Patent Application
20070229937
This invention relates to the generation of electromagnetic waves in the terahertz frequency range.
Continuous wave generators that generate signals in the so-called “terahertz region”, which is also known as the “terahertz gap”, e.g., between 300 gigahertz and 10 terahertz, are useful for various applications, e.g., spectrum analyzer applications. Prior art terahertz wave generators employed two continuous wave lasers with a frequency difference between them in the terahertz range. The two lasers were combined using an optical coupler or a beam combiner to produce a single beam with a beat frequency equal to the difference between the two laser beams, i.e., with a beat frequency in the terahertz range. The single beam was then directed to a photomixer, which responded to the input optical signal to produce an electrical signal with the same frequency as the beat frequency. The photomixer is biased, and coupled to an antenna, so that the resulting electrical signal causes an electromagnetic wave having a frequency in the terahertz range to propagate from the antenna.
Disadvantageously, the use of two lasers is unduly expensive. Also, it is very difficult to get the two lasers to match precisely in order to produce a terahertz wave with small frequency fluctuations, especially over time, given that as it operates each laser tends to drift somewhat in frequency.
Another prior art approach to generating a continuous terahertz wave is to employ a mode-locked laser that generates a periodically modulated signal having a carrier frequency that is much higher, e.g., in the vicinity of 200 terahertz, than the modulation rate, which is typically between 10 and 40 gigahertz. The modulation rate that results is typically that of a radio frequency (RF) signal. The modulated signal is supplied as an input to the mode-locked laser. The resulting output signal is represented in the frequency domain as a comb signal, with the comb teeth representing frequency components within the signal having a frequency spacing equal to the modulation rate. A filter is employed to select two of the frequency components with a desired spacing, e.g., a spacing of a magnitude that is within the terahertz gap. This difference corresponds to the beat frequency of the selected frequency components. The selected frequency components are directed to a photomixer, which responds to the input selected frequency components to produce an electrical signal with the same frequency as the beat frequency between them. The photomixer is biased, and coupled to an antenna, so that the resulting electrical signal causes and electromagnetic wave having a frequency in the terahertz range to propagate from the antenna.
Disadvantageously, each mode-locking laser is designed for a single particular mode-locking frequency, which may only be varied by a small amount. Thus, the mode-locked laser cannot be easily tuned to any desired modulation frequency, because the frequency of the RF signal must be within the mode-locking range of the laser.
I have recognized that the generation of terahertz waves can be improved, in accordance with the principles of the invention, by generating optical frequency components that have a beat frequency in the terahertz gap using a continuous wave (cw) laser, a modulator coupled to the output of the cw laser, and a nonlinear optical waveguide coupled to the output of the modulator. More specifically, the cw laser output signal is modulated by the modulator to produce an optical signal with equally spaced frequency components, i.e., a comb frequency spectrum, but not necessarily of equal magnitude. The modulator may employ a radio frequency signal, e.g., typically between and 100 GHz, as the modulating signal. The modulated optical signal is supplied to the nonlinear optical waveguide in which it undergoes self-phase modulation (SPM). As a result, at least two further frequency components are produced that are separated from each other by a desired frequency in the terahertz gap and have a sufficient magnitude. Note that the separation between the two further frequency components is a multiple of the frequency of the modulating signal. Thereafter, an optical filter then selects the two frequency components, which are directed to a photomixer, that responds to the selected frequency components to produce an electrical signal with the same frequency as the beat frequency, i.e., the difference in frequency, between them. The photomixer is biased and, optionally, coupled to an antenna. If so, the resulting electrical signal causes an electromagnetic wave having a frequency in the terahertz range to propagate from the antenna.
Advantageously, inexpensive cw lasers that are typically used for telecommunications applications, e.g., cw lasers at 1.5 μm and 1.3 μm, and conventional electroabsorption modulators may be employed, thereby resulting in an overall low system cost.
In the drawing:
The following merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.
Thus, for example, it will be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the invention. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudocode, and the like represent various processes which may be substantially represented in computer readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown.
The functions of the various elements shown in the FIGs., including any functional blocks labeled as “processors”, may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared.
In the claims hereof any element expressed as a means for performing a specified function is intended to encompass any way of performing that function. This may include, for example, a) a combination of electrical or mechanical elements which performs that function or b) software in any form, including, therefore, firmware, microcode or the like, combined with appropriate circuitry for executing that software to perform the function, as well as mechanical elements coupled to software controlled circuitry, if any. The invention as defined by such claims resides in the fact that the functionalities provided by the various recited means are combined and brought together in the manner which the claims call for. Applicant thus regards any means which can provide those functionalities as equivalent as those shown herein.
Unless otherwise explicitly specified herein, the drawings are not drawn to scale. Also, unless otherwise explicitly specified here, all optical elements or systems that are capable of providing specific function within an overall embodiment disclosed herein are equivalent to one another for purposes of the present disclosure.
In the description, identically numbered components within different ones of the FIGs. refer to the same components.
Terahertz waves are generated, in accordance with the principles of the invention, by generating optical frequency components that have a beat frequency in the terahertz gap using a continuous wave (cw) laser, a modulator coupled to the output of the cw laser, and a nonlinear optical waveguide coupled to the output of the modulator.
More specifically, cw laser 101 provides a constant output signal at a prescribed wavelength. For example, cw laser 101 may be an inexpensive cw laser such as is typically used for telecommunications applications, e.g., cw lasers at 1.5 μm and 1.3 μm. The output signal from cw laser 101 is modulated by modulator 103 to produce an optical signal made up of short pulses that have equally spaced frequency components, i.e., a comb frequency spectrum, but not necessarily of equal magnitude. Modulator 103 may be a conventional electroabsorption modulator. As is well known to those of ordinary skill in the art, such an electroabsorption modulator is driven by a sinusoidal electrical signal that is generated in the arrangement of
The modulated optical signal output by modulator 104 is supplied to nonlinear optical waveguide 105, in which it undergoes self phase modulation (SPM). As a result, at least two further frequency components are produced that 1) are separated by a desired frequency in the terahertz gap and 2) have a sufficient magnitude. Note that the separation between the two further frequency components is a multiple of the frequency of the modulating signal supplied by frequency generator 104.
Thereafter, optical filter 107 selects the two desired frequency components and directs them to photomixer 109. Photomixer 109 may be a so-called Auston switch.
Preferably, each of the selected frequency components has approximately the same power, in order to maximize the efficiency of photomixer 109. Also, preferably, optical filter 107 selects the two frequency components such that they have essentially the same polarization. If the selected frequency components do not have similar polarization, their interaction in photomixer 109 will be reduced. Indeed, if the selected frequency components are orthogonal, no interaction will occur, and hence photomixer 109 will not produce any signal. To achieve selection of frequency components that have essentially the same polarization, optical filter 107 may include at least one polarization controller, which can change the polarization of selected light to a desired polarization.
Photomixer 109 responds to the selected frequency components by producing an electrical signal with the same frequency as the beat frequency, i.e., the difference in frequency, between the two selected frequency components. To this end, photomixer 109 may be biased, e.g., with a continuous voltage. The output of photomixer 109 may be coupled to an antenna, e.g., antenna 111, so that the resulting electrical signal causes an electromagnetic wave having a frequency in the terahertz range to propagate from antenna 111, e.g., radiated into space. Alternatively, the resulting electrical signal may be captured by a focusing device, e.g., a lens or an antenna, and supplied to a waveguide, e.g., in order to contain the terahertz waves that are produced on a chip.
