An atomic clock comprising at least one light source adapted to provide an optical beam , at least one photo detector and a vapor cell comprising a first optical window , said optical beam being directed through said vapor cell for providing an optical frequency reference signal, said photo detector being adapted to detect said optical frequency reference signal and to generate at least one reference signal, wherein - said atomic clock further comprises a first optical waveguide arranged to said first optical window , - said first optical waveguide is a substantially flat optical waveguide, defining a first surface arranged on said first optical window and a second surface parallel to the first surface , and comprising a first edge at one of the longitudinal extremities of the first optical waveguide , - said first optical waveguide comprises an incoupling surface facing said light source and arranged to incouple at least a portion of said optical beam into said first optical waveguide , said portion providing a first guided light beam propagating into said first optical waveguide , said first optical waveguide being sized and shaped so that said first guided light beam undergoes at least three internal reflections in said first optical waveguide , - a first outcoupler is located in said first surface , facing said first optical window and being parallel thereto, so as to outcouple a first outcoupled light beam being a portion of the guided light beam that is transmitted to said vapor cell through said first optical window , - said first incoupling surface and said first outcoupler are separated by a distance D chosen to expand said first guided light beam having a cross-section in the plane of the outcoupler that is smaller, equal or greater than the cross-section of said first optical window and to provide said first outcoupled light beam , - the thickness (t) of the atomic clock , defined in the direction of the normal to said first surface , is smaller than 15 mm. - said atomic clock further comprises a first optical waveguide arranged to said first optical window , - said first optical waveguide is a substantially flat optical waveguide, defining a first surface arranged on said first optical window and a second surface parallel to the first surface , and comprising a first edge at one of the longitudinal extremities of the first optical waveguide , - said first optical waveguide comprises an incoupling surface facing said light source and arranged to incouple at least a portion of said optical beam into said first optical waveguide , said portion providing a first guided light beam propagating into said first optical waveguide , said first optical waveguide being sized and shaped so that said first guided light beam undergoes at least three internal reflections in said first optical waveguide , - a first outcoupler is located in said first surface , facing said first optical window and being parallel thereto, so as to outcouple a first outcoupled light beam being a portion of the guided light beam that is transmitted to said vapor cell through said first optical window , - said first incoupling surface and said first outcoupler are separated by a distance D chosen to expand said first guided light beam having a cross-section in the plane of the outcoupler that is smaller, equal or greater than the cross-section of said first optical window and to provide said first outcoupled light beam , - the thickness (t) of the atomic clock , defined in the direction of the normal to said first surface , is smaller than 15 mm. <EOS>