Amplification-free electro-optical oscillator

Abstract

An electro-optical oscillator includes, in part, a modulator, a signal splitter, N photodiodes with N being an integer greater than one, a signal combiner, and a filter. The modulator modulates an optical signal in accordance with a feedback signal. The splitter splits the modulated optical signal into N optical signals each delivered to a different one of N photo-diodes. Each of the N photo-diodes converts the optical signal it receives to a current signal. The signal combiner combines the N current signals received from the N photo-diodes to generate a combined current signal. The filter filters the combined current signal and generates the feedback signal. The electro-optical oscillator optionally includes, in part, N variable optical gain/attenuation components each amplifying/attenuating a different one of the N optical signals generated by the splitter.

Claims

1. An electro-optical oscillator comprising: an optical modulator adapted to generate a pair of modulated differential optical signals from a received optical signal; a splitter adapted to split the modulated differential optical signals into a first N differential optical signals and a second N differential optical signals, the splitter delivering each of the first N optical signals to a different one of first N optical paths, and delivering each of the second N optical signals to a different one of second N optical paths, N being an integer greater than one; a first N photo-diodes each adapted to convert a different one of the first N optical signals to a current signal; a second N photo-diodes each adapted to convert a different one of the second N optical signals to a current signal; a first signal combiner adapted to combine the N current signals received from the first N photo-diodes to generate a differentially positive signal; a second signal combiner adapted to combine the N current signals received from the second N photo-diodes to generate a differentially negative signal; a first filter adapted to filter the differentially positive signal to generate a first feedback signal; said first filter adapted to apply the first feedback signal to the optical modulator; and a second filter adapted to filter the differentially negative signal to generate a second feedback signal; said second filter adapted to apply the second feedback signal to the optical modulator.

2. The electro-optical oscillator of claim 1 further comprising: first N variable optical gain/attenuation components each disposed in a different one of the first N optical paths and adapted to amplify/attenuate the optical signal delivered to the path; and second N variable optical gain/attenuation components each disposed in a different one of the second N optical paths and adapted to amplify/attenuate the optical signal delivered to the path.

3. The electro-optical oscillator of claim 2 further comprising: first N variable delay components each disposed in a different one of the first N optical paths and adapted to delay the optical signal delivered to the path; and second N variable delay components each disposed in a different one of the second N optical paths and adapted to delay the optical signal delivered to the path.

4. The electro-optical oscillator of claim 1 further comprising: first N variable delay components each disposed in a different one of the first N optical paths and adapted to delay the optical signal delivered to the path; and second N variable delay components each disposed in a different one of the second N optical paths and adapted to delay the optical signal delivered to the path.

5. The electro-optical oscillator of claim 1 further comprising: at least one variable delay component disposed between the optical modulator and the splitter and adapted to delay the optical signal delivered to the splitter.

6. A method of generating an oscillating signal, the method comprising: modulating an optical signal to generating a pair of modulated differential signals; splitting a first one of the pair of modulated differential signals into a first N differential optical signals each delivered to a different one of first N optical paths; splitting a second one of the pair of modulated differential signals into a second N differential optical signals each delivered to a different one of second N optical paths; converting the optical signal delivered to each of the first N optical paths to generate a first N current signals; converting the optical signal delivered to each of the second N optical paths to generate a second N current signals; combining the first N current signals to generate a differentially positive current signal; combining the second N current signals to generate a differentially negative current signal; filtering the differentially positive current signal to generate a first feedback signal; filtering the differentially negative current signal to generate a second feedback signal; modulating the optical signal in accordance with the first feedback signal to generate the first one of the pair of modulated differential signals; and modulating the optical signal in accordance with the second feedback signal to generate the second one of the pair of modulated differential signals.

7. The method of claim 6 further comprising: amplifying/attenuating the optical signal delivered to each of the first and second N optical paths.

8. The method of claim 7 further comprising: delaying the optical signal delivered to each of the first and second N optical paths.

9. The method of claim 6 further comprising: delaying the optical signal delivered to each of the first and second N optical paths.

10. The method of claim 6 further comprising: delaying the pair of modulated differential signals prior to splitting the pair of modulated differential signals.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a block diagram of an electro-optic oscillator, as known in the prior art.

(2) FIG. 2 is a block diagram of an electro-optic oscillator, in accordance with one embodiment of the present invention.

