# US 5,710,849 A · Taper shapes for flatband response and sidelobe suppression in grating assisted optical coupler filters

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- Number: US 5,710,849 A
- Document ID: US5710849A
- Status: Granted
- Kind: A
- Application number: 08/551,470
- Filed: 1995-11-01
- Granted: 1998-01-20
- Earliest claimed: 1995-11-01
- Assignees: Northern Telecom Limited
- Inventors: Brent Little; Chi Wu; Wei-Ping Huang
- Web page: https://patentgrep.com/patent/US5710849A

## CPC

- G02B 6/12007: {forming wavelength selective elements, e.g. multiplexer, demultiplexer}
- G02B 2006/12107: {Grating}
- G02B 2006/12147: {Coupler}
- G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS

## Abstract

Improvements in grating-assisted optical filter couplers are described. In particularly, taper shapes for the gratings-assisted codirectional couplers are determined to provide an out-of-band sidelobe suppression ratio of greater than -30 dB, and a passband response that is flat and has a transmission of greater than 99.55% of the transmission at the passband center, over a wavelength range of 25% of the half-power bandwidth. The interaction strength defining the optimized spectral characteristics is given by K.sub.1 (z)=A.sub.1 e.sup.-.alpha..sbsp.1.sup.(z-z.sbsp.i.sup.).spsp.2 +A.sub.2 e.sup.-.alpha..sbsp.2.sup.(z-z.sbsp.2.sup.).spsp.2 A.sub.3 e.sup.-.alpha..sbsp.3.sup.(z-z.sbsp.3.sup.).spsp.2 0<z<1 where K.sub.1 (z) is the normalized interaction strength along the propagation direction, and the constant coefficients are A.sub.1 =8.774; A.sub.2 =-3.45; A.sub.3 =1.183; .alpha..sub.1 =51.87; .alpha..sub.2 =34.49; .alpha..sub.3 =20.63; z.sub.1 =0.28; z.sub.2 =0.60; z.sub.3 =0.75. where z is the normalized distance and is related to the physical distance Z=Z/L.sub.c, where L.sub.c is the coupler length. At the positions z=0.462 and z=0.777, where the function K.sub.1 changes sign, the grating period is moved forward or backwards by one-half period. The specified interaction strength K.sub.1 is produced in practice by modulation of one of the physical parameters of a grating assisted coupler, for example by: (i) modulating the space relationship between two waveguides, (ii) modulating the refractive index contrast of the grating, (iii) modulating the duty cycle of the grating, (iv) modulating the height of a surface corrugation grating, (v) modulating the width of the grating regions, or (vi) modulating the ridge width of coupled ridge waveguides.

## Claims

### Claim 1

A grating-assisted filter coupler comprising: a first waveguide for receiving an optical signal having at least one predetermined wavelength, and
a second waveguide in a spaced relationship to the first waveguide,
a grating region providing selective coupling of said predetermined wavelength between the first waveguide and the second waveguide,
the coupling between first and second waveguides being tailored by a taper function defining an interaction strength given by
K.sub.I (z)=A.sub.1 e.sup.-.alpha..sbsp.1.sup.(z-z.sbsp.i.sup.).spsp.2 +A.sub.2 e.sup.-.alpha..sbsp.2.sup.(z-z.sbsp.2.sup.).spsp.2 +A.sub.3 e.sup.-.alpha..sbsp.3.sup.(z-z.sbsp.3.sup.).spsp.2 0<z<1
where: K.sub.I (z) is the normalized interaction strength along the propagation direction;
A.sub.1, A.sub.2, A.sub.3 ; .alpha..sub.1, .alpha..sub.2, .alpha..sub.3 ; z.sub.1, z.sub.2, z.sub.3 ; are sets of constant coefficients determined to provide a specific level of sidelobe suppression and passband response;
and z is the normalized propagation distance along the coupler.

### Claim 2 (depends on 1)

A grating assisted filter coupler according to claim 1 and having a passband center at a selected wavelength characterized by a half-power bandwidth, wherein constant coefficients are determined to provide an out-of-band sidelobe suppression ratio of greater than -30 dB, and a passband response that is flat and has a transmission of greater than 99.55% of the transmission at the passband centre, over a wavelength range of 25% of the half-power bandwidth.

### Claim 3

A grating-assisted filter coupler comprising a first waveguide for receiving an optical signal having at least one predetermined wavelength, and a second waveguide in a spaced relationship to the first waveguide,
a grating region providing selective coupling of said predetermined wavelength between the first and second waveguide along a coupler length L.sub.c,
the coupling being determined by an interaction strength function defining spectral characteristics of the filter coupler, determined by
K.sub.I (z)=A.sub.1 e.sup.-.alpha..sbsp.1.sup.(z-z.sbsp.i.sup.).spsp.2 +A.sub.2 e.sup.-.alpha..sbsp.2.sup.(z-z.sbsp.2.sup.).spsp.2 +A.sub.3 e.sup.-.alpha..sbsp.3.sup.(z-z.sbsp.3.sup.).spsp.2 0<z<1
where K.sub.I (z) is the normalized interaction strength along the propagation direction, and the constant coefficients are
A.sub.1 =8.774; A.sub.2 =-3.45; A.sub.3 =1.183;
.alpha..sub.1 =51.87; .alpha..sub.2 =34.49; .alpha..sub.3 =20.63;
z.sub.1 =0.28; z.sub.2 =0.60; z.sub.3 =0.75;
where z is the normalized distance and is related to the physical distance Z=zL.sub.c, where L.sub.c is the coupler length.

### Claim 4 (depends on 3)

A grating assisted filter coupler according to claim 3 wherein the specified interaction strength is produced by defining the space relationship between the first and second waveguides as a function of Z.

### Claim 5 (depends on 3)

A grating assisted filter coupler according to claim 3 wherein the grating region is characterized by a refractive index contrast modulation and the specified interaction strength is produced by defining the refractive index contrast modulation of the grating region as a function of Z.

### Claim 6 (depends on 3)

A grating assisted filter coupler according to claim 3 wherein the specified interaction strength is produced by defining a duty cycle of the grating region as a function of Z.

### Claim 7 (depends on 3)

A grating assisted filter coupler according to claim 3 wherein the specified interaction strength is produced by defining the height of a surface corrugation grating region as a function of Z.

### Claim 8 (depends on 3)

A grating assisted filter coupler according to claim 3 wherein the specified interaction strength is produced by defining a width of the grating region as a function of Z.

### Claim 9 (depends on 3)

A grating assisted filter coupler according to claim 3 wherein at least one of said first and second waveguides is a ridge waveguide and the specified interaction strength is produced by defining a ridge width of the at least one ridge waveguide as a function of Z.