Coupling prisms for light waveguides applications

Customer wrote:

I am looking for coupling prisms for light waveguides applications of index 1.78. I am using thin films of index 1.78 and wanted to use prism film coupler experiment. Please for clear understanding see the fig 3b in attached file. If you have half prisms or triangular prisms with prism angles 60° please give details of that. I will go for order.

referenced article recommends to use rutile prism.

The prism material should be as hard as possible to avoid damaging the prism base by repeatedly pressing films on it. Some suitable materials are the various high-index glasses, crystals like SrTiO3 and TiO2 (rutile), and Si and Ge for theIR (infrared). In the case of rutile, a uniaxial birefringent crystal, the optical axis should be oriented parallel to the edge of the angle E. Only this orientation allows us to use in Eq. (4) a constant index n, which is the ordinary index for TM polarization, and the extraordinary index for TE. It need hardly be mentioned that the prism faces enclosing the angle E should have a good optical polish. They must be flat to about λ/2 in order to define the angle E with sufficient accuracy. The prism may be replaced, in principle, by a hemispherical cylinder. The input beam then would pass the cylindrical surface at nearly normal incidence for all angles , thus extending the N range and simplifying the evaluation. An analysis of the hemispherical coupler shows, however, that it is critical with respect to small lateral displacements of the input beam, which must point exactly to the center of the cylinder. The prism-coupler, in comparison, is insensitive to parallel displacements of the input beam or of the prism.


Measurement of Thin Film Parameters with a Prism Coupler
R. Ulrich and R. Torge

The prism coupler, known from experiments on integrated optics, can be used to determine the refractive index and the thickness of a light-guiding thin film. Both parameters are obtained simultaneously and with good accuracy by measuring the coupling angles at the prism and fitting them by a theoretical dispersion curve. The fundamentals and limitations.of this method are discussed, its practical use, and mathematical procedures for the evaluation.


Del Mar Photonics offer variety of rutile prisms. We have standard 5x5x5 right angle prisms always in stock and can supply many other stanadsr and custom prisms as well as hemispherical cylinder couplers.

Rutile (TiO2) coupling prisms and their applications - buy online - download brochure

Del Mar Photonics offers optical elements made of high quality synthetically grown Rutile Titanium Dioxide crystals. Rutile’s strong birefringency, wide transmission range and good mechanical properties make it suitable for fabrication of polarizing cubes, prisms and optical isolators. Boules having high optical transmission and homogeneity are grown by proprietary method. Typical boules have 10 - 15 mm in dia. and up to 25 mm length. Optical elements sizes - from 2 x 2 x 1 mm to 12.7 x 12.7 x 12.7 mm. Laser grade polish quality is available for finished elements. So far we the largest elements that we manufactured are 12 x15 x 5 mm, in which optical axis is parallel to 15 mm edge, 5 mm is along beam path, 12 x 15 mm faces polished 20/10 S/D, one wave flatness, parallelism < 3 arc.min. (better specs. available on request).

Standard Specifications (buy online):

Rutile (TiO2) coupling prism
Material: single crystal TiO2
Sizes: 5x5x5 mm +/- 0.2 mm
Angles:45-45-90 deg.+/-10 arcmin
Polish quality: 20/10Scratch/Dig
Surface flatness: lambda @ 633nm
Parallelism: <5arcmins
Orientation: Z-axis along prism thickness +/- 15 arcmin

Examples of research done or proposed to be done using rutile coupling prisms


Nonlinear Optics in Whispering Gallery Mode Resonators

By Irina Novikova, Matt Simons and David Gribbin, the College of William and Mary

The reliable and efficient generation of an electromagnetic field with non-classical statistics, such as "squeezed" light or single-photon wave-packets, is important for a number of applications from reduced measurement uncertainty to new secure quantum information protocols. Nonlinear processes in optical crystals, such as second-harmonic generation and spontaneous parametric down-conversion, are currently the best and the most common ways to produce non-classical light. However, traditional experimental arrangements with bulk nonlinear crystals are rather involved and usually require high-power lasers and high-quality optical cavities.
Our research group explores the potential for using high-Q crystalline whispering gallery mode resonators (WGMRs) to accomplish low-threshold nonlinear frequency conversion. Such cavities support the modes of light traveling along the circumference of a polished disk (or sphere) through total internal reflection (TIR). Because no actual mirror is used, extremely high quality factors can be achieved in WGMR. Theoretically, the lifetime of a photon inside the cavity is limited by the scattering on the impurities. This limit depends on a crystal, but generally the quality factor ranges from 109 in LiNbO3 to over 1013 in CaF2. More realistically, the Q-factor of a microresonator is limited by the quality of surface polishing. The efficient coupling of laser radiation in a WGM in a crystalline disc is possible by means of frustrated total internal reflection in a coupling prism, as shown in Figure 1. If the rim of a disk is close enough to the reflecting surface of the prism, an evanescent wave tunnels across the gap and that light excites one or several WGMs. It is crucial that the index of refraction of the prism is higher than that of the disc, and thereby the optical coupling is achieved at the critical angle. Rutile with its high refractive index makes a perfect material for coupling to most nonlinear crystals.