(3) FIG. 3 is a block diagram of an electro-optic oscillator, in accordance with one embodiment of the present invention.

(4) FIG. 4 is a block diagram of an electro-optic oscillator, in accordance with one embodiment of the present invention.

(5) FIG. 5 is a block diagram of an electro-optic oscillator, in accordance with one embodiment of the present invention.

(6) FIG. 6 is a block diagram of an electro-optic oscillator, in accordance with one embodiment of the present invention.

(7) FIG. 7 is a block diagram of an electro-optic oscillator, in accordance with one embodiment of the present invention.

(8) FIG. 8 is a block diagram of an electro-optic oscillator, in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION

(9) An electro-optical oscillator, in accordance with one embodiment of the present invention, has a substantially reduced phase noise. The electro-optical oscillator dispenses the need for such operations as electrical amplification, thereby achieving a phase noise that is generally independent of the oscillation frequency. The substantially lower phase noise of an electro-optical oscillator, in accordance with embodiments of the present invention, makes it suitable for use in applications operating in mm-wave and THz frequency ranges.

(10) FIG. 2 is a simplified block diagram of an electro-optic oscillator 200, in accordance with one embodiment of the present invention. Electro-optic oscillator 100 is shown as including, in part, a modulator 130, a variable delay component 102, a filter 140, a multitude of variable gain/attenuation components 220.sub.1, 220.sub.2 . . . 220.sub.N, and a multitude of photodiodes 225.sub.1, 225.sub.2 . . . 225.sub.N.

(11) The optical signal generated by optical signal source (e.g., a laser) 112 is modulated by optical modulator 130 shown as having disposed therein an optical phase modulator 132 and a combiner 134. The modulated optical signal opt_mod is delayed by optical delay component 102 and split by optical signal splitter 106 into N optical signals each delivered to a different one of N optical paths 110.sub.1, 110.sub.2 . . . 110.sub.N-1, 110.sub.N—collectively and alternatively referred to herein as optical paths 110. Disposed in each optical path 110.sub.i, where i is an integer ranging from 1 to N, is a variable gain/attenuation component 220.sub.i adapted to amplify the optical signal it receives and deliver the amplified/attenuated optical signal to an associated photo-diode 120, disposed in that path.

(12) In the following, for simplicity, the same reference number may be used to identify both the path through which a signal travels, as well as to the signal which travels through that path. For example, reference numeral 110.sub.1 may be used to refer to the path so identified in FIG. 1, or alternatively to the signal that travels through this path. Furthermore, in the following, the terms divider, splitter, coupler, or combiner are alternatively used to refer to an element adapted to split/divide a signal to generate more signals and/or couple/combine a multitude of signals to generate one or more signals. Such a component is also alternatively referred to herein as splitter/coupler. Furthermore, although the embodiments of the present invention are described with reference to photodiodes, it is understood that any device that converts an optical signal to an electrical signal may also be used; accordingly, all such devices are referred to herein as photodiodes.

(13) As described above, each variable gain/attenuation component 220.sub.i is adapted to vary the gain/attenuation value of the optical signal it receives and deliver the amplified/attenuated signal to an associated photodiode 225.sub.i. Each photo-diode 225.sub.i is adapted to convert the optical signal it receives from its associated variable gain/attenuation component 220.sub.i to an electrical current signal 235.sub.i. Combiner 250 is adapted to receive and combine currents 235.sub.i to generate a current signal I.sub.RF that is delivered to filter 140. Filter 140 converts current I.sub.RF to a voltage signals and filters out the undesirable frequency components of the signal to generate voltage signal V.sub.RF applied to optical phase modulator 132.

(14) Because, in accordance with the present invention, electro-optic oscillator 200 splits the modulated signal opt_mod into a multitude of optical signals 110.sub.i each of which is amplified by an optical amplifier 220.sub.i and subsequently converted to an electrical signal via an associated photodiode 225.sub.i, the phase-noise or linewidth of the oscillator has a substantially reduced dependence on the oscillation frequency. For example, assuming that the modulator and photodiodes operate at 80 GHz, and the relative noise intensity of the laser 112 is −140 dB/Hz, a phase noise better than −140 dBc/Hz at 1 MHz offset (at 80 GHz) may be achieved. Furthermore, because in accordance with the present invention electro-optic oscillator 100 uses a multitude of photodiodes 225.sub.i to generate current I.sub.RF, the overall gain of electro-optic oscillator 200 may be equal to or higher than those of the conventional electro-optic oscillators using electrical signal amplification.

