| Polarization |
Polarized light refers to a state in which the vibration direction of light is biased. Light is a transverse wave (the electric field and magnetic field oscillate in a plane perpendicular to the direction of travel), so in the case of perfect polarization, it can be represented by the composition of two orthogonal vibration components. When polarization is represented by the combination of two orthogonal linearly polarized lights with the same amplitude, a phase difference of 0 or 180° (π) result in linearly polarized light and a phase difference of ±90° (π/2) result in circularly polarized light. And other phase differences result in elliptically polarized light.
- ●Polarization extinction ratio (PER)
- The polarization extinction ratio (PER) is used as a value that indicates how close the linearly polarized light is to perfection. The polarization extinction ratio is measured as the ratio between the maximum and minimum values when the measurement light is transmitted through a polarizer and the polarizer is rotated to measure the output light amount. The polarization extinction ratio (PER) is generally expressed in dB and is calculated by the following formula.
 - ●Degree of polarization (DOP)
- The actual polarization state is often a mixture of perfect polarization (linear polarization/circular (elliptical) polarization) and non-polarization (random). Degree of polarization (DOP) is an index indicating such a polarization state, and is defined as follows using Stokes parameters.
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| Polarizer |
An optical element that can obtain linearly polarized light by transmitting light. The performance of a polarizer is represented by the value of the polarization extinction ratio of transmitted light. |
| Polarization dependence and polarization dependence compensation |
The surface reflection (transmission) characteristics when light is obliquely incident on a glass substrate or beam splitter are generally different for P-polarized light and S-polarized light. Therefore, in optical systems using these BSs, the intensity of reflected/transmitted light fluctuates when the polarization state of incident light changes (polarization dependence). It is very difficult to eliminate this polarization dependency by designing a dielectric multilayer film in a wide wavelength range. Therefore, the polarization characteristics can be compensated by arranging and using the tilt directions of the two beam splitters having the same characteristics in the orthogonal directions. (Because the component that was P-polarized in the first-stage BS becomes S-polarized in the second-stage BS, and the component that was S-polarized in the first-stage BS plane becomes P-polarized in the second-stage BS plane.) Whether the beam splitter is used for reflection-reflection or transmission-transmission, polarization characteristics can be compensated by making the BS tilt directions orthogonal to each other. |
| Waveguide mode of optical fiber |
Here, the waveguide mode of light propagating in the optical fiber will be described. The waveguide mode (transverse mode) of a fiber changes depending on the size of the core, the refractive index of the core/cladding, and the wavelength. The number of modes varies depending on the type of fiber, and some have 1 to several thousand or more modes. In general, there are single mode fibers (propagating only the fundamental mode) and multimode fibers.
- ●The standardized frequency V
- The number of modes that can exist in the optical fiber can be determined by the standardized frequency V (V parameter). The standardized frequency V is defined by the following formula.
 - ●Cutoff wavelength λc
- In the case of step index, under the condition that the refractive index distribution of the fiber is V≦2.405, it becomes a single mode fiber in which only the fundamental mode is excited. The wavelength λc at this time is called (theoretical) cutoff wavelength.
 - Further, when the normalized frequency is sufficiently large, the number of modes that can be propagated in the step index type multimode fiber can be approximated by the following formula.
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| SMF (Single mode optical fiber) |
An optical fiber with the characteristic that only the fundamental mode is excited at the wavelength of light used. |
| MMF (Multi mode optical fiber) |
A fiber that can excite multiple modes at the wavelength used. There are two main types of multimode fibers with different structures.
- SI type MMF : A multimode fiber with a flat core refractive index profile.
- GI type MMF : A multimode fiber with characteristics that the refractive index distribution of the core gradually decreases from the center to the periphery. Compared to SI type MMF, mode dispersion (difference in propagation velocity due to mode difference) is smaller, so the spread of signal pulse is suppressed and longer distance transmission is possible.
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| Insertion loss |
The ratio of the incident power to the optical component and the output power from the optical component is calculated by the following formula. It represents the amount of attenuation of optical power when an optical component is inserted in the optical path.
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| Silicon photonics |
A technology to fabricate optical circuits (optical waveguides, optical switches, wavelength filters, optical modulators, light receivers, and light emitters) on Si substrates using the microfabrication technology of semiconductor manufacturing. Therefore, it becomes possible to fabricate a hybrid device in which optical and electronic components are integrated on the same chip.
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| Silicon waveguide |
An optical circuit constructed by fabricating a Si wire waveguide on an SOI substrate with a thickness and width of sub micron as a Si wire waveguide. Since the Si wire waveguide has a very strong optical confinement effect, a bending radius on the order of µm is possible, and a very small optical circuit can be realized. Generally, a spot size converter or a grating coupler is used for input and output light into and from the waveguide. As applications to passive devices, optical switches using optical couplers and Mach-Zehnder interferometers, wavelength filters using ring interferometers, etc. can be realized.
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| Polymer optical waveguide |
An optical waveguide manufactured using a polymer as the material for the optical waveguide. The wavelength used is generally in the 850 nm band, which has good matching with VCSEL. Although SI type multimode waveguides are generally used as the structure, GI type and SMF type ones are being developed. It is expected to be applied to short-distance optical wiring boards and opto-electric hybrid boards.
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| LD(Laser diode) light source |
The following are the LD light sources. A Fabry-Pérot LD that uses the reflection on both end faces of the active layer (light-emitting layer) that has a waveguide structure as a cavity for laser oscillation. Distributed feedback type laser (DFB) that performs laser oscillation at a single wavelength using a diffraction grating. Vertical Cavity Surface Emitting Laser (VCSEL) with a cavity formed vertically on a substrate. A large number of products are being sold that combine laser light from an LD element with an optical fiber to make a fiber emission type, and use an output stabilizing light source for temperature control. Although it depends on the structure of the LD element, it is generally a light source with a narrow wavelength width and good coherence in terms of time and space.
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| SLD(Superluminescent diode) light source |
Like the FP-LD, the SLD generates light in the waveguide, but unlike the FP-LD, because it has a structure that does not form a cavity, it has a broadband wavelength width in which spontaneous emission is amplified by stimulated emission. Since the light is emitted from the end of the waveguide, the spatial coherence is high, but the wavelength coherence is wide, and thus the temporal coherence is low. Utilizing these features, SLD light sources are used in OCT (coherence tomography) and optical fiber gyroscope (FOG).
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