An N-channel wavelength division multiplexing, WDM, optical subassembly, OSA, (300, 600A, 600B), comprising: a plurality of active optical devices coupled to a carrier; a wavelength division multiplexer, MUX, or a wavelength division demultiplexer, DEMUX, including a plurality of elements; an optical block positioned above the plurality of active optical devices and coupled to the carrier, wherein the optical block includes: a bottom with a plurality of lenses formed in the bottom that are aligned with the plurality of active optical devices; the optical block further including: a first side that extends up from the bottom, the first side defining a first plurality of ledges; a second side that extends up from the bottom and is opposite the first side, the second side defining a second plurality of ledges; a port (204, 206) that extends forward from the bottom and the first and second sides (316, 318); and an optical block cavity defined by the bottom and the first and second sides (316, 318) that extends rearward from the port (204, 206), wherein each one of the first plurality of ledges together with a corresponding one of the second plurality of ledges is configured to support a respective one of the plurality of elements above a corresponding one of the plurality of active optical devices and a corresponding one of the plurality of lenses and at an angle of 45 ± 2 degrees to incoming light; and the plurality of elements comprising a plurality of thin film filters; and wherein the MUX or the DEMUX and the plurality of thin film filters are positioned in the optical block cavity in an optical path between the port of the optical block and the plurality of active optical devices. a plurality of active optical devices coupled to a carrier; a wavelength division multiplexer, MUX, or a wavelength division demultiplexer, DEMUX, including a plurality of elements; an optical block positioned above the plurality of active optical devices and coupled to the carrier, wherein the optical block includes: a bottom with a plurality of lenses formed in the bottom that are aligned with the plurality of active optical devices; the optical block further including: a first side that extends up from the bottom, the first side defining a first plurality of ledges; a second side that extends up from the bottom and is opposite the first side, the second side defining a second plurality of ledges; a port (204, 206) that extends forward from the bottom and the first and second sides (316, 318); and an optical block cavity defined by the bottom and the first and second sides (316, 318) that extends rearward from the port (204, 206), wherein each one of the first plurality of ledges together with a corresponding one of the second plurality of ledges is configured to support a respective one of the plurality of elements above a corresponding one of the plurality of active optical devices and a corresponding one of the plurality of lenses and at an angle of 45 ± 2 degrees to incoming light; and a bottom with a plurality of lenses formed in the bottom that are aligned with the plurality of active optical devices; the optical block further including: a first side that extends up from the bottom, the first side defining a first plurality of ledges; a second side that extends up from the bottom and is opposite the first side, the second side defining a second plurality of ledges; a port (204, 206) that extends forward from the bottom and the first and second sides (316, 318); and an optical block cavity defined by the bottom and the first and second sides (316, 318) that extends rearward from the port (204, 206), wherein each one of the first plurality of ledges together with a corresponding one of the second plurality of ledges is configured to support a respective one of the plurality of elements above a corresponding one of the plurality of active optical devices and a corresponding one of the plurality of lenses and at an angle of 45 ± 2 degrees to incoming light; and a first side that extends up from the bottom, the first side defining a first plurality of ledges; a second side that extends up from the bottom and is opposite the first side, the second side defining a second plurality of ledges; a port (204, 206) that extends forward from the bottom and the first and second sides (316, 318); and an optical block cavity defined by the bottom and the first and second sides (316, 318) that extends rearward from the port (204, 206), wherein each one of the first plurality of ledges together with a corresponding one of the second plurality of ledges is configured to support a respective one of the plurality of elements above a corresponding one of the plurality of active optical devices and a corresponding one of the plurality of lenses and at an angle of 45 ± 2 degrees to incoming light; and the plurality of elements comprising a plurality of thin film filters; and wherein the MUX or the DEMUX and the plurality of thin film filters are positioned in the optical block cavity in an optical path between the port of the optical block and the plurality of active optical devices.