# US 8,571,421 B2 · Highly tunable, low jitter optical clock generation

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- Number: US 8,571,421 B2
- Document ID: US8571421B2
- Status: Granted
- Kind: B2
- Application number: 12/679,259
- Filed: 2008-09-22
- Published: 2010-09-16
- Granted: 2013-10-29
- Earliest claimed: 2007-09-20
- Assignees: FTL Systems, Inc.
- Applicants: John C. Willis; Ruth A. Betcher
- Inventors: John C. Willis; Ruth A. Betcher
- Web page: https://patentgrep.com/patent/US8571421B2

## CPC

- H04B 10/50: Transmitters
- H04L 7/0008: {Synchronisation information channels, e.g. clock distribution lines}
- H04B: TRANSMISSION
- H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M)

## Abstract

An apparatus is disclosed for the optical generation of clock signals with tunable frequency and low jitter. A laser source serves as both the carrier used to transmit the clock signal for use by other optical, electronic or hybrid circuit elements and the original modulation time base. A fraction of the original laser source undergoes one or more stages of frequency division before being recombined as a modulation signal with the remaining laser beam. Transmission of the resulting signal via single mode fiber and dividers retains the low jitter properties of the modulated signal. By starting with a clock signal of optical frequency then dividing downward in frequency, comparatively high frequency clocks may be generated, notably in the GigaHertz and TeraHertz frequency ranges.

## Claims

### Claim 1

An optical clock generator, comprising:
a coherent light source;
an energy splitter to receive output from the coherent light source;
a time lens to scale a frequency of the coherent light source received from the energy splitter;
an intensity modulator to receive output from the time lens and from the coherent light source to generate an optical clock signal derived from and in a phase relationship with the frequency of the coherent light source.

### Claim 2 (depends on 1)

The optical clock generator of claim 1, further comprising:
the intensity modulator being optically controlled from the output of the time lens.

### Claim 3 (depends on 1)

The optical clock generator of claim 1, further comprising:
a polarizer arranged between the coherent light source and the energy splitter thereby restricting the plane of polarization of the coherent light.

### Claim 4 (depends on 3)

The optical clock generator of claim 3, further comprising:
the time lens and the intensity modulator integrated as a lithographic opto-electronic micro-electronic structure.

### Claim 5 (depends on 1)

The optical clock generator of claim 1, wherein the coherent light source is a continuous wave laser.

### Claim 6 (depends on 1)

The optical clock generator of claim 1, wherein a scaling factor of the time lens is digitally-controlled.

### Claim 7 (depends on 2)

The optical clock generator of claim 2, wherein the intensity modulator is a Kerr cell.

### Claim 8 (depends on 2)

The optical clock generator of claim 2, wherein the intensity modulator is a Mach-Zehnder modulator.

### Claim 9 (depends on 1)

The optical clock generator of claim 1, wherein light energy not used to generate the optical clock is ducted for power transmission.

### Claim 10 (depends on 1)

The optical clock generator of claim 1, wherein light energy not used to generate the optical clock is ducted for power cooling.

### Claim 11 (depends on 1)

The optical clock generator of claim 1, further comprising:
a wavelength locking device arranged to receive the output of the coherent light source and to transmit a narrow wavelength range of light to the optical splitter.

### Claim 12

A method to generate a clock signal, comprising:
receiving electromagnetic energy from an energy source;
deriving a lower-frequency modulation frequency from the electromagnetic energy;
recombining the lower-frequency modulation frequency with the electromagnetic energy to generate a clock signal have a phased relationship with the electromagnetic energy.

### Claim 13 (depends on 12)

The method of claim 12, wherein the step of deriving a lower-frequency modulation frequency from the electromagnetic energy further comprises expanding the electromagnetic energy in time.

### Claim 14 (depends on 12)

The method of claim 12, wherein:
the electromagnetic energy is optical energy, the energy source is a continuous wave coherent light source, and the clock signal is an optical clock signal.

### Claim 15 (depends on 14)

The method of claim 14, further comprising:
polarizing the optical energy prior to deriving the lower-frequency modulation frequency.

### Claim 16 (depends on 12)

The method of claim 12, further comprising:
narrowing a range of frequencies of the electromagnetic energy output from the energy source.