Optical: systems and elements – Optical frequency converter – Harmonic generator
Reexamination Certificate
1999-08-25
2001-10-16
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical frequency converter
Harmonic generator
C372S021000, C372S022000
Reexamination Certificate
active
06304366
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the generation of photonic signals at frequencies other than the input signal. In particular, it relates to second or higher harmonic generation, sum, and difference frequency conversion, Raman processes and generic parametric amplification near the photonic band edge.
2. Related Art
In recent years, advances in photonic technology have generated a trend toward the integration of electronic and photonic devices. These devices offer an array of advantages over conventional electronic devices. For example, they can provide enhanced speed of operation, reduced size, robustness to environmental changes. such as rapid temperature variations, and increased lifetime and ability to handle high repetition rates. These structures can be made of metals, semiconductor materials, ordinary dielectrics, or any combination of these materials.
The theoretical and experimental investigations of photonic band gap (PBG) structures is evidence of the widely recognized potential that these new materials offer. In such materials, electromagnetic field propagation is forbidden for a range of frequencies, and allowed for others. The nearly complete absence of some frequencies in the transmitted spectrum is referred to as a photonic band gap (PBG), in analogy to semiconductor band gaps. This phenomenon is based on the interference of light; for frequencies inside the band gap, forward- and backward-propagating components can cancel destructively inside the structure, leading to complete reflection.
For example, recent advancements in PBG structures have been made in the development of a photonic band edge nonlinear optical limiter and switch. See, “Optical Limiting and Switching of Ultrashort Pulses in Nonlinear Photonic Band-Gap Materials”, M. Scalora, et al.,
Physical Review Letters
73:1368 (1994) (incorporated by reference herein in its entirety). Also, advancements in photonic technology have been achieved with the development of the nonlinear optical diode. See, “The Photonic Band-Edge Optical Diode”, M. Scalora, et al.,
Journal of Applied Physics
76:2023 (1994), which is incorporated by reference herein in its entirety. In addition, the physical processes involved in the photonic signal delay imparted by a uniform PBG structure are described in detail in Scalora, et al., “Ultrashort pulse propagation at the photonic band edge: large tunable group delay with minimal distortion and loss.” Phys. Rev. E Rapid Comm. 54(2), R1078-R1081 (August 1996), which is incorporated by reference herein in its entirety.
The frequency conversion of coherent light sources, such as lasers, has been investigated for many years, because of the desirability to expand the ranges of available output wavelengths. Many different processes have been utilized, including Raman-shifting, harmonic generation, and quasi-phase-matching techniques. Also important are frequency up-and down-conversion, and the more general issue of obtaining laser radiation at frequencies generally not accessible with a more direct process.
Harmonic generation involves the non-linear interactions between light and matter using a suitable non-linear material that can generate harmonics at multiples of the pump signal frequency. Conventional non-linear materials include potassium dihydrogen phosphate (KDP), &bgr;-barium borate (BBO), lithium triborate (LBO), lithium niobate (LiNbO
3
), and the like. However, the utility of these types of non-linear crystals for efficient frequency conversion often depends on proper adjustment of parameters such as non-linear coefficients, phasematching capabilities, walkoff angle, and angular acceptance.
For example, lithium niobate is conventionally used for second harmonic (SH) generation because its nonlinear &khgr;
(2)
coefficient is larger than most other materials. In addition, the effective magnitude of &khgr;
(2)
can be enhanced further by a process called polling. Typically, a certain length of LiNbO
3
material, ordinarily a few millimeters to a few centimeters, is subdivided in sections each on the order of a few microns in thickness. Then, a strong, static electric field is applied to the material such that the direction of the electric field is reversed in each successive section. In effect then, the field leaves a permanent impression behind, similar to the impression that visible light leaves on a photographic plate, which causes the sign of the &khgr;
(2)
to reverse in a predetermined way in each successive section throughout the length of the material. As a consequence of alternating the sign of the nonlinear index of refraction, a technique that is also referred to as quasi-phase-matching (QPM), SH generation from a similar length of material that is not quasi-phase-matched can be orders of magnitude smaller than the phase-matched case.
The reason for this kind of material processing can be explained as follows. For SH generation, a field at twice the original frequency is generated. In addition to its dependence on field strength, the index of refraction of any material also depends on frequency. For typical SH up-conversion, the indices of refraction may differ by as much as 10% or more; this means that the speed of light in the material may differ by that amount, causing the two waves, the fundamental and the SH, to get out of phase. As it turns out, by modulating the &khgr;
(2)
,the waves tend to remain in phase, which defines the QPM phenomenon, thus yielding enhanced SH generation.
However, QPM devices utilized in frequency conversion are typically on the order of a 1-2 centimeters (cm) in length. What is needed is a device that performs frequency conversion of a light source that is compact in size, has sufficient conversion efficiency, and can be manufactured by conventional techniques.
SUMMARY OF THE INVENTION
The present invention provides a new device and method to produce photonic signals at frequencies other than the frequency of the incident pump beam or pulse. The photonic band gap (PBG) device comprises a plurality of first and second material layers. The first and second material layers are arranged such that the PBG device exhibits a photonic band gap structure. The photonic band gap structure exhibits a transmission band edge corresponding to the pump signal frequency. A second photonic signal at a second frequency is generated by an interaction of the input photonic signal with the arrangement of layers. The second photonic signal is an harmonic of the pump signal and can be either transmitted through the device or reflected out the input region of the PBG device.
According to one embodiment of the present invention, the first and second layers are arranged in a periodically alternating manner. In addition, the PBG device can further comprise one or more periodicity defects in order to produce other harmonics of the pump signal.
According to the present invention, a method for the frequency conversion of a pump pulse comprises selecting a desired frequency for the pump signal to produce a second signal at a desired harmonic frequency. Next, a PBG structure is provided, wherein the arrangement of layers comprising the PBG structure is similar to the structure described above. The method further comprises inputting the desired pump signal into the PBG structure in order to produce an ouput signal at a desired harmonic of the pump signal frequency.
The generated signals can be in the form of either a continuous wave (cw) signal, if the pump beam is a cw signal, or a pulsed signal, if the pump beam is pulsed. The frequency conversion process for the PBG device is orders of magnitude more efficient than any other ordinary QPM device of comparable size. This conversion efficiency can be achieved with the utilization of a photonic band gap (PBG) structure. An incident pump beam or pulse is applied to the PBG device near the photonic band edge transmission resonance, in close proximity to the photonic band gap. The output signal (of different frequency or wavelength) that is generated in the PBG device
Bloemer Mark J.
Scalora Michael
Epps Georgia
Sterne Kessler Goldstein & Fox P.L.L.C.
Thompson Tim
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