# US 5,708,671 A · Tunable gigihertz all-optical clock generator and method using same

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- Number: US 5,708,671 A
- Document ID: US5708671A
- Status: Granted
- Kind: A
- Application number: 08/633,514
- Filed: 1996-04-17
- Granted: 1998-01-13
- Earliest claimed: 1996-04-17
- Assignees: Semi-Custom Logic, Inc.
- Inventors: Sueh-Wen Siao; Zoaib Z. Rangwala
- Web page: https://patentgrep.com/patent/US5708671A

## CPC

- H03B 28/00: Generation of oscillations by methods not covered by groups H03B5/00 - H03B27/00, including modification of the waveform to produce sinusoidal oscillations (analogue function generators for performing computing operations G06G7/26; use of transformers for conversion of waveform in AC-AC converters H02M5/18)
- H01S 5/0658: {Self-pulsating}
- H03B: GENERATION OF OSCILLATIONS, DIRECTLY OR BY FREQUENCY-CHANGING, BY CIRCUITS EMPLOYING ACTIVE ELEMENTS WHICH OPERATE IN A NON-SWITCHING MANNER; GENERATION OF NOISE BY SUCH CIRCUITS (generators adapted for electrophonic musical instruments G10H; masers or lasers H01S; generation of oscillations in plasma H05H)
- H01S: DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL

## Abstract

A fully integrated photonic IC for optical clock generation using no electronic current modulation. A first and second laser stripes are joined by a Kerr effect non-linear directional coupler, NLDC. The first stripe is pumped with a DC current for laser generation, and the second stripe has a reflector at an end facet for generation of mode selective optical feedback. The DC current generates relaxation oscillation in both stripes and the Kerr effect of the NLDC causes the electric fields in both stripe to interact constructively so as to shift the relaxation oscillation to undamped oscillation. The optical clock generator is tuned by varying the applied DC current.

## Claims

### Claim 1

A self pulsating optical semiconductor apparatus comprising:
a plurality of semiconductor layers disposed over a substrate, one of said semiconductor layers forming an active region for lightwave generation and propagation under lasing conditions;
first and second waveguides fabricated in said semiconductor layers and being optically coupled to each other by a nonlinear coupling layer, said first waveguide being in direct optical communication with said active region and said second waveguide having reflector means for establishing optical feedback; and
means for applying an electrical forward bias to said layers for generating lightwaves in said active region.

### Claim 2 (depends on 1)

The apparatus of claim 1 wherein said nonlinear coupling layer forms a nonlinear directional coupler.

### Claim 3 (depends on 1)

The apparatus of claim 1 wherein said nonlinear coupling layer has a non-linear Kerr effect.

### Claim 4 (depends on 1)

The apparatus of claim 1 wherein said nonlinear coupling layer is formed from a material and is configured such that its index of refraction varies with respect to the intensity of said lightwaves.

### Claim 5 (depends on 1)

The apparatus of claim 1 wherein said nonlinear coupling layer has a normalized coupling strength that is not more than .alpha.p/(1+2p) and not less than (1+2p)/.alpha.T where p is equal to (1/2)g'N.sub.th .tau..sub.p (P/P.sub.th -1), .alpha. is a line width enhancement factor and T is a ratio of a charge carrier's lifetime and a photon's lifetime.

### Claim 6 (depends on 1)

The apparatus of claim 1 wherein said reflector means is mode selective.

### Claim 7 (depends on 1)

The apparatus of claim 1 wherein said reflector means is one of a mirror facet, a grated reflector and a distributed Bragg reflector.

### Claim 8 (depends on 1)

The apparatus of claim 1 wherein said first and second waveguides form an in-plane, non-linear, vertical directional coupler.

### Claim 9 (depends on 1)

The apparatus of claim 1 wherein said first waveguide is an electrically pumped laser and said second waveguide is an optically pumped laser.

### Claim 10

A self pulsating optical semiconductor apparatus comprising:
a directional coupler joining first and second waveguides with a common resonant cavity therebetween;
a gain medium located within said first waveguide; and
means located within said common resonant cavity for nonlinearly varying the spatial profile of a laser beam within said common cavity with respect to the intensity of said laser beam.

