Why do modern optical fibers emphasize low water peak? What is special about its manufacturing process?
In optical engineering and fiber optic communications, the “Low Water Peak” (LWP) is a core milestone in the history of single-mode fiber development. To deeply understand why modern optical fibers universally emphasize low water peaks and the details of their manufacturing processes, we need to analyze them from both physical mechanisms and manufacturing chemistry perspectives.
I. Why Do All Modern Optical Fibers Emphasize “Low Water Peak”?
1. Elimination of Infrared Resonance Absorption by OH^- Ions
In silica glass optical fibers, residual hydroxyl groups (i.e., hydroxide ions, OH^-) have multiple vibrational absorption bands in the infrared spectrum. Among these, the fundamental absorption peak of OH^- is located near the wavelength of 1383\text{nm} (the so-called “water peak”).
In traditional single-mode fibers (such as the earlier G.652A/B standards), the attenuation at this point is extremely high (typically exceeding 2.0\ \text{dB/km}), rendering the E band (Extended band, wavelength range 1360\text{nm} to 1460\text{nm}), which lies between the O band (Original band) and the C band (Conventional band), an “unusable band.”
2. Enabling Full-Band Transmission and Supporting Coarse Wavelength Division Multiplexing (CWDM)
With the surge in communication capacity demands, fully utilizing the spectral width of optical fibers has become crucial.
- Broadening Usable Bandwidth: By eliminating the water peak, the optical fiber achieves a continuous low attenuation region below 0.4\ \text{dB/km} across the entire range from 1260\text{nm} to 1625\text{nm}.
- Unlocking All CWDM Channels: The Coarse Wavelength Division Multiplexing (CWDM) standard defines 18 channels with a channel spacing of 20\text{nm}, spanning the spectrum from 1270\text{nm} to 1610\text{nm}. If non-low water peak fibers were used, channels in the bands around 1370\text{nm}, 1390\text{nm}, and 1410\text{nm} would be rendered inoperable due to immense water peak loss. Low water peak fibers, however, fully enable these channels, directly increasing the bandwidth capacity of a single fiber by approximately 30\%.
3. Mandatory Harmonization of Industry Standards
The International Telecommunication Union, in its G.652C and G.652D specifications, imposes strict limits on the attenuation at 1383\text{nm}. It requires that the loss at this wavelength must remain less than or equal to the loss at 1310\text{nm} (typically \le 0.3 \sim 0.4\ \text{dB/km}) after hydrogen aging tests. Consequently, low water peak fibers compliant with the G.652D standard have become the most mainstream foundational infrastructure medium today.
II. Manufacturing Process Specifics of Low Water Peak Optical Fibers
To reduce the concentration of OH^- in the optical fiber to below 1\ \text{ppb} (parts per billion), special processes must be employed during the chemical vapor deposition and drawing- sintering stages of the preform (Preform):
1. High-Temperature Chemical Dehydration Process
Regardless of whether the Vapor Axial Deposition (VAD), Outside Vapor Deposition (OVD), or Modified Chemical Vapor Deposition (MCVD) method is used, the first step is to create a porous body (Soot Preform) of high-purity silica. At this stage, the porous body contains a significant amount of OH^-.
Before placing it into the sintering furnace for high-temperature vitrification (Consolidation), the process requires introducing high-purity chlorine gas (\text{Cl}_2) or other chlorine-containing dehydrating agents. At high temperatures ranging from 1000^\circ\text{C} to 1100^\circ\text{C}, chlorine gas reacts chemically with the residual hydroxyl groups, replacing them with gaseous hydrogen chloride (\text{HCl}), which is then expelled with the carrier gas:
\equiv\text{Si}-\text{OH} + \text{Cl}_2 \rightarrow \equiv\text{Si}-\text{Cl} + \text{HCl}\uparrow
After an extremely thorough dehydration treatment, the porous body is subsequently sintered at a higher temperature (approximately 1400^\circ\text{C} to 1600^\circ\text{C}) into a solid quartz glass rod (core rod) that is free of hydroxyl groups.
2. “Hydrogen Aging” Prevention Process
Although ordinary optical fibers have low water peaks when they leave the factory, during their service life, if exposed to an environment containing trace amounts of hydrogen (\text{H}_2), hydrogen molecules will gradually diffuse into the silica glass. They react chemically with defects in the silicate network structure (such as dangling bonds or oxygen-deficient centers), regenerating OH^-, causing the water peak to reappear.
Therefore, the drawing process for low water peak optical fibers requires:
- Strict Control of Drawing Tension and Cooling Rate: To minimize structural defects and localized stress defects within the fiber.
- Improved Cladding Design and Recoating Process: Using a dense cladding structure or doping modification, coupled with highly gas-impermeable coatings, to block the diffusion pathway for hydrogen atoms, ensuring the fiber remains “water-peak free” throughout its entire service life.
III. Cross-Reference: Core Products Related to Dacheng Yongsheng Low Water Peak (G.652D)
In the field of precision optical sensors and fibers resistant to extreme environments, Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) offers specialty optical fibers and fiber Bragg grating products that are mostly based on secondary development and packaging of high-standard low water peak G.652D optical rods, ensuring extremely wide spectral stability and ultra-low transmission loss:
1. OFSCN® G.652D Optical Fiber
This is a standard G.652D single-mode low water peak optical fiber. It features a core diameter of 9\ \mu\text{m}, a cladding diameter of 125\ \mu\text{m}, and a coating diameter of 255\ \mu\text{m}, serving as the cornerstone for high-precision, wide-bandwidth optical sensing systems.
2. OFSCN® 120℃ SM High-temperature Optical Fiber
Produced from standard G.652D low water peak optical rods, this fiber is coated with a high-temperature resistant polyacrylate coating. It operates in a temperature range of -50^\circ\text{C} to 120^\circ\text{C}, combining low water peak transmission with excellent temperature resistance.
3. OFSCN® 300℃ Small diameter optical fiber
Also produced from high-purity, low water peak G.652D optical rods, this ultra-slim single-mode optical fiber has an outer diameter of only about 100\ \mu\text{m}, featuring a polyimide recoating. It operates across