A phase retarder, comprising a polarization beam splitter, a first air arm, and a second air arm, wherein the polarization beam splitter is configured to decompose a beam incident on a first side wall of the polarization beam splitter into a first light component propagated in a first direction and a second light component propagated in a second direction, wherein the first direction is perpendicular to the second direction, and a polarization state of the first light component is perpendicular to a polarization state of the second light component; the first air arm is disposed on a second side wall of the polarization beam splitter, and is configured to receive the first light component and reflect the first light component back to the polarization beam splitter; and the second air arm is disposed on a third side wall of the polarization beam splitter, and is configured to receive the second light component and reflect the second light component back to the polarization beam splitter, so that interference light is formed after the second light component interferes with the first light component, and the interference light is emitted from a fourth side wall of the polarization beam splitter, wherein a first optical path of the first air arm is not equal to a second optical path of the second air arm, the first side wall and the third side wall are disposed opposite to each other, and the second side wall and the fourth side wall are disposed opposite to each other, wherein the first air arm comprises a first etalon and a first polarizer, the first etalon has a first air cavity and a first reflective surface, and the first polarizer is disposed in the first air cavity, wherein the first polarizer is configured to change the polarization state of the first light component emitted from the second side wall, wherein the first light component is propagated to the first reflective surface after being transmitted through the first polarizer; and the first reflective surface is configured to reflect the received first light component to the first polarizer, so that the first polarizer changes the polarization state of the first light component again and then propagates the first light component to the polarization beam splitter, to enable the first light component to be propagated in the first direction, wherein the second air arm comprises a second etalon and a second polarizer, the second etalon has a second air cavity and a second reflective surface, and the second polarizer is disposed in the second air cavity; the second polarizer is configured to change the polarization state of the second light component emitted from the third side wall, wherein the second light component is propagated to the second reflective surface after being transmitted through the second polarizer; and the second reflective surface is configured to reflect the received second light component to the second polarizer, so that the second polarizer changes the polarization state of the second light component again and then propagates the second light component to the polarization beam splitter, to enable the second light component to be propagated in the first direction, and wherein an inner surface of the first reflective surface of the first etalon and an inner surface of a front wall that is provided on the second side wall are inclined surfaces with predetermined angles, and an inner surface of the second reflective surface of the second etalon and an inner surface of a front wall that is provided on the third side wall are inclined surfaces with predetermined angles.