Optical Circuits of Fiber-optic Devices

Fiber optics is a branch of optics that studies the physical phenomena that arise and flow in optical fibers, as well as products of the precision engineering industries, which include components based on optical fibers how fiber optics work.

Fiber optic devices include: lasers, amplifiers, multiplexers, demultiplexers, and others. Fiber optic components include: insulators, mirrors, connectors, splitters, and others. The basis of a fiber-optic device is its optical design - a set of fiber-optic components connected in a specific sequence. Optical circuits can be closed or open, with or without feedback.

Optical circuits of fiber-optic devices:

Laser

The figure shows the simplest diagram of a fiber-optic laser, where A is an active fiber, D is a pump diode, and M1 and M2 are mirrors. As in the case of conventional lasers, there is a cavity with an active medium formed by an active fiber and mirrors. Mirrors provide feedback. One of the mirrors can be 100% reflective. Then the radiation will come out only from the opposite end of the resonator. There can be several pump diodes, and they can be located on different sides of the resonator.

Amplifier

The figure shows the simplest diagram of a fiber optic amplifier. It is similar to that of a laser except that the mirrors are replaced with insulators to suppress feedback. Insulators only allow light to pass in one direction.

Fiber Optic Component Arrangement:

Mirrors and filters

A mirror is a component that reflects radiation of a certain frequency with a certain reflection coefficient. The filter, in turn, transmits radiation of a certain frequency, usually in a narrow frequency range, and absorbs or scatters the rest of the radiation. For the manufacture of mirrors and filters, diffraction gratings are used, deposited on a section of the fiber core. The analogue of the stroke is performed by ultraviolet illumination, which changes the properties of the fiber at the place of irradiation. One and the same diffraction grating for different signal frequencies will be either a mirror or a filter.

Insulators

Insulators allow radiation to pass in only one direction. In electricity, their analogue is diodes. Optical isolators operate on the Faraday effect and are not fully fiber optic. The working area of ​​the insulator consists of two polarizers and a Faraday cell (a magnet with a special crystal embedded in it). In the cavity of the magnet, and therefore in the crystal, a magnetic field is created, which rotates the polarization of the transmitted radiation by 45 degrees. The axes of the polarizers are also rotated at an angle of 45 degrees relative to each other. Due to this, radiation in one direction passes unhindered (and at the same time becomes linearly polarized), and does not pass in the other direction, because delayed by the output polarizer. The input and output of the insulator are made in the form of sections of optical fiber, so that this component can be used in optical circuits. To improve the characteristics of the insulator, lenses are placed after exiting the fiber and before entering another fiber, which make the beam parallel, which significantly reduces losses.

Connectors & Splitters

They are two parallel fibers, devoid of a shell and in contact with each other. Fiber contact and fixation is achieved at high temperatures - above the fiber melting point. Thus, the fiber sections are fused together. Depending on the length of the common section, as a result of wave interference, an arbitrary division ratio of the output signal can be obtained over the two output fibers.

Active fiber

A fiber capable of amplifying or generating a signal of a specific frequency. This is achieved by introducing rare-earth impurities into the silica fiber, depending on the required amplification frequency. Thus, ytterbium (Yb) impurities give amplification at wavelengths of 1.06 and 1.3 µm, and erbium (Er) impurities at a wavelength of 1.5 µm. The gain peak is determined by the transparency peak of a particular impurity.

Passive fiber

Non-reinforcing fiber. It is used to connect fiber-optic components to each other, as well as to increase the total length of the optical circuit, if necessary.

Pump diodes

As in the case of conventional lasers, pumping of the active medium is required to start amplification and generation. Semiconductor laser diodes are used to pump active fibers. At the exit from the semiconductor crystal, the laser beam is collimated and introduced into the fiber. The choice of the wavelength of the pump diodes is due to the absorption peaks of active fibers, which fall in narrow ranges in the regions of 0.81 mm, 0.98 mm, and 1.48 mm. For ytterbium fibers, pumping is most efficient in the range 0.95-0.98 mm. By looking at the ratio of pump and signal wavelengths, one can determine the maximum possible efficiency of lasers and amplifiers. For ytterbium fibers, it will be 0.95: 1.06 = 90%. In practice, the efficiency is of course lower.

Non-linear effects in optical fibers

2nd harmonic generation

With sufficient radiation power in the optical fiber, the radiation frequency is doubled. For example, in ytterbium fibers operating at a wavelength of 1.06 μm, the second harmonic generation leads to the appearance of radiation with a wavelength of 0.53 μm. This radiation is in the visible range and is an emerald green light.

Fiber Optic Applications

Optical fibers can be used for medical purposes, for example, inserting a camera into a patient's body to transmit an image of an organ or lesion to an external television camera, which eliminates the need for examination using surgery. In automobiles, they are used to supply light from a common source to various dashboards. Optical fibers link computers, robots, television sets, and telephones in many factories and offices.