Any back reflection light from the fiber line is isolated to prevent damage on the LD chip. In an optical transmitter, a free-space isolator is placed between the DFB LD chip and optical fiber. Comparing to FP LDs, DFB LDs have better monochromaticity, higher output power and higher modulation rate, which enable its more applications. The major application of the free-space isolators is inlaser diodes (LDs).Ī FP LD (Fabry-Perot laser diode) emits randomly polarized light, while DFB LD (distributed feedback laser diode) emits LP light. Thus it can’t be employed in-line (optical fiber)because the SOP (state of polarization) of optical signal in optical fiber is random. It requires the forward light to be LP polarized and the optical vector aligned to the first polarizer. However, it is polarization dependent, which limits its applications. Thus the forward light is let pass, while the backward light is isolated.įree-space isolator is characterized by simple structure and low cost. It is blocked by polarizer #2 finally because its optical vector is perpendicular to the transmission axis. For the backward light, it first passes polarizer #2 and then rotated by the FR. It passes polarizer #2 with little loss finally because its optical vector is aligned to the transmission axis. Fig.1 Structure of free-space optical isolatorįor the forward light(linearly polarized (LP)),itfirst passes polarizer #1 and then rotated by the FR. The transmission axes of the two polarizers are aligned with 45º angle and the FR has a fixed rotatory angle of 45º in a saturated magnetic field. The structure of a free-space optical isolator is shown in Fig.1, which comprises two polarizers, a Faraday rotator (FR) and a magnet ring. However, the device structures and characteristics are variable, which are detailed as follow. The basic and common principle for optical isolators is Faraday effect. Optical isolators are widely used in optical fiber communication systems, optical fiber sensing systems and fiber lasers.
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