(15) FIG. 3 is a simplified block diagram of an electro-optic oscillator 300, in accordance with another embodiment of the present invention. Electro-optic oscillator 300 is similar to electro-optic oscillator 200 except that electro-optic oscillator 300 includes an optical delay component 205.sub.i in each of its optical paths 110.sub.i. Each optical delay component 205.sub.i is adapted to delay the optical signal it receives and deliver the delayed optical signal to an associated variable gain/attenuation component 220.sub.i.

(16) FIG. 4 is a simplified block diagram of an electro-optic oscillator 400, in accordance with another embodiment of the present invention. Electro-optic oscillator 400 is similar to electro-optic oscillator 300 except that electro-optic oscillator 400 does not include variable gain/attenuation components in its optical paths 110.sub.i. To achieve the desired level of power and compensate for the lack of gain/attenuation components, electro-optic oscillator 400 uses a laser 222 having a power that is higher than the power of the lasers used in embodiments 200 and 300. For example, the lasers used in embodiments 200 and 300 may have a power of 10 mW. The laser used in embodiments 400 may have a power of 800 mW. Electro-optic oscillator 400 is also shown as including a main variable delay components 102 disposed between optical splitter 106 and optical modulator 130.

(17) FIG. 5 is a simplified block diagram of an electro-optic oscillator 500, in accordance with yet another embodiment of the present invention. Electro-optic oscillator 500 is similar to electro-optic oscillator 400 except that electro-optic oscillator 500 does not include any optical delay component 205.sub.i in its optical paths 110.sub.1. To achieve the desired level of power, electro-optic oscillator 300 uses a laser 222 having a power that is higher than the power of the lasers used, for example, in embodiment 300. For example, the lasers used in embodiments 500 may have a power of 800 mW.

(18) FIG. 6 is a simplified block diagram of an electro-optic oscillator 600, in accordance with yet another embodiment of the present invention. Electro-optic oscillator 600 is similar to electro-optic oscillator 400 except that electro-optic oscillator 600 does not include main optical delay components 102 of embodiment 400.

(19) FIG. 7 is a simplified block diagram of an electro-optic oscillator 700, in accordance with another embodiment of the present invention. Electro-optic oscillator 700 generates an oscillating signal differentially, as described further below. The optical signal generated by optical source 402, which may be a laser, is split into a pair of optical signals 452 and 454 that are respectively delivered to optical phase modulators 402 and 404. Optical phase modulators 402, 404 together with combiner 408 form an optical modulator 330. As described further below, differential feedback voltages V.sub.RF.sup.+, V.sub.RF.sup.− are used to modulate optical signals 452 and 454 using optical phase modulators 402 and 404, respectively. The modulated optical signals are received by combiner 408 which in response delivers the optically modulated signals V.sub.in.sub._mod.sup.+ and V.sub.in.sub._mod.sup.− to signal splitter 406. Signal splitter 406 splits each of the differential signals V.sub.in.sub._mod.sup.+ and V.sub.in.sub._mod.sup.− into N signals, where N is an integer greater than one. Accordingly, as shown, signal V.sub.in.sub._mod.sup.+ is split into N signals 410.sub.1.sup.+, 410.sub.2.sup.+ . . . 410.sub.N.sup.+, representing differentially positive signals. Likewise, signal V.sub.in.sub._mod.sup.− is split into N signals 410.sub.1.sup.−, 410.sub.2.sup.− . . . 410.sub.N.sup.−, representing differentially negative signals.

(20) Each of the 2N optical paths is shown as including a variable delay component 405.sub.i. For example, path 410.sub.1.sup.+ is shown as including a variable delay component 405.sub.1.sup.+ and path 410.sub.1.sup.− is shown as including a variable delay component 405.sub.1.sup.−. Likewise, path 410.sub.N.sup.+ is shown as including a variable delay component 405.sub.N.sup.+ and path 410.sub.N.sup.− is shown as including a variable delay component 405.sub.N.sup.−. Each optical delay component 405.sub.i.sup.+ is adapted to delay the optical signal 410.sub.i.sup.+ it receives in accordance with the delay value selected for optical delay component 405.sub.i.sup.+. Likewise, each optical delay component 405.sub.i.sup.− is adapted to delay the optical signal 410.sub.i.sup.− it receives in accordance with the delay value selected for optical delay component 405.sub.i.sup.−.

