Patent Application
20090154508
The present application is a regular application of U.S. Provisional Patent Application Ser. No. 61/013,050 filed on Dec. 12, 2007; the complete disclosure of which are hereby incorporated herein by reference for all purposes.
(A) Field of the Invention The present invention relates to a light-generating apparatus with a broadband pumping laser and a quasi-phase matching (QPM) waveguide, and more particularly, to a light-generating apparatus with a broadband pumping laser and a broadband wavelength-converting waveguide for converting a pumping light into a broadband conversion light by using the sum frequency generation and the second harmonic generation mechanisms.
(B) Description of the Related Art
Quasi-phase matching (QPM) is a technique for phase matching nonlinear optical interactions in which the relative phase is corrected at regular intervals using a structural periodicity built into the nonlinear medium, and the most popular case of interest in modern QPM technology is called frequency doubling or second harmonic generation (SHG).
Obtaining a meaningful power transfer between an coherent pumping wave and its frequency doubled second harmonic generation allows the production, for example, of coherent green or blue light by the passage of near infra-red radiation from a solid state laser through a non-linear ferroelectric crystal. Since coherent pumping radiation is easier to produce by laser action than coherent radiation, quasi-phase matching devices with second harmonic generation (QPM-SHG) ability have been widely used for high-efficiency wavelength conversion to generate visible lasers.
The most important design aspect of QPM-SHG devices including a ferroelectric single-crystal is the ability to produce periodic polarization-inversion domains with accuracy. Much inventive effort has been expended in finding ways of preparing the periodically poled structure such as the proton-exchanging method, the electron beam-scanning method, the electric voltage applying method, and others, which enables the generation and conversion of new laser wavelengths via material's nonlinearity under a specific QPM condition of temperature and pumping wavelength.
However, the tolerance for the QPM condition is very narrow, any insufficiency in inversion period results in failure to achieve the objective of producing small-sized, high-efficiency devices. Furthermore, the QPM-SHG wavelength conversion in general has a narrow temperature bandwidth and is sensitive to variations in temperature. Thus, it is common to use a temperature controlling apparatus to stabilize the device temperature for high-efficiency wavelength conversion (See: Michele Belmonte et al., J. Opt. A: Pure Appl. Opt. 1 (1999) 60-63.).
Even though there are several methods for preparing the periodically poled structure in which inverted lattices are nearly uniform in the direction of the thickness of the crystal, there is still a significant problem associated with the necessary of having the pumping radiation propagate in a tightly focused beam to provide adequate power density within the region of wave overlap. In bulk material, the pumping beam cannot be tightly focused since the propagation wave will diffract, resulting in low conversion efficiency. For these reasons, therefore, it is difficult to produce ideal QPM-SHG devices using this conventional method.
In addition, the use of lasers in a projection display enables the creation of vibrant images with extensive color coverage that is unachievable with conventional sources. One major obstacle is a phenomenon called speckle, which originates from the visible laser (See: Jahja I. Trisnadi, Proc. SPIE Vol. 4657, p. 131-137, Projection Displays VIII, Ming H. Wu; Ed.). Speckle arises when coherent light scattered from a rough surface, such as a screen, is detected by a square-law (intensity) detector that has a finite aperture such as an observer's eye. The image on the screen appears to be quantized into small areas with sizes equal to the detector resolution spot. The detected spot intensity varies randomly from darkest, if contributions of the scattering points inside the spot interfere destructively, to brightest if they interfere constructively. This spot-to-spot intensity fluctuation is referred as speckle. The characteristic granular size of the speckle is therefore the same as the size of the detector resolution spot.
One aspect of the present invention provides a light-generating apparatus with a broadband pumping laser on a broadband QPM waveguide through at least a broadband sum frequency generation (SFG) to achieve temperature insensitive and speckle-free (speckle reduction) wavelength-converting device for high efficiency visible light.
A light-generating apparatus according to this aspect of the present invention comprises a broadband pumping laser configured to emit a broadband pumping light having a bandwidth substantially larger than 10 nanometers and a broadband wavelength-converting device with an acceptance bandwidth larger than 0.5 nanometers. The broadband wavelength-converting device includes a domain-inverted structure configured to convert the broadband pumping light into at least one conversion light by using at least a sum frequency generation mechanism and at least one waveguide positioned in the domain-inverted structure, and the waveguide has an input end configured to receive the broadband pumping light and an output end configured to output the conversion light.
Since the broadband pumping light has the bandwidth substantially larger than 10 nanometers and the acceptance bandwidth of the broadband wavelength-converting device is preferably larger than 0.5 nanometers, the conversion light is substantially a broadband incoherent light, which can prevent the speckle problem when it is used as light source of the display system. In addition, the broadband pumping laser provides the broadband pumping light and the acceptance bandwidth of the broadband wavelength-converting device is also wide enough such that there are always at least two corresponding bands in the acceptance bandwidth of the broadband wavelength-converting device for converting two portions of the broadband pumping light into the conversion light by using the sum frequency generation mechanism, even when the environmental temperature is not constant. Consequently, the light-generating apparatus does not need an expensive temperature-controlling system and thus it is temperature-insensitive.
