Note: Measuring grating periods by diffraction method with a fore-end light path comprising fused fiber couplers and fiber port collimators

Based on the Littrow and Littman configurations for autocollimation, two methods are proposed herein with a fore-end light path comprising fused fiber couplers and fiber port collimators to measure the diffraction grating period. The experimental data from a uniform-line-spacing grating with a nominal value of 1200 lines/mm agreed well with theoretical values and with measurement errors of the Littrow and the Littman configurations of 1.05 × 10−4 and 4.99 × 10−5, respectively. Both methods were demonstrated to reduce the diameter of the laser spot and improve the alignment precision of the measurement system.