The temporal characteristics of a distributed feedback dye laser are investigated both theoretically and experimentally. It is shown that the feedback is provided by the gain modulation and that the effect of refractive index modulation is negligible. The solution of the coupled rate equations predicts the generation of picosecond pulses. The mechanism of short pulse formation is self Q -switching which differs fundamentally from mode locking. A novel experimental arrangement for pumping the distributed feedback laser is described which makes it possible to obtain transform-limited pulses even with a partially coherent pumping source such as the N 2 laser. The experimental results show excellent qualitative and satisfactory quantitative agreement with the computer solutions. A very simple and reliable method for generation of tunable, nearly transform-limited single pulses with 36 ps duration (FWHM) is demonstrated experimentally. The feasibility of picosecond and subpicosecond pulse generation in the entire visible and near IR region is discussed.

Tunable picosecond pulse generation by an N 2 laser pumped self Q-switched distributed feedback dye laser

Kiselev [10] has solved the boundary equations for an asymmetric slab waveguide with a shallow sinusoidal corrugation, and derived explicit, closed expressions for the TE radiation loss coefficients of various waveguide geometries. For many practical applications in integrated optics, the Rayleigh assumption justifying the boundary-matching procedure and the approximations of linear perturbation analysis employed in this derivation are both satisfied. We have extended this procedure to TM modes and found that the expressions for both TE and TM modes in all of the geometries so treated agree identically with those derived by coupled-mode theory. The dependence of the TM radiation loss coefficient in each waveguide geometry on the grating period, radiation angle, refractive index profile, and mode polarization is illustrated graphically, and compared with its TE counterpart.

Radiation loss coefficients of asymmetric dielectric waveguides with shallow sinusoidal corrugations

The conventional rate equations for a Q-switched laser are augmented to explicitly include the effects of time- and level-dependent pumping, thermalization among the sublevels in the upper and lower multiplets, and multiplet relaxation in a homogeneously broadened four-level laser medium. To make the numerical computations more generally valid, we introduce a number of dimensionless variables. We show that the initial set of five coupled differential equations can be reduced to a simple set of two coupled equations for the inversion density and photon flux. Via numerical modeling, we have investigated the manner in which both thermalization and lower multiplet relaxation affect Nd:YAG laser characteristics such as output energy and temporal waveform. Our numerical results confirm earlier predictions that the Q-switched Nd:YAG laser output energy increases monotonically by a factor of 3.33 as one progresses from the assumption of slow to rapid thermalization and by an additional factor of 1.46 if one further assumes a terminal multiplet relaxation which is fast relative to the resonator photon decay time. We also find that the laser pulsewidth is substantially broadened when the resonator photon decay time is comparable to the thermalization, and to a lesser extent, the terminal multiplet relaxation times.

Effects of thermalization on Q-switched laser properties

Waveguide grating couplers allow the essential function of waveguide access to be integrated together with optical processing functions on a monolithic planar substrate. Grating coupling technology is topologically and, to a large extent, technologically compatible with the planar processes which define the optical processing waveguide circuit. Poor coupling efficiency, highly dispersive character, absence of user-friendly modelling tools and fabrication difficulties have long prevented this technology from being implemented into practical sensors and microsystems. This paper makes a review of the points where decisive technological progress has been made, illustrates some useful features of waveguide coupling gratings and underlines some of the difficulties which the designer may encounter.

https://iopscience.iop.org/article/10.1088/0963-9659/5/4/010/pdf

Coupling gratings as waveguide functional elements

We have measured the terminal level lifetime (τ11/2) for the 1-μm neodymium transition in several laser media, using a novel pump (2.41-μm) and probe (1.06-μm) technique. This method allows us to populate the 4I13/2 level directly and subsequently to monitor the terminal level 4I11/2 population as a function of time by observing the change in integrated fluoresence of the 0.88-μm emission for each time delay between the pump and the probe pulses. We developed a computer model to analyze the data and determined the upper and lower limits for the τ11/2 lifetime. The results for some of the materials investigated are 115–225 ps for Nd:Y3Al5O12, 250–450 ps for Nd:LG-750 (phosphate glass), 535–740 ps for Nd:LG-660 (silicate glass), 896–1900 ps for Nd:YAlO3, and 10.5–20 ns for Nd:YLiF4. In addition, we found the lifetimes to be independent of the neodymium doping concentration for the phosphate and silicate glass samples investigated.

Direct measurements of the terminal laser level lifetime in neodymium-doped crystals and glasses

A theoretical analysis is made of the influence of interference on the emission of surface light waves from a corrugated part of a thin-film waveguide. It is shown that the interference effects are strongest when the film is corrugated on the substrate side. A strong dependence of the attenuation coefficient of a wave on the film thickness and its modulation period is confirmed experimentally. Recommendations are made on the efficient coupling of light into the film.

Emission of surface light waves from a corrugated part of a thin-film waveguide

An investigation was made of resonant reflection of a surface wave and simultaneous coupling of light out of a corrugated section of a waveguide with a period equal to the wavelength of light in the waveguide. It was found theoretically and experimentally that the reflection coefficient could be close to unity. The ratio of the power coupled out of the corrugated section to the power incident on that section was highest when the reflection coefficient was ~ 0.3?0.4. An efficient method for making contact between a miniature laser and a corrugated section of a thin-film waveguide was tested experimentally.

Emission and reflection of light by a corrugated section of a waveguide

An investigation is made of the influence of the polarization of a light wave on its emission from a corrugated section of a waveguide. It is shown that the asymmetry in the dependence of the attenuation coefficient on the direction of emission into the media adjoining a waveguide is important for E waves.

Emission of E waves from a corrugated section of a waveguide

The relaxation time of the transition from the lower laser level 4I11/2 to the ground state 4I9/2 of the Nd3+ ions was determined experimentally for a crystal of Y3Al5O12:Nd3+. The relaxation time was measured by two independent methods. The value obtained, τ21 ≤ 3·10–7 sec, was associated with the very broad (up to ~ 900 cm–1) phonon spectrum of the garnet crystal.

MEASUREMENT OF THE RELAXATION TIME τ21 OF A Y3Al5O12:Nd3+ CRYSTAL

A simple method is suggested for calculation of reflection, radiation and transmission coefficients for the distributed feedback structure in the second diffraction order. The method is based on a slight difference between coefficients of reflectionR and radiationI of the surface wave for λ=λ∞ (where λ∞ is the light wavelength corresponding to a precise resonance for the grating length I→∞) and those for λ=λl (where λl is the light wavelength corresponding to the resonance for the finite grating length). The simplicity of the method makes it possible to use it for optimization of the distributed feedback structure by a number of parameters. The technique can be used in the case of thin-film and diffused waveguides for both TE and TM modes.

A A Zlenko

Papers

Emission of surface light waves from a corrugated part of a thin-film waveguide

Emission and reflection of light by a corrugated section of a waveguide

Emission of E waves from a corrugated section of a waveguide

MEASUREMENT OF THE RELAXATION TIME τ21 OF A Y3Al5O12:Nd3+ CRYSTAL

Calculation and optimization of parameters of radiating distributed feedback structures