This application claims benefit of priority to U.S. Provisional Patent Application No. 61/349,427 filed May 28, 2010 which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to optoelectronic oscillators of improved performance providing high RF frequency stability and reduced noise. These find application in a wide variety of fields, wherever signal generation is required at high respective frequencies with reduced noise, such as radar.
2. Background of the Invention
Microwave generators with multi gigahertz frequency, high spectral purity and high RF frequency stability have applications in many areas such as communication, radar and metrology. Different approaches can be used to obtain high spectral purity and high repetition rate signals. Conventional electronic approaches rely on using a high quality factor (Q) resonator in order to get high spectral purity. Generating microwave signals with electro-optical systems have been studied previously [1,2]. The current optoelectronic oscillator (hereinafter “OEO”) design, introduced by Yao and Maleki, use optical fibers as the high Q element, and have attracted great attention due to their extraordinary spectral purity [3,4]. Several different types of OED's are demonstrated successfully [5] such as the coupled optoelectronic oscillator [6], multi loop OEO architectures [7-9], and OEO with photonic filters that use atomic cells [10]. A standard OEO loop is shown in
OED's of the type described find application in a variety of devices and methods. Radar and signal intelligence are standard applications. OEO generated signals may be used as carriers and are important in clock recovery, and in communication broadcasting and receiving. A single loop OEO is described in detail in U.S. Pat. No. 5,723,856 and a multi-loop OEO is shown in U.S. Pat. No. 5,777,778
Most OED's employ an RF Filter as the mode selector for signal generation. See for instance, [11] and U.S. Pat. No. 5,777,778. In this invention, a F-P interferometer, or etalon, is used in replacement of the RF Filter. In one embodiment, a 10.287 GHz optoelectronic oscillator is demonstrated which uses a 1000 finesse Fabry-Perot etalon as the mode selector instead of an RF filter. The results are compared with a standard optoelectronic loop with an RF filter. The substitution of the RF filter with the optical filter results in a higher RF frequency stability and lower phase noise.
We describe an optoelectronic loop design which uses a high finesse Fabry-Perot etalon as the mode selector instead of an RF filter. The inclusion of the Fabry-Perot etalon instead of the RF filter results in less phase noise, due to the higher Q, and also results in higher RF frequency stability due to the ultralow temperature dependency of the Fabry-Perot etalon. The results of the OEO with a Fabry-Perot etalon are compared to the conventional OEO with the RF filter. When the same laser source, intensity modulator, optical delay length and RF amplifier are used, the OEO with the Fabry-Perot etalon results in lower phase noise and higher RF frequency stability.
The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate exemplary embodiments of the invention, and, together with the general description given above and the detailed description given below, serve to explain the features of the invention.
The conventional OEO design shown in
Our inventive OEO design, using a Fabry-Perot etalon, is shown in
The schematic of the OEO with the Fabry-Perot etalon is shown in
The RF spectrum of the OEO is shown in
The phase noise and RF frequency stability performance of the OEO is also measured and compared with the standard OEO. In the standard OEO, the Fabry-Perot etalon is replaced by an RF filter with other components and parameters kept constant. The RF filter used in the experiment has a 3 dB bandwidth of 13 MHz, centered at 10.287 GHz and has an insertion loss of 5 dB. The total optical delay is kept constant by adding an appropriate amount of fiber to the cavity.
The comparison of the phase noise of the OEO with an RF filter and the OEO with an etalon is shown in
The RF stability of the oscillation frequency is also measured using a real time RF spectrum analyzer in the spectrogram mode and compared for the two (2) OEO designs (
This disclosure demonstrates an optoelectric oscillator (OEO) which uses an ultra stable 1000 finesse Fabry-Perot etalon as the oscillator mode selector. The selection of the modes is performed in the optical domain by the use of the periodic etalon transfer function. The OEO oscillation frequency is defined by the free spectral range of the etalon. The performance of the new OEO is compared with the standard OEO design with an RF filter. When all the other parameters are kept constant, except the mode selector, the OEO with the Fabry-Perot etalon results in a better phase noise performance and higher RF frequency stability. The resonance bandwidth of the etalon can be narrowed easily by using a higher finesse etalon which allows inserting very long optical fiber delays to the OEO. When higher finesse etalons are used, a CW laser frequency locking system such as Pound-Drever-Hall (PDH) method may be required to lock the laser frequency to the etalon resonance peak.
While the present invention has been disclosed both generically, and with reference to specific alternatives, those alternatives are not intended to be limiting unless reflected in the claims set forth below. The invention is limited only by the provisions of the claims, and their equivalents, as would be recognized by one of skill in the art to which this application is directed
This work was supported by the Defense Advanced Research Programs Agency under the PHOBIAC program and National Science Foundation under contract DMR 0120967. The United States Government may enjoy certain rights in the invention(s) claimed herein.
Number | Name | Date | Kind |
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5723856 | Yao et al. | Mar 1998 | A |
5777778 | Yao | Jul 1998 | A |
6845108 | Liu et al. | Jan 2005 | B1 |
7492795 | Delfyett et al. | Feb 2009 | B1 |
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Number | Date | Country | |
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20110292486 A1 | Dec 2011 | US |
Number | Date | Country | |
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61349427 | May 2010 | US |