H. G. Danielmeyer

Fluorescence in neodymium ultraphosphate

The properties of the Nd-ultraphosphate NdP 5 O 14 (NdUP), a crystalline material that appears to be a promising candidate for an efficient Nd laser, are described. The Nd concentration of 4.1021/cm3is ∼60 times higher than the upper limit useful for doped laser crystals ( \simI percent) like Nd:YAG. Despite the much higher Nd concentration the linewidths and cross sections of the major transitions are shown to be very similar to those of Nd:YAG. Therefore, it is expected that the gain per unit length in NdUP is also ∼60 times higher than in Nd:YAG. The crystal structure, the absorption and fluorescence spectra, level assignments, and various other features of NdUP are reported. Other rare earths form similar insoluble stable compounds. Most of them are transparent from the UV to the near IR except for the absorption bands of the metal ions.

Nd‐ultraphosphate laser

Laser action at λ=1.051 μm with a bandwidth of 1.4 nm is reported in a 35‐μm‐thick platelet of a neodymium‐ultraphosphate crystal (NdUP) placed within an optical resonator. Longitudinally pumped with a pulsed rhodamine 6G laser (λ=0.58 μm) the room‐temperature threshold was 40 μJ and the quantum efficiency slope was ∼18%. Also in glass of the same chemical composition laser action was observed.

Fluorescence quenching in Nd:YAG

We show that quenching of the Nd fluorescence is in principle not associated with the Nd3+ ion but with the host. The process is due to near-field electric dipole interaction between Nd pairs, and cross relaxation via the4I1 5/2 manifold. We present for the first time the complete fluorescence spectrum and level scheme of Nd∶YAG, and find that Nd∶YAG has an exceptional level configuration which boosts cross relaxation. Our results encourage the search for new Nd laser materials which have a slightly different position of the4I1 5/2 manifold so that higher Nd concentrations can be achieved for integrated-optics applications. Finally, we report on implications of energy migration which we found to be quite effective in Nd∶YAG.

Anisotropy in the laser performance of NdP5O14

We report, for the first time in a biaxial laser material, on the distribution of fluorescence cross sections in the index ellipsoid. The largest effective fluorescence cross section of pure NdP5O14 at room temperature (2.0±0.1) ×10−19 cm2 for light at 1.0512 μ wavelength polarized parallel to the 2 axis which coincides with the crystallographic b axis. We predict from the cross sections the cw thresholds and polarizations of the four highest‐gain laser transitions and verify the results quantitatively by operating one crystal in all major polarizations. We measured the major phase and group refractive indices and the thermal expansion coefficient of the optical lengths near 1.05 μ wavelength. We also discuss the twin domains which have different orientations of the crystallographic and optic axes. In imperfect crystals they can easily be created by thermal stresses which can drastically increase the laser threshold.

FREQUENCY CONTROL OF A PULSED PARAMETRIC OSCILLATOR BY RADIATION INJECTION

A singly resonant optical parametric oscillator (SRO) is pumped by a high power, pulsed source and is tuned approximately to a predetermined desired frequency. Radiation of that frequency is injected into a mode of the SRO from a stabilized, low-power injection source. The output of the SRO is a pulsed, high-power signal with frequency equal to the predetermined frequency to a high degree of accuracy.

Energy transfer and the complete level system of NdUP

We report on the complete fluorescence spectrum (52 transitions) and the full level system of neodymium ultraphosphate (NdUP) single crystals. Below 30 K the actual linewidths are below 0.1 Å indicating that there is one well defined site for all Nd ions which can be expected from a stoichiometric material. The position of the4I1 5/2 manifold does not allow for cross relaxation which explains the relatively long fluorescent lifetime of NdUP in spite of its high Nd-concentration. We find, however, lifetime shortening at high excitation rates which limits the optical gain of NdUP. We also find a very high rate of energy migration in NdUP which leads us to predict spontaneous single frequency oscillation for cw operation near threshold.

Continuous oscillation of a (Sc, Nd) pentaphosphate laser with 4 milliwatts pump threshold

We report on room temperature cw oscillation of a scandium neodymium pentaphosphate (Sc: NdP5O14) laser pumped with an argon laser. The threshold pump power is, to our knowledge, the lowest obtained so far at room temperature with any laser material.

Effects of Drift and Diffusion of Excited States on Spatial Hole Burning and Laser Oscillation

The rate equation for the population inversion of homogeneously broadened 4‐level lasers is solved for the cases of small and large mobility of excited states. Considered are drift and diffusion, i.e., collective and random motions along the laser axis. Both drift and diffusion smooth spatial holes which are burned into the population inversion by standing‐wave fields. A drift results also in a drag effect, i.e., a phase lag between the population inversion and the standing‐wave field. For homogeneously broadened lasers, sufficient smoothing of spatial holes will result in spontaneous full‐power single‐frequency operation. This is examined by calculating the unsaturated gains of secondary modes as a function of the excitation, drift, and diffusion parameters. Spatial hole burning is insignificant for gas lasers because of their large carrier diffusion. Furthermore, ionic drift in gas‐ion lasers alone could effectively quench spatial hole burning. This is in contrast to dye lasers where drifts, i.e., longi...

Stabilized efficient single-frequency Nd:YAG laser

It is shown how intracavity etalons can be optimally designed for axial mode selection and frequency stabilization of gas and crystal lasers. Relevant laser parameters, required etalon properties, and expected losses are determined. A simple frequency stabilization scheme based on birefringent etalons is described, which does not require laser modulation. Major problems associated with stable single-frequency operation of solid-state lasers are discussed. Using an optimally designed crystal quartz etalon, a Nd:YAG laser could be stabilized to 10-7in frequency and 2 percent in amplitude. The linearly polarized single frequency TEM 00 output was 150 mW, which compares to an unpolarized 12-mode output of 400 mW for the bare laser.

Diode‐Pump‐Modulated Nd:YAG Laser

We have investigated the response of a luminescent diode‐pumped cw Nd:YAG laser to a sinusoidal modulation of the pump intensity. For small modulation depths we found exact agreement with the relaxation oscillation resonance predicted by the linearized treatment of the coupled rate equations. For large modulation depths we obtained regular spikes at repitition rates between approximately 4 and 6 kHz with peak powers up to 100 times the cw power. Both the relaxation oscillation resonance (∼ 10–20 kHz depending on pump power) and the spiking could be self‐excited by means of a feedback loop.