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The demand for communication bandwidth is endless. Recently, through the improvement of optical communication TDM or WDM system technology,
It has been able to significantly increase the communication bandwidth, and the future high-speed communication transmission network will most likely be a combination of these two optical communication systems. In recent years, the obvious progress of "Fiber Grating" in the manufacturing technology is rapidly impelling the design trend of these two optical communication technology systems.
According to the prediction of the Canadian CRC Research Center: In the next few years, the world market value of fiber grating will increase to the scale of 100 million US dollars, mainly due to the impact of fiber grating widely used in optical communications. Published in
Vol.6,1996 has made a brief introduction to fiber grating. In this issue, we will further discuss the process technology of fiber grating and its application trend in optical communication system.
Fiber Bragg Grating Process Technology Trends Fiber Bragg Grating is a single-mode fiber core with a diameter of about 10 microns, which is exposed to periodic strong ultraviolet light stripes. These ultraviolet light stripes are mainly interfered by an excimer laser with a wavelength of 248nm or 193nm. Or through the phase mask.
This interference fringe will form a periodic refractive index change in the fiber core. When the wavelength of the incident light meets the Bragg condition,
The scattered light will return along the original optical path to form an optical filter. In terms of practicality and mass production, United in 1989
Technologies developed the "holographic interferometry" to greatly improve the practicality of fiber gratings; followed by 1993 in Hill
The development of the "phase mask method" has greatly increased the feasibility of mass production of fiber gratings.
The main performance parameters of fiber grating include: reflection wavelength, central wavelength reflectivity, spectral line width and loss.
These performance parameters are affected by structural parameters such as refractive index changes, grating length, and grating period.
Generally speaking, the operating wavelength of a fiber grating depends on the grating period; the operating bandwidth and reflectivity of the fiber grating depends on the length of the grating and the amount of change in the refractive index of the fiber core. The longer the grating, the narrower the bandwidth and the higher the reflectivity, and the change in the refractive index of the fiber core is more related to the quality of the product and the manufacturing cost. Therefore, the improvement of manufacturing technology is mainly to increase the amount of refractive index change.
It is generally believed that during the manufacture of fiber gratings, the change in the refractive index of the fiber is due to the Ge-Si misbonding in the fiber core
(wrong bonds) Broken due to ultraviolet light irradiation, reducing the stress caused by Ge-Si wrong bonds, this process causes the refractive index of the fiber to change. Others claim that the GeO defect in the fiber core absorbs ultraviolet light, causing the color center
The absorption spectrum of the optical fiber is changed and the refractive index of the optical fiber is changed.
Therefore, the general method to increase the refractive index change is to increase the concentration of germanium. For example: boron doping can reduce the numerical aperture, thereby allowing the germanium content to be increased to enhance its optical rotation. Using a hydrogen flame to bake the section to be photosensitive back and forth for about 20 minutes at a temperature as high as 1700 ° C can increase the germanium defects in the fiber, expose the same ultraviolet light, and change the refractive index by more than ten times.
Exposure in a hydrogen-filled environment can also increase the sensitivity of the light and reduce the time required for exposure. This is because ultraviolet photons break the chemical bond at the doping point. This excited state bond reacts with nearby H2 molecules to form The Si-OH bond and new germanium defects are accompanied by an increase in refractive index. The process is to place the bare fiber in high-pressure hydrogen at hundreds of atmospheric pressure for hundreds of hours, and then perform UV light exposure, you can get larger than the fiber core,
The refractive index increase of the refractive index difference of the fiber shell. The difference in refractive index can be on the order of 10-2, however the optical sensitivity of hydrogen loading is temporary,
UV exposure must be performed immediately after the fiber is removed from the high-pressure compartment.
In addition to the doping concentration, the amount of refractive index change in the fiber is also related to many parameters, such as: irradiation wavelength, fiber type, fiber temperature, and the previous history and exposure power, exposure time, etc. of the fiber. Among them, when the pulse energy of ultraviolet light is less than 10mJ, an I-type grating can be formed, and when the energy of the pulse is greater than 40mJ, it can form II
Type grating.
Type â… grating is unstable, it can be erased by blue light or green light, or it can be erased by putting it at 450 ℃ for a few seconds. If a grating with the same refractive index changes is formed at the erased place, the exposure required will be greater than the previous exposure.
The type II grating is very stable and can pass the 1800 ° C for 10 hours without changing the refractive index. Ⅱ
The formation principle of the type grating is different from the type I grating. The light pulse greater than 40mJ causes a temperature rise of several thousand degrees in the fiber.
Will make ultraviolet absorption increase rapidly.