Beside, using waveguides can further enhance nonlinear efficiency mixing as compared to bulk devices, by tightly confining the light over long distances. The tightly focused optical wave will often diffract when it propagates in a bulk device, so single-pass high conversion efficiency cannot be achieved. In waveguides, the mode profile is confined to a transverse dimension in the order of the wavelength, and hence high optical intensities can be maintained over considerable distance to improve the conversion efficiency by two to three orders of magnitude as compared to bulk devices. Also, the nonlinear mixing efficiency is quadratically proportional to the interaction length of the waveguide device (linear proportional for bulk devices), thus the fabrication of long, uniform and low-loss waveguide is essential for highly efficient wavelength-converting device.
Moreover, a tapered waveguide configuration within the crystal can be used to achieve high conversion efficiency without tightly focus the beam to increase the mode overlapping between the interaction lights (pumping light and conversion light) and the material nonlinearity (the polarization waves induced within the material).
The objectives and advantages of the present invention will become apparent upon reading the following description and upon reference to the accompanying drawings in which:
The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.
The domain-inverted structure 50 includes a plurality of first domains 52 having a first polarity 52′ and a plurality of second domains 54 interlaced in the first domains 52, with the second domains 54 having a second polarity 54′ opposite to the first polarity 52′. The domain-inverted structure 50 may have a non-uniform period (Λ); for example, the period varies along a propagation direction of the broadband pumping light 22. The domain-inverted structure 50 includes a first portion having a first period (Λ1) and a second portion having a second period (Λ2) different from the first period (Λ1). In addition, the output end 43 of the broadband wavelength-converting device 30 may be optionally cascaded with another broadband wavelength-converting device configured to convert the conversion light 44 (for example, 532 nm) and the pumping light 22 (for example, 1064 nm) into a ultra-violet (UV) light.
Referring to
The acceptance bandwidth of the broadband wavelength-converting device 30 is wide enough to include at least two narrow bands 80B and 80C; in addition, the broadband pumping laser 20 provides the broadband pumping light 22 also including several narrow bands 70A-70I. Consequently, the two narrow bands 80B and 80C of the broadband wavelength-converting device 30 can be used to convert the two narrow bands 70B and 70C of the broadband pumping light 22 into the conversion light 44 by the using the sum frequency generation (SFG) mechanism. Moreover, the narrow band 80A of the broadband wavelength-converting device 30 can be used to convert the narrow band 70A of the broadband pumping light 22 into the conversion light 44 by the using the second harmonic generation (SHG) mechanism.
Referring to
In other words, the relative shift of the acceptance bandwidth of the broadband wavelength-converting device 30 is smaller than the bandwidth of the broadband pumping laser 20, even when the environmental temperature is not constant. Consequently, the light-generating apparatus 10 does not need an expensive temperature-controlling system and thus it is temperature-insensitive. Consequently, the output power of the conversion light 44 maintains at a high level even the environmental temperature varies from 0 to 100° C. In contrast, the prior art spends effort on combining the narrow band laser and the narrow band wavelength-converting device; however, the output power drops dramatically even the variation of the environmental temperature is within 10° C., that is way the prior art needs to use a high performance temperature controlling apparatus to stabilize the device temperature for high-efficiency wavelength conversion.
In addition to the sum frequency generation mechanism, there is always a corresponding band in the acceptance bandwidth of the broadband wavelength-converting device 30 for converting a portion of the broadband pumping light 22 into the conversion light 44 using a second harmonic generation mechanism. Furthermore, it is much easier to prepare the broadband wavelength-converting device 30 with domains having non-uniform width, as compared to the preparation of the wavelength-converting device with domains having uniform width.
In particular, the high conversion efficiency of the waveguide 40 mostly couples with an issue of high loss of pumping when guiding the power of the broadband pumping light 22 into the waveguide 40. According to the embodiments of the present invention, through the broadband pulsed high power pumping laser 20, a low loss waveguide 40 is effective to couple much more power to enable a higher specific output even with a lower conversion efficiency. In addition, using broadband pulsed high power pumping laser 20 allows the light-generating apparatus 10 to effectively couple more power into the waveguide 40 and this enables higher specific output even with lower conversion efficiency.
It will be appreciated by those skilled in the art having the benefit of this disclosure that this invention provides an adjustable and versatile gun rest apparatus having numerous uses and applications. It should be understood that the drawings and detailed description herein are to be regarded in an illustrative rather than a restrictive manner, and are not intended to limit the invention to the particular forms and examples disclosed. On the contrary, the invention includes any further modifications, changes, rearrangements, substitutions, alternatives, design choices, and embodiments apparent to those of ordinary skill in the art, without departing from the spirit and scope of this invention, as defined by the following claims. Thus, it is intended that the following claims be interpreted to embrace all such further modifications, changes, rearrangements, substitutions, alternatives, design choices, and embodiments.
| Number | Date | Country | |
|---|---|---|---|
| 61013050 | Dec 2007 | US |
