H. P. Weber

Metal-clad optical waveguides: analytical and experimental study.

Planar optical waveguides consisting of thin dielectric films with metal cladding have been investigated theoretically and experimentally. A computer program was devised to provide the phase and attenuation constants and wavefunctions for TE and TM modes in symmetric and asymmetric guides. Approximate expressions suitable for slide-rule calculation were also derived. Numerical results and illustrations are given for films of photoresist with Al, Ag, and Au cladding. Direct measurements of the attenuation and phase constants at 0.633 microm of numerous experimental waveguides are in reasonable agreement with theory. Attenuations <1 dB/cm, which is sufficiently small for application in devices, were measured. Calculated wavefunctions illustrate the mismatch of modes at transitions between unclad and metal-clad waveguides. Experimentally, we find substantial losses at such abrupt junctions. They can be overcome by simple tapered transitions.

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.

Loss Measurements in Thin-Film Optical Waveguides

An apparatus for the measurement of attenuation of light in thin-film waveguides is described. It involves a prism that rides on a liquid film along the guide and couples the light out of the guide. A spatial resolution of ~0.5 mm and an accuracy of 0.02 dB/cm were achieved.

Solution-deposited thin films as passive and active light-guides.

Light-guiding thin films of various materials have been deposited on glass substrates from liquid solutions by slow evaporation of the solvent. The attenuation of the guided light wave in some of these films is as low as that in the best films prepared by other methods. Film deposition from solution is done at or near room temperature. Therefore, this technique allows us to dope the thin films with organic molecules, e.g., laser dyes. The preparation and light-guiding properties of the doped and undoped films are described. The materials studied were epoxy, lead-silica, polyurethane, and Kodak photoresist KPR. Films of the first and second material have shown losses as low as 0.3 dB/cm at wavelengths of 0.633 microm and 1.064 microm, respectively. Films of polyurethane have been prepared with rhodamine 6G doping. When pumped with a pulsed nitrogen laser, the doped films showed optical gains of up to 100 dB/cm. The KPR films have an attenuation of ~1 dB/cm at 1.064 microm. This low optical loss, in combination with the hotographic sensitivity of the KPR, offers the possibility of fabricating integrated optical circuits directly in the photoresist films.

POLY(METHYL METHACRYLATE) DYE LASER WITH INTERNAL DIFFRACTION GRATING RESONATOR

We describe a compact monochromatic (0.1 A) dye laser, which consists of a pair of Bragg phase gratings photodielectrically induced inside a bulk sample of poly (methyl methacrylate), PMMA, doped with rhodamine 6G. The laser frequency can be selected by choice of grating period from the usual broad (∼200 A) emission spectrum of the dye.

Photolocking‐A new technique for fabricating optical waveguide circuits

Low‐loss dielectric optical waveguides have been made by a new process in which a photochemical reaction is used to lock or fix a dopant in a polymer film with a lower refractive index, and unreacted dopant is then removed by heating. Narrow guides ([inverted lazy s]4 μm wide, Δn [inverted lazy s] 1%) were formed by laser beam writing at λ=364 nm or by contact printing at λ=313 nm, and had losses of 0.20±0.05 dB/cm at λ=633 nm.

Multicomponent photopolymer systems for volume phase holograms and grating devices

Novel photopolymer systems for the fabrication of high-resolution volume phase holograms and grating devices are reported. Previously reported techniques use a single monomer (or a mixture of similar monomers) and rely solely on density modulation. In contrast, we have found it advantageous to use a mixture of components chosen to have differing reactivities and polarizabilities, which results in a composition modulation. During the image-forming exposure the more reactive monomer is polymerized while the less reactive species is excluded from the irradiated regions. Two-way diffusion, which must be invoked to explain our results, leads to a modulation of the chemical composition and hence of the polarizability of the final material. In some systems the polarizability effect was strong enough to exceed the density effect, even when these factors were in opposite directions. As in previous systems, an over-all exposure is used to fix the images. With appropriate monomer systems, stable images were obtained even when one component was unreactive. The peak-to-peak refractive index differentials achieved in our systems were as high as 1.5%. This is an order of magnitude larger than that reported for initially all liquid systems and 50% larger than that reported for related solid materials. The use of a liquid system enables us to fill small cavities with our materials and then record a high-resolution image. We have made gratings of >3000 lines mm(-1) inside hollow fibers, with 80 microm i.d., filled with a photopolymer material. For white light guided in the core, the expected band-stop filter response was observed.

A Thin‐Film Ring Laser

We report the operation and characteristics of a ring laser formed by a single‐mode light‐guiding thin film. The rhodamine 6G doped polyurethane film is coated on the surface of a cylindrical glass rod. This geometry establishes feedback for laser oscillation around the circumference of the rod. A nitrogen laser serves as pump source.

Thin birefringent polymer films for integrated optics

The fabrication of low‐loss (0.1 dB/cm) thin birefringent polymer films on glass substrates is reported. A birefringence of 4.7 × 10−3 was measured in polystyrene. In such films, TE and TM modes can be made degenerate by controlling the film thickness. Degeneracy over distances of up to 3 cm was achieved. Such guides may simplify the construction of modulators and other devices requiring degenerate mode operation.