A coupled system for light of one or more optical signals, the coupled system comprising: a silicon photonic integrated circuit, Si PIC, having: a first waveguide (212, 716) formed in the Si PIC in a PIC process, the first waveguide having a first refractive index n1 and a tapered end; and at least one second waveguide (208, 712) formed in the Si PIC in the same Si PIC process as the first waveguide, each having a second refractive index n2; and an interposer being attached separately to the Si PIC, the interposer being configured to input and/or output the light of the one or more optical signals and comprising a third waveguide (224, 730), the third waveguide having a third refractive index n3 and a coupler portion, the interposer being: a polymer interposer (704, 2002, 2102, 2202, 2404) having a polymer substrate and having the third waveguide formed thereon as a polymer waveguide; or a polymer on glass interposer (2802, 2900), including: a glass substrate; a polymer cladding coupled to the glass substrate; and a polymer core, the polymer cladding and the polymer core collectively forming the third waveguide; wherein: the tapered end of the first waveguide (212, 716) is adiabatically coupled to a coupler portion of one of the at least one second waveguide (208, 712); a tapered end of one of the at least one second waveguide (208, 712) is adiabatically coupled to the coupler portion of the third waveguide (224, 730) of the interposer (104, 704); n1>n2>n3; the coupled system is configured to adiabatically couple light of the one or more optical signals between the first waveguide (212, 716) and the at least one second waveguide (208, 712) and between the at least one second waveguide (208, 712) and the third waveguide (224, 730). a silicon photonic integrated circuit, Si PIC, having: a first waveguide (212, 716) formed in the Si PIC in a PIC process, the first waveguide having a first refractive index n1 and a tapered end; and at least one second waveguide (208, 712) formed in the Si PIC in the same Si PIC process as the first waveguide, each having a second refractive index n2; and a first waveguide (212, 716) formed in the Si PIC in a PIC process, the first waveguide having a first refractive index n1 and a tapered end; and at least one second waveguide (208, 712) formed in the Si PIC in the same Si PIC process as the first waveguide, each having a second refractive index n2; and an interposer being attached separately to the Si PIC, the interposer being configured to input and/or output the light of the one or more optical signals and comprising a third waveguide (224, 730), the third waveguide having a third refractive index n3 and a coupler portion, the interposer being: a polymer interposer (704, 2002, 2102, 2202, 2404) having a polymer substrate and having the third waveguide formed thereon as a polymer waveguide; or a polymer on glass interposer (2802, 2900), including: a glass substrate; a polymer cladding coupled to the glass substrate; and a polymer core, the polymer cladding and the polymer core collectively forming the third waveguide; a polymer interposer (704, 2002, 2102, 2202, 2404) having a polymer substrate and having the third waveguide formed thereon as a polymer waveguide; or a polymer on glass interposer (2802, 2900), including: a glass substrate; a polymer cladding coupled to the glass substrate; and a polymer core, the polymer cladding and the polymer core collectively forming the third waveguide; wherein: the tapered end of the first waveguide (212, 716) is adiabatically coupled to a coupler portion of one of the at least one second waveguide (208, 712); a tapered end of one of the at least one second waveguide (208, 712) is adiabatically coupled to the coupler portion of the third waveguide (224, 730) of the interposer (104, 704); n1>n2>n3; the coupled system is configured to adiabatically couple light of the one or more optical signals between the first waveguide (212, 716) and the at least one second waveguide (208, 712) and between the at least one second waveguide (208, 712) and the third waveguide (224, 730). the tapered end of the first waveguide (212, 716) is adiabatically coupled to a coupler portion of one of the at least one second waveguide (208, 712); a tapered end of one of the at least one second waveguide (208, 712) is adiabatically coupled to the coupler portion of the third waveguide (224, 730) of the interposer (104, 704); n1>n2>n3; the coupled system is configured to adiabatically couple light of the one or more optical signals between the first waveguide (212, 716) and the at least one second waveguide (208, 712) and between the at least one second waveguide (208, 712) and the third waveguide (224, 730).