The quality factor is a measure of the lifetime of energy stored in the cavity - the longer the lifetime, the more energy can build up in the cavity. For a very high-Q cavity even a low input light intensity can turn into a very intense field in the resonator. For example, a WGM cavity with a Q-factor of 1010 will support a photon for a millisecond (10-3 s), and that is significantly higher than the round trip time (typically in nanosecond scale). In our experiments we have achieved the quality factor of 107 as shown in Figure 2.
Significantly higher values will be achieved with improved polishing. This long cavity lifetime combined with small mode volume makes the crystalline WGMR attractive for the quantum optics applications. In particular, we are interested in observing narrow-band low-threshold second-harmonic generation as a first step toward the generation of heralded single photons. Our ultimate goal is to produce high-quality WGM discs that convert a laser light at 795nm to 397nm and vice versa, since 795nm is the wavelength of the D1 spectral line in Rubidium. Such nonclassical light will be organically integrated with the atomic quantum memory and slow light experiments conducted by our group. However, as the first step we are practicing polishing LiNbO3 disks and observing nonlinear conversion of 1064 nm pump laser light into 532nm second harmonic, as shown in Figure 3. Efficient frequency conversion can be achieved since the non-critical phase-matching (i.e. the matching of the refractive indices for the fundamental and doubled optical frequencies) is possible by tuning the temperature of the nonlinear material. The pump and generated fields are orthogonally polarized, and thus the dichroic rutile prism offers additional benefit for separating them, as they couple out of the WGM disc at significantly different angles.

 Irina Novikova
Telephone: (757) 221-3693
FAX: (757) 221-3540
Office: Millington 249
Department of Physics
College of William&Mary
P.O. Box 8795
Williamsburg, VA 23187-8795
Research group web-site


Research description for a Rutile coupling prism in the development of electrically pumped organic semiconductor thin film lasers.

This description will give a brief insight into our research at the Light Technology Institute of the University of Karlsruhe (TH), Germany. One of our research fields is the development of electrically pumped organic semiconductor thin film lasers. Due to the complex behavior of these lasers numerous electrical and optical characterization is necessary. One of the most important optical properties of these organic semiconductor laser structures is the attenuation coefficient of the multilayer waveguide, which has to be carefully optimized to reduce waveguide losses 1. The first step in the optimization process is the numerical simulation of the anticipated waveguide design. Next, the optimized sample structure is fabricated and characterized in our attenuation measurement setup. This measurement is done as follows:
A Rutile coupling prism is pressed onto the waveguide. A laser beam is then coupled into the prism so that total reflection occurs inside the prism at the interface to the waveguide. In the vicinity of the waveguide the overlapping incident and reflected beam generate a standing wave. The evanescent field of that standing wave penetrates into the waveguide.

Evanescent field coupling

Under a certain angle and if the phase match conditions are fulfilled, the evanescent field stimulates a mode that is guided by the waveguide. The phase match condition can only be achieved when the refractive index of the prism is at least as high as the effective refractive index of the waveguide. Owing to its high refractive index, Rutile is an ideal material for use as a coupling prism in such a prism-coupler waveguide attenuation measurement setup.

Beam coupling into waveguide

A small fraction of the guided light is scattered out of the waveguide. The intensity of this scattered light is assumed to be proportional to the intensity of the guided light. Thus the intensity distribution inside the waveguide along the propagation direction can be directly determined through measuring the intensity of the scattered light.

Streak caused by scattering inside the waveguide

The intensity distribution is detected with a computer controlled, cooled CCD-Camera. Finally the attenuation coefficient is extracted from the measured data.

Intensity distribution measured with CCD-Camera

The following two figures show the setup that was used for the measurements.

Schematic of the Setup

Photography of the Setup

Additionally, it is possible with our setup to measure the refractive index and the thickness of waveguides that support a minimum of two guided modes. These parameters can be extracted from the dependency between coupling angle and effective refractive index.

Keywords: Prism, Coupling, Thin film waveguide, Waveguide losses, scattering, Effective refractive index, Organic semiconductor lasers, Polymer, Small molecule, Evanescent field, CCD-Camera, Coupling angle

1 M. Reufer, J. Feldmann, P. Rudati, A. Ruhl, D. Müller, K. Meerholz, C. Karnutsch, M. Gerken, and U. Lemmer, Appl. Phys. Lett. 86, 221102 (2005).


MSc.Christian Karnutsch
Lichttechnisches Institut
Universität Karlsruhe (TH)
Geb. 30.34
Kaiserstraße 12
D-76131 Karlsruhe

Raum: 126
Telefon: +49 721 608 7742
Telefax: +49 (0)721 608 - 2590


Optical Waveguiding in Ferroelectric Na0.5K0.5NbO3 Thin Films

Prism coupling as a non destructive tool for optical characterization of (Al,Ga) nitride compounds

Inter-laboratory Measurements of the optical losses in Ferroelectric thin films by prism coupling method

rutile Brewster prism

Del Mar Photonics


Standard Specifications (buy online):

Rutile (TiO2) coupling prism
Material: single crystal TiO2
Sizes: 5x5x5 mm +/- 0.2 mm
Angles:45-45-90 deg.+/-10 arcmin
Polish quality: 20/10Scratch/Dig
Surface flatness: lambda @ 633nm
Parallelism: <5arcmins
Orientation: Z-axis along prism thickness +/- 15 arcmin