Conventionally, in optical communications using an optical transmission member such as an optical fiber and an optical waveguide, an optical module including a light emitting element such as a surface-emitting laser (e.g. a vertical cavity surface emitting laser (VCSEL)) has been used. Such an optical module includes an optical receptacle that operates such that light containing communication information emitted from a light emitting element is incident on an end surface of the optical transmission member.
An optical module that is intended to adjust the optical output or stabilize the output performance of a light emitting element against temperature variation, and includes a detection element for checking (monitoring) the intensity and the quantity of the emission light emitted from the light emitting element is known (see, for example, PTL 1).
PTL 1 discloses an optical module including a photoelectric conversion device in which a light-emitting element and a detection element are disposed on a substrate, and an optical receptacle disposed on the substrate of the photoelectric conversion device and configured to optically couple the light-emitting element and an end surface of an optical transmission member.
FIG. 1A is a sectional view schematically illustrating a configuration of optical module 10 disclosed in PTL 1, and FIG. 1B is a partially enlarged sectional view illustrating a configuration of light separation part 33 of optical module 10. FIG. 1B is a partially enlarged sectional view of a region indicated with the broken line in FIG. 1A. In FIGS. 1A and 1B, the hatching on the cross-section of optical receptacle 30 is omitted to illustrate light paths in optical receptacle 30.
As illustrated in FIG. 1A, optical module 10 disclosed in PTL 1 includes photoelectric conversion device 20 and optical receptacle 30. Optical receptacle 30 includes first optical surface 31 that allows incidence of emission light L emitted from light-emitting element 21; reflection surface 32 that reflects, toward optical transmission member 22 side, emission light L having entered optical receptacle 30 from first optical surface 31; light separation part 33 that separates emission light L reflected by reflection surface 32 into monitor light Lm directed toward detection device 24 side and signal light Ls directed toward optical transmission member 22 side; transmission surface 34 that allows, to reenter optical receptacle 30, signal light Ls emitted out of optical receptacle 30 at light separation part 33; second optical surface 35 that emits signal light Ls having entered optical receptacle 30 from transmission surface 34 such that the light is collected at end surface 23 of optical transmission member 22; and third optical surface 36 that emits monitor light Lm toward detection device 24. In addition, light separation part 33 is formed as a part of the internal surface of recess 37 formed in optical receptacle 30.
As illustrated in FIG. 1B, light separation part 33 of optical receptacle 30 disclosed in PTL 1 includes divided reflection surface 33a that totally reflects a part of incident light toward third optical surface 36 as monitor light Lm, and divided transmission surface 33b that allows another part of incident light to pass therethrough toward second optical surface 35 as signal light Ls. Divided reflection surface 33a and divided transmission surface 33b alternate in the inclination direction of divided reflection surface 33a. 
In optical module 10 disclosed in PTL 1, emission light L which is emitted from light-emitting element 21 and is incident on first optical surface 31 is reflected by reflection surface 32 toward light separation part 33. Emission light L reaching light separation part 33 is separated into monitor light Lm and signal light Ls by divided reflection surface 33a and divided transmission surface 33b. Monitor light Lm separated by light separation part 33 (divided reflection surface 33a) is emitted from third optical surface 36 toward the light-receiving surface of detection element 24. On the other hand, signal light Ls separated by light separation part 33 (divided transmission surface 33b) is emitted out of optical receptacle 30 through light separation part 33, and then reenters optical receptacle 30 from transmission surface 34. The signal light Ls having reentered optical receptacle 30 from transmission surface 34 is emitted from second optical surface 35 toward end surface 23 of optical transmission member 22.