In addition, when the type II grating is formed, due to strong Bragg reflection, the wavelength below the Bragg resonance wavelength is tens of nm
There is an additional loss band within the range, which results from the guided mode coupling into a fiber shell mode. To eliminate this loss, you can start with the structure of the optical fiber and add an interlayer between the core and the fiber shell. The interlayer is co-doped with germanium and fluorine, so that its refractive index is equivalent to that of the fiber shell. The germanium content of the interposer is as large as the germanium content in the fiber core. In this way, when the optical fiber is exposed, the refractive index changes of the core and the fiber shell are the same, so that the refractive index changes in the mode field region of the guided mode transmission are the same, and no additional coupling loss will occur.
Finally, in the formation of ultraviolet gratings, the refractive index changes with the increase of the exposure, but at the same time, the resonance wavelength drifts toward the long wavelength. The reason is that the formation of a grating in the hydrogen-loaded fiber consumes hydrogen molecules in the core region of the fiber, which causes the hydrogen molecules in the fiber shell to diffuse toward the core region and out of the fiber, thereby increasing the refractive index and increasing the wavelength. The drift of the hydrogen-carrying fiber Bragg increases the difficulty of manufacturing the grating. The drift of the wavelength depends on the concentration of hydrogen molecules in the fiber and how many hydrogen molecules are consumed during exposure. However, there is almost no drift in the wavelength of the ultraviolet grating formed by the hydrogen flame.
FBG optical communication application trends The main function of fiber grating is to filter. When the broadband optical signal passes through the fiber grating, the grating can effectively reflect the incident light whose wavelength meets the Bragg conditions, and the light of other wavelengths is not affected by the grating And through,
Can be made into wavelength selective distribution mirror or filter. By controlling the grating period, a narrow-band periodic grating with a bandwidth from 0.05 nm to a chirp broadband grating with a bandwidth of 12 nm can be made.
(2) Split-wave multitasking and demultiplexing multitasking use the filtering characteristics of fiber gratings, which can connect fiber gratings of different wavelengths in series to separate different wavelengths to form a WDM component.
There are currently two more feasible methods for manufacturing WDM components: one is to use optical circulators in series with fiber gratings of different wavelengths to separate different wavelengths, but this method is more labor-intensive to manufacture. Another method is to use the optical coupler and the fiber grating to form a Mach-Zehnder interferometer structure, and then connect the signals of different wavelengths to or separate from the transmission line after being connected in series with each other.
The WDM component made of fiber grating has nothing to do with the polarity of the input light, and is not sensitive to external temperature changes,
It can effectively split and multiplex and demultiplex multiple signals with a signal spacing of 100GHz in the 1550nm wavelength range.
(3) The fiber laser is made into a 1550nm wavelength grating at both ends of a certain length of erbium-doped fiber, and the two gratings are equivalent to a resonant cavity. When the pump laser is excited, erbium ions will be generated by 980nm or 1480nm. The gain is amplified to form an optical fiber laser.
Due to the frequency selection effect of the grating, the resonant cavity can only feed back light of a specific wavelength, output a single-frequency laser, and then output a signal laser with a narrow linewidth, high power, and low noise through an optical isolator.
The advantages of fiber laser are fiber grating compatibility, output stability and spectral purity. Compared with semiconductor lasers, fiber lasers have higher optical output power, lower relative intensity noise, extremely narrow linewidth, and wider tuning range. The line width of the fiber laser can be less than 2.5KHz, which is obviously better than the distributed feedback of the line width of 10MHz
(DFB) Laser.
In addition, a very important parameter in WDM transmission system is tunability. Fiber laser is not only easy to tune, but also the tuning range (> 50nm) is much larger than semiconductor laser (1 ~ 2nm).
(4) Fiber grating DFB laser uses fiber grating as the external cavity mirror of semiconductor diode to produce fiber grating with excellent performance
DFB laser not only has a narrow line width of the output laser, which is easy to couple with the optical fiber system, but also can control the frequency and mode of the output laser by applying a longitudinal tensile force to the grating. It has been proved by experiments that the laser output with line width less than 50kHz and chirp less than 50Hz can be obtained by using 1.2Gb / s direct modulation and grating control.
Compared with the fiber grating laser, the resonant cavity of the fiber grating DFB laser is less affected by temperature, so its output mode is more stable than the fiber grating laser.
(5) Dispersion compensator dispersion is one of the main factors that limit the capacity of optical communication, and many dispersion compensation methods have been discovered. However, compared with other methods, the fiber Bragg grating dispersion compensator has an all-fiber type, low loss, small size and light weight.
Many advantages such as low cost, flexibility and convenience.