### Claim 11 (depends on 10)

The self pulsating optical semiconductor apparatus of claim 10 wherein said first waveguide is an electrically pumped laser and said second waveguide is an optically pumped laser.

### Claim 12 (depends on 10)

The self pulsating optical semiconductor apparatus of claim 10 further comprising reflector means for establishing optical feedback located at an end facet of said second waveguide.

### Claim 13 (depends on 12)

The self pulsating optical semiconductor apparatus of claim 12 wherein said reflector means for establishing optical feedback is one of a mirror facet, grated reflector and distributed Bragg reflector.

### Claim 14 (depends on 12)

The optical semiconductor apparatus of claim 12 wherein said reflector means is mode selective.

### Claim 15 (depends on 10)

The optical semiconductor apparatus of claim 10 wherein said common cavity is configured such that propagating lightwaves within said common resonant cavity have antisymmetric mode interaction.

### Claim 16 (depends on 10)

The optical semiconductor apparatus of claim 10 wherein said common resonant cavity has a normalized coupling strength that is not more than .alpha.p/(1+2p) and not less than (1+2p)/.alpha.T where p is equal to (1/2)g'N.sub.th .tau..sub.p (P/P.sub.th -1), .alpha. is a line width enhancement factor and T is a ratio of a charge carrier's lifetime and a photon's lifetime.

### Claim 17 (depends on 10)

The optical semiconductor apparatus of claim 10 wherein said common cavity has a structure such that its index of refraction varies with respect to the intensity of said laser beam.

### Claim 18 (depends on 10)

The optical semiconductor apparatus of claim 10 further comprising means for applying an electric bias to said gain medium for generating said laser beam within said gain medium.

### Claim 19

A self pulsating laser comprising:
a waveguide having an active region;
means for applying a electric bias to said active region for lightwave generation;
an intensity-dependent external cavity optically coupled to said waveguide; and
means for generating mode selective lightwave reflection in said intensity-dependent external cavity.

### Claim 20 (depends on 19)

The self pulsating laser of claim 19 wherein said means for generating mode selective lightwave reflection is one of a grated reflector, a mirror facet and a distributed Bragg reflector.

### Claim 21 (depends on 19)

The self pulsating laser of claim 19 wherein said intensity-dependent external cavity is made from a material and is configured such that its index of refraction varies with respect to the intensity of said lightwaves.

### Claim 22 (depends on 19)

The self pulsating laser of claim 19 wherein said intensity-dependent external cavity has a normalized coupling strength that is not more than .alpha.p/(1+2p) and not less than (1+2p)/.alpha.T where p is equal to (1/2)g'N.sub.th .tau..sub.p (P/P.sub.th -1), .alpha. is a line width enhancement factor and T is a ratio of a charge carrier's lifetime and a photon's lifetime.

### Claim 23

A method of operating a self pulsating optical apparatus having a resonant cavity, a gain region, means for applying a DC electric bias to said gain region for generating propagating lightwaves, and means for generating mode selective light reflection at an end reflector comprising the steps of:
inducing intensity relaxation oscillation of said lightwaves in response to an application of said DC electric bias to said gain region; and
varying the index of refraction of said resonant cavity to vary the temporal and spatial profile of said lightwaves.

### Claim 24 (depends on 23)

The method of claim 23 wherein said index of refraction is varied in response to an intensity beat of the propagating lightwaves.

### Claim 25 (depends on 23)

The method of claim 23 wherein said varying of the index of refraction causes a constructive gain interaction between the intensity relaxation oscillation of the lightwaves in said resonant cavity.

### Claim 26 (depends on 23)

The method of claim 23 wherein said varying of the index of refraction introduces a modulating phase shift of the propagating lightwaves causing them to interfere constructively.

### Claim 27 (depends on 23)

The method of claim 23 further including the step of tuning said self pulsating optical apparatus by adjusting said DC electric bias to cause a change in the frequency of said relaxation oscillation.

### Claim 28 (depends on 27)

The method of claim 27 further defined by increasing the oscillation frequency with increasing DC bias.

### Claim 29 (depends on 23)

The method of claim 23 further defined by establishing a frequency range for said self pulsating optical apparatus with selection of coupling strength between the resonant cavity and the reflector.