A calibration device consisting of a gas cell constructed so one window (16) is a Fabry-Perot etalon. The gas cell absorption lines are used to calibrate the Fabry-Perot etalon characteristics. Thus the device can be used to calibrate an optical instrument over a broad range of frequencies that are generated by the Fabry-Perot etalon with the accuracy determined by the stable gas absorption lines. A gas cell in combination with other frequency artifact generators such a Fiber-Bragg grating (30). The artifact generator has artifacts both within the gas cell spectrum as well as beyond. The transmission characteristics within the gas cell spectrum are used to calibrate the artifact at frequencies outside this range.