An optical apparatus comprising: a plurality of transmitters; a multiplexer comprising: a first Bragg grating having a first input port coupled with a first transmitter of the plurality of transmitters, a first add port coupled with a second transmitter of the plurality of transmitters, and a first output port configured to output a first optical signal comprising a first two wavelengths; a second Bragg grating having a second input port coupled with a third transmitter of the plurality of transmitters, a second add port coupled with a fourth transmitter of the plurality of transmitters, and a second output port configured to output a second optical signal comprising a second two wavelengths; and a third Bragg grating having a third input port configured to receive the first two wavelengths, a third add port configured to receive the second two wavelengths, and a third output port configured to output a third optical signal comprising the first two wavelengths and the second two wavelengths; characterized in that the multiplexer further comprises a fourth Bragg grating directly coupled with the first output port, wherein a fourth output port of the fourth Bragg grating is directly coupled with one of the third input port and the third add port; and an optical absorber coupled with a fourth add port of the fourth Bragg grating. a plurality of transmitters; a multiplexer comprising: a first Bragg grating having a first input port coupled with a first transmitter of the plurality of transmitters, a first add port coupled with a second transmitter of the plurality of transmitters, and a first output port configured to output a first optical signal comprising a first two wavelengths; a second Bragg grating having a second input port coupled with a third transmitter of the plurality of transmitters, a second add port coupled with a fourth transmitter of the plurality of transmitters, and a second output port configured to output a second optical signal comprising a second two wavelengths; and a third Bragg grating having a third input port configured to receive the first two wavelengths, a third add port configured to receive the second two wavelengths, and a third output port configured to output a third optical signal comprising the first two wavelengths and the second two wavelengths; characterized in that the multiplexer further comprises a fourth Bragg grating directly coupled with the first output port, wherein a fourth output port of the fourth Bragg grating is directly coupled with one of the third input port and the third add port; and a first Bragg grating having a first input port coupled with a first transmitter of the plurality of transmitters, a first add port coupled with a second transmitter of the plurality of transmitters, and a first output port configured to output a first optical signal comprising a first two wavelengths; a second Bragg grating having a second input port coupled with a third transmitter of the plurality of transmitters, a second add port coupled with a fourth transmitter of the plurality of transmitters, and a second output port configured to output a second optical signal comprising a second two wavelengths; and a third Bragg grating having a third input port configured to receive the first two wavelengths, a third add port configured to receive the second two wavelengths, and a third output port configured to output a third optical signal comprising the first two wavelengths and the second two wavelengths; characterized in that the multiplexer further comprises a fourth Bragg grating directly coupled with the first output port, wherein a fourth output port of the fourth Bragg grating is directly coupled with one of the third input port and the third add port; and an optical absorber coupled with a fourth add port of the fourth Bragg grating.