isolator using yttrium iron garnet (YIG) single crystal as Faraday rotator. Typical insertion loss and isolation of developed isolators are 0.6 dB and 35 dB respectively at λ=1.31 μm and the size is 6ϕ×6 mm. The isolators, which could be used in the wavelength range of 1.31 to 1.55 μm, are also fabricated with insertion loss of less than 1.1 dB and isolation of more than 35 dB.

Transverse interactions in 2-D feedback non-linear optical systems

Abstract: The new dynamic phenomena in 2-D feedback non-linear optical systems are investigated theoretically. Typical numerical results for phase-switching waves and suppression of phase distortions phenomenon are shown.

Effective cut-offs for modes on helical fibres

Abstract: Every mode of a single-mode or multimode helical fibre is always leaky but, for practical purposes, can be treated as being bound with an effective cut-off wavelength. The leakage loss for each mode is quantified, showing that, for fixed core offset and source wavelength, the cut-off pitch increases with increasing mode order. The value of the cut-off pitch for each mode is in agreement with experimental measurements.

Mechanical and hydrogen characteristics of hermetically coated optical fibre

Abstract: Corning's hermetically coated fibre has been evaluated for mechanical and hydrogen performance under severe test conditions using large sample sizes. The results indicate that the carbon coating is truly hermetic, and that it provides an effective barrier for preventing hydrogen and water from reaching the glass surface. As a result the hermetically coated fibre possesses superior resistance to fatigue and hydrogen permeation over standard fibre. These attributes make this hermetically coated fibre and excellent candidate for use in terrestrial communication and undersea cables, as well as in other special applications where a harsh environment is present.

Prismatic pulse compression: beam deviations and geometrical perturbations

Abstract: A generalized multiple-prism dispersion theory is utilized to provide a numerical description of dispersive behaviour in prismatic arrays designed for laser pulse compression. In addition, there is an assessment of the effect on overall dispersion caused by either beam deviations or by geometrical perturbations of the prisms.

Two-photon absorption and resonant non-phase-matched second-harmonic generation in CdSe

Abstract: Exciting a hexagonal CdSe crystal with picosecond Nd: glass laser pulses, two-photon absorption and resonant non-phase-matched second-harmonic generation occur simultaneously. Using different crystal orientations, all components of the secondharmonic susceptibility tensor (non-vanishing components ared 31,d 33 andd 15) and some components of the two-photon absorption susceptibility tensor χ (3)′′ (−ωL; ωL, ωL, −ωL) are determined.

Three-dimensional step discontinuities in planar waveguides: Angular-spectrum representation of guided wavefields and generalized matrix-operator formalism

Abstract: For obliquely incident guided modes the light propagation in slab waveguides with step discontinuities can be described by means of an angular-spectrum representation of the electromagnetic fields together with a generalized matrix-operator formalism. By means of a discretization of the radiation and evanescent modes, the computation of the reflection and transmission coefficients can be reduced to the numerical solution of algebraic equations. The TE-TM coupling, which takes place for oblique incidence, and the different evanescent field effects are discussed in detail.

Spectroscopy of multisites Cr3+ and Nd3+ in yttrium aluminium garnet

Abstract: Cr3+ and Nd3+ non-equivalent crystal field sites were observed. By use of a high-resolution dye laser,2E and4A2 splittings, zero-phonon4A2 →2E,2T1,4T2 transitions and vibronic levels of2E doublets of Cr3+ non-equivalent centres were recorded. Energy transfers from Cr3+ multisites to Nd3+ were studied by site-selective spectroscopy and fluorescence decays.

Holographic investigations on photoinduced changes in dichromated gelatin

Abstract: The investigations conducted resulted in the determination of the order of the primary photochemical reaction and of the photoinduced changes in the material parameters in the stages of latent image and after development. The necessity of using the multiwave coupled-wave approach is demonstrated, taking into account the characteristic curve of the material for a correct description of the diffraction efficiency behaviour.

Dye laser pulse shortening and stabilization by Q-switching

Abstract: The properties of passive Q-switched short-cavity dye lasers (SCDL) and passive Q-switched distributed-feedback dye lasers (DFDL) are investigated theoretically by using rate-equation models. The calculations, in agreement with experimental results, predict a significant decrease of the pulse energy fluctuation and the pulse duration of both the SCDL and DFDL, when passive Q-switching is used.