The optical fiber laid early had the smallest dispersion around the wavelength of 1310nm; however, in order to cooperate with the use of EDFA,
At present, most of them use the 1550nm band. However, at 1550nm, the blue component in the negative dispersion region is faster than the red component, which will cause obvious dispersion problems. One solution is to install fiber grating dispersion compensation components at appropriate distances. The principle is that the chirp grating has different Bragg wavelengths at different points, so that the end of the grating period is in front, the red light component is reflected at the front of the grating, and the blue light component is reflected at the end of the grating, so the blue light component is more than the red light component Over a longer distance, which creates a time difference between red and blue light. After passing through the grating, the lagging red light will catch up with the blue light, thereby producing dispersion compensation. At present, the chirp grating dispersion compensator has been used to achieve 200km
Standard fiber dispersion compensation.
Manufacturer News
The Photosil single-mode fiber produced by Spectran is specially made for the manufacture of fiber gratings. It can expose high-reflectivity fiber gratings greater than 20dB in two minutes. Its numerical aperture and mode field diameter are also similar to standard fibers, so its splice loss Quite low.
The phase mask can copy a large number of fiber gratings, but the mask itself is expensive to manufacture. QPS Canada sells low-cost standard phase masks with different wavelengths. Its latest product is a phase mask that can be used to manufacture 125mm long gratings. In addition, the experiment of fiber grating is time-consuming and expensive, and the use of computer simulation can accelerate product design. QPS also works with Power
Matrix Technology co-developed SuperBragg software to design uniform and chirp fiber gratings.
3M's new product is to connect a fiber grating to the end of the 980nm laser diode, so that a small number of wavelengths of 980nm
The light is reflected back into the resonant cavity of the laser diode to lock the wavelength, so it can be used to stabilize the pump laser excitation wavelength of EDFA. The reflectivity of this fiber grating is 2 ~ 5%, the temperature sensitivity is 0.012nm / ℃, and the pressure The degree is 100 kpsi.
Lucent Technology has applied fiber grating technology to six new commercial products including 980nm
Stabilizer, 980 nm pump reflector, 1480 pump reflector, YAG reflector, 1550 nm signal
reflector and ASE (amplified STImulated Emission) suppression filter, the first three products are mainly used to improve the performance of fiber amplifiers, YAG reflector is used for fiber laser, 1550 nm signal reflector is used for DWDM system and ASE (amplified STImulated Emission) suppression filter Used to eliminate 1530nm ASE noise caused by the use of fiber amplifiers.
Bragg Photonics, Inc. sells fiber gratings in the range of 1300nm and 1500nm with a channel spacing of 1nm.
DWDM meets ITU standards, and Bragg Photonics, Inc. also sells long-period fiber gratings for EDFA
Flatten the gain curve.
The LOA and CablopTIc companies produce long-period fiber gratings for use in dispersion compensators, which have a period of up to 120 microns after multiple exposures.
In order to overcome the temperature instability, Melles Griot Company embeds the fiber grating in the temperature adjustment package, its temperature stability is 0.035nm / ℃, and its wavelength spacing is up to 200GHz required by the ITU agreement DWDM.
When designing DWDM, a stable light source is required. E-TEK uses fiber grating to stabilize the laser light source. Its wavelength is accurate to plus or minus 0.08nm, which meets the specifications required by ITU. Its temperature stability is 0.01nm / ℃ and wavelength stability is 0.005nm. /
12hours, the line width is less than 100KHz.
Conclusion The optical characteristics of fiber gratings are incomparable with traditional filters. The application of fiber gratings in optical communication is very extensive. It is likely to become a necessary component of WDM and make WDM widely used in user circuits. However, from the perspective of commodities, the market can only be opened after solving some technical problems. For example, the channel spacing of WDM needs to be standardized, and the fiber grating is more susceptible to temperature changes and its temperature stability range needs to be improved.
In the application of fiber dispersion compensation and flat EDFA optical gain curve, fiber grating seems to be more feasible than other technologies, but it is difficult to launch dispersion compensation standard products because the optical amplifier spacing has not been determined. In addition, in the application of sensors, its market will gradually grow and reduce the cost of manufacturing fiber gratings.
As far as the current situation in China is concerned, there are currently academic institutions such as the Institute of Telecommunications, the Optoelectronics Institute of the Industrial Technology Research Institute, and the National Taiwan University, and several manufacturers have invested in technology research and development. In terms of use, in addition to the use in optical communications, there are also many units keen on engineering sensor applications. On the whole, as Wang Lun, a professor at the National Taiwan University, pointed out: Although China now has mass production technology for fiber gratings, it still has to overcome the patent problems in order to successfully occupy a place in the world market.
Fiber Bragg Grating Production and Application (2)
Fiber Bragg Grating Production and Application (2)