(21) The optically delayed signal in each path 410.sub.1.sup.+/410.sub.i.sup.− is received by an associated photo-diode 420.sub.i.sup.+/420.sub.i.sup.− adapted to convert the received optical signal to an electrical signal 435.sub.i.sup.+/435.sub.i.sup.−. For example, photo-diode 420.sub.1.sup.+ converts the optical signal it receives from variable delay component 405.sub.1.sup.+ to an electrical signal 435.sub.1.sup.+. Likewise, for example, photo-diode 420.sub.i.sup.− converts the optical signal it receives from variable delay component 405.sub.1.sup.− to an electrical signal 435.sub.1.sup.−. Signal combiner/coupler 458 is adapted to combine the differentially positive current signals 435.sub.1.sup.+, 435.sub.2.sup.+ . . . 435.sub.N.sup.+ generated respectively by photo-diodes 420.sub.1.sup.+, 420.sub.2.sup.+ . . . 420.sub.N.sup.+ to generate differentially positive current signal I.sub.RF.sup.+. In a similar manner, signal combiner/coupler 456 is adapted to combine the differentially negative current signals 435.sub.1.sup.−, 435.sub.2.sup.− . . . 435.sub.N.sup.− generated by photo-diodes 420.sub.1.sup.−, 420.sub.2.sup.− . . . 420.sub.N.sup.− to generate differentially negative current signal I.sub.RF.sup.−.

(22) Filter 140.sub.1 is adapted to convert current I.sub.RF.sup.+ to a voltage signal and filter out the undesirable frequency components of the signal to generate voltage signal V.sub.RF.sup.+ applied to optical phase modulator 402. Likewise, filter 140.sub.2 is adapted to convert current I.sub.RF.sup.− to a voltage signal and filter out the undesirable frequency components of the signal to generate voltage signal V.sub.RF.sup.− applied to optical phase modulator 402. The optical delay components 405.sub.i.sup.+ and 405.sub.i.sup.− disposed in paths 410.sub.i.sup.+ and 410.sub.i.sup.− form a finite impulse response (FIR) filter thereby relaxing the characteristics that would be otherwise required from filters 140.sub.1 and 140.sub.2.

(23) FIG. 8 is a simplified block diagram of an electro-optic oscillator 800, in accordance with another embodiment of the present invention. The optical signal generated by optical source 402, which may be a laser, is split into a pair of optical signals 452 and 454 that are respectively delivered to optical phase modulators 402 and 404. Differential feedback voltages V.sub.RF.sup.+, V.sub.RF.sup.− are used to modulate optical signals 452 and 454 using optical phase modulators 402 and 404, respectively. Optical phase modulators 402, 404 together with combiner 408 form an optical modulator 330. The modulated optical signals are received by combiner 408 which in response delivers the optically modulated signals V.sub.in.sub._mod.sup.+ and V.sub.in.sub._mod.sup.− to signal splitter 406. Signal splitter 406 splits each of the differential signals V.sub.in.sub._mod.sup.+ and V.sub.in.sub._mod.sup.− into N signals, where N is an integer greater than one. Accordingly, as shown, signal V.sub.in.sub._mod.sup.+ is split into N signals 410.sub.1.sup.+, 410.sub.2.sup.+ . . . 410.sub.N.sup.+, representing differentially positive signals. Likewise, signal V.sub.in.sub._mod.sup.− is split into N signals 410.sub.1.sup.−, 410.sub.2.sup.− . . . 410.sub.N.sup.−, representing differentially negative signals.

(24) Each of the 2N optical paths is shown as including a variable delay component 405.sub.i and a variable gain/attenuator 455.sub.i. For example, path 410.sub.1.sup.+ is shown as including a variable delay component 405.sub.1.sup.+ and a variable gain/attenuator 455.sub.1.sup.+; path 410.sub.1.sup.− is shown as including a variable delay component 405.sub.1.sup.− and a variable gain/attenuator 455.sub.1.sup.−. Likewise, path 410.sub.N.sup.+ is shown as including a variable delay component 405.sub.N.sup.+ and a variable gain/attenuator 455.sub.N.sup.+; and path 410.sub.N.sup.− is shown as including a variable delay component 405.sub.N.sup.− and a variable gain/attenuator 455.sub.N.sup.−.

