Nonlinear optical device

Abstract

There is provided a non-linear optical device for enhancing the bandwidth accessible in the nonlinear generation of an optical signal. The device comprises a planar optical waveguide, the planar optical waveguide being operative to generate an optical output from an optical input having an input bandwidth by means of a non-linear optical process, the optical output having a wavelength within an accessible bandwidth, wherein the planar optical waveguide is operative to enhance the accessible bandwidth such that the ratio of the accessible bandwidth to the input bandwidth is at least 4. The device is particularly applicable to broad optical continuum generation, but may also be used in a parametric oscillator or amplifier arrangement with broad tuning range. The planar waveguide geometry permits easy integration in more complex photonic integrated circuits such as a Michelson interferometer for low coherence interferometry based optical coherence tomography.

Description

FIELD OF THE INVENTION The present invention relates to nonlinear optical devices and in particular to a planar waveguide device for nonlinear optical signal generation with large accessible bandwidth, including optical continuum generation. BACKGROUND TO THE INVENTION Optical sources which can generate radiation over a wide wavelength range currently have many applications in scientific research, engineering and medicine. Often the application requires the coherence properties associated with laser radiation and so various laser materials have been developed which exhibit a broad gain bandwidth when suitably pumped. A good example is titanium doped sapphire, which is characterized by a gain bandwidth covering the range 650-1100 nm. The coherence properties of lasers based on broadband material systems may be further enhanced in a number of ways. The technique of modelocking permits the utilization of much of the available laser bandwidth to obtain a repetitive train of short (or ...

AI Technical Summary

Benefits of technology

"The present invention provides a non-linear optical device that enhances the bandwidth of an optical input by generating an optical output through a non-linear process in a planar waveguide. The device has a ratio of the accessible bandwidth to the input bandwidth of at least 4, preferably at least 10. The device can be designed with a high refractive index core layer to enhance the bandwidth and optical confinement. The device can also be designed with a tapered region to mitigate coupling in from other devices. The device can also include other structures for pre-processing or post-processing the optical input and output. The device can also compress the optical input and output to increase the peak power and pre-compensate for pulse broadening. The device can also modify the optical dispersion characteristics of the waveguide to optimize device performance and enhance the accessible bandwidth."

Problems solved by technology

However, as many laser sources only operate over narrower, well-defined wavelength ranges, nonlinear optical processes have been employed to generate other wavelengths using the output from available laser sources.

However, as will now be described, all of these have their attendant drawbacks.

However, due the low nonlinear coefficient associated with many materials, the characteristic threshold intensity is high and so a high peak intensity laser source is required.

Consequently, there is a high attendant risk of surface or bulk damage to the sample unless it is a material exhibiting a high damage threshold such as sapphire, which also exhibits good stability of CG.

Nevertheless, the associated pulse energy damage threshold is also reduced and so end facet damage may occur, requiring the cleaving of a new facet or provision of a new fiber entirely.

In addition, the stability of CG in optical fibers is typically low, the overall size may limit the compactness of the source and the optical mode properties are not easily compatible with the planar waveguide devices used in photonic integrated circuits.

A further problem is maintaining the polarization (electric field orientation) of the light generated, which affects its usefulness in applications.

Another issue associated with CG is the characteristic optical dispersion (variation of refractive index with wavelength) of the device in which the continuum is generated.

But for waveguides, the situation is more complex, with the total dispersion also depending on waveguide and modal dispersion.

However, despite the improvement in CG bandwidth, the microstructured fibers still suffer from the drawbacks associated with more conventional fibers, as outlined above.

In addition, there are many materials that can not be fabricated in bulk or fiber form and are therefore unavailable for CG in these configurations.

However, although the process was accompanied by nonlinear spectral broadening, which resulted in an optical output having a bandwidth broader than that of the input pulse, the degree of spectral broadening was not sufficient to generate an optical continuum.

Although improved performance is obtained in terms of threshold power and conversion efficiency, the available tuning range is still limited.

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