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Simple Introduction to Several Types of Optical Fiber Gratings

Uniform optical fiber grating


The refractive index variation period of uniform optical fiber grating is generally in the order of 0.1 μm. It can reflect light of a specific wavelength in the incident light, with narrow reflection bandwidth. In the field of sensors, uniform optical fiber grating can be used to make temperature sensors, strain sensors and other sensors. In the field of optical communication, uniform optical fiber grating can be used to make bandpass filters, multiplexers/demultiplexers, and wavelength division multiplexers/demultiplexers.


Uniform long period fiber grating


The refractive index variation period of uniform long period fiber grating is generally in the order of 100 μm. It can couple the guided mode in the core over a certain range of wavelengths to the cladding mode and dissipate it. In the field of sensors, long period fiber grating can be used to make microbend sensors, refractive index sensors and other sensors. In the field of optical communication, long period fiber grating can be used to make erbium-doped fiber amplifier gain flattening filters, mode converters, and bandstop filters.


Apodized optical fiber grating


For a certain length of uniform optical fiber grating, there are a series of side peaks on both sides of the main peak in the reflection spectrum, which are generally called the edge modes of the grating. If the grating is used in some devices such as dense wavelength division multiplexers that require higher suppression ratio of the edge modes, the existence of these side peaks is a negative factor that seriously affects the channel isolation of the device. To reduce the edge modes of the grating, a commonly used method is apodization, which modulates the refractive index modulation amplitude of the fiber grating with specific functions. The fiber grating modulated by apodization is called apodized optical fiber grating, and its edge modes in the reflection spectrum are significantly reduced.


Phase-shifted fiber grating


Phase-shifted fiber grating is composed of multiple sections of uniform fiber grating with different lengths and M-1 connecting regions connecting these gratings (M>2). Phase-shifted fiber grating has a transmission window in its reflection spectrum and can be directly used as a bandpass filter.


Sampled optical fiber grating


Sampled fiber grating, also known as superstructure fiber grating, is composed of multiple sections of fiber grating with the same parameters cascaded with equal spacing. In addition to being used as a comb filter, sampled fiber grating can also be used in system multiplexing/demultiplexing devices. Unlike other multiplexers/demultiplexers, the ones constructed by sampled fiber grating can simultaneously multiplex/demultiplex multiple channels with the same spacing.


Chirped optical fiber grating


Chirped fiber grating refers to a fiber grating in which the amplitude of the refractive index variation or the period of the refractive index variation gradually increases (decreases) along the fiber axis. Different wavelengths of incident light can be reflected at different positions along the axis of the chirped fiber grating. Therefore, the characteristic of chirped fiber grating is wide reflection spectrum and gradually varying group delay in the reflection bandwidth, and the slope of the group delay curve is the dispersion value of the fiber grating. Therefore, chirped fiber grating can be used as a dispersion compensator.


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