(25) Each optical delay component 405.sub.i.sup.+ is adapted to delay the optical signal 410.sub.i.sup.+ it receives in accordance with the delay value selected for optical delay component 405.sub.i.sup.+. Likewise, each optical delay component 405.sub.i.sup.− is adapted to delay the optical signal 410.sub.i.sup.− it receives in accordance with the delay value selected for optical delay component 405.sub.i.sup.−. Each variable gain/attenuator 455.sub.i.sup.+ is adapted to vary the gain or attenuation level of the optical signal it receives from its associated optical delay component 405.sub.i.sup.+ in accordance with the gain/attenuation value selected for the gain/attenuation component 455.sub.i.sup.+. Similarly, each variable gain/attenuator 455.sub.i.sup.− is adapted to vary the gain or attenuation level of the optical signal it receives from its associated optical delay component 405.sub.i.sup.− in accordance with the gain/attenuation value selected for the gain/attenuation component 455.sub.i.sup.−. For example, optical delay component 405.sub.i.sup.+ delays optical signal 410.sub.i.sup.+ in accordance with its selected delay. Likewise, variable gain/attenuator 455.sub.1.sup.+ is adapted to vary the gain/attenuation level of the optical signal it receives from optical delay component 405.sub.1.sup.+ in accordance with the gain or attenuation value selected for amplifier/attenuator 455.sub.1.sup.+.

(26) The optically delayed and amplified/attenuated signal in each path 410.sub.i.sup.+/410.sub.i.sup.− is received by an associated photo-diode 420.sub.i.sup.+/420.sub.i.sup.− adapted to convert the received optical signal to an electrical signal 435.sub.i.sup.+/435.sub.i.sup.−. For example, photo-diode 420.sub.1.sup.+ converts the optical signal it receives from variable gain/attenuator 455.sub.1.sup.+ to an electrical signal 435.sub.1.sup.+. Likewise, for example, photo-diode 420.sub.1.sup.− converts the optical signal it receives from variable gain/attenuator 455.sub.1.sup.− to an electrical signal 435.sub.1.sup.−. Signal combiner/coupler 458 is adapted to combine the differentially positive current signals 435.sub.1.sup.+, 435.sub.2.sup.+ . . . 435.sub.N.sup.+ generated respectively by photo-diodes 420.sub.1.sup.+, 420.sub.2.sup.+ . . . 420.sub.N.sup.+ to generate differentially positive current signal I.sub.RF.sup.+. In a similar manner, signal combiner/coupler 456 is adapted to combine the differentially negative current signals 435.sub.1.sup.−, 435.sub.2.sup.− . . . 435.sub.N.sup.− generated by photo-diodes 420.sub.1.sup.−, 420.sub.2.sup.− . . . 420.sub.N.sup.− to generate differentially negative current signal I.sub.RF.sup.−.

(27) The optical delay components 405.sub.i.sup.+/405.sub.i.sup.− and gain/attenuators 455.sub.i.sup.+/455.sub.i.sup.− disposed in paths 410.sub.i.sup.+ and 410.sub.i.sup.− form a finite impulse response (FIR) filter thereby relaxing the characteristics that would be otherwise required from filters 140.sub.1 and 140.sub.2. In one example, FIR filtering may be achieved by setting τ.sub.k=τ.sub.1+kΔτ, where Δτ defines the band-pass filter center frequency and N, which is the number of parallel optical paths, N, defines the order of the filter.

(28) The above embodiments of the present invention are illustrative and not limitative. The invention is not limited by the type of optical phase modulator, signal splitter, signal combiner, optical delay component, optical gain attenuator, or photo-diode. The invention is not limited by the frequency or bandwidth of the optical signal modulated by the electrical signal. The invention is not limited by the number of paths into which the optical signal is split. The invention is not limited by the type of integrated circuit in which the present invention may be disposed. Nor is the invention limited to any specific type of process technology, e.g., CMOS, Bipolar, or BICMOS that may be used to manufacture the embodiments of the present invention. Other additions, subtractions or modifications are obvious in view of the present invention and are intended to fall within the scope of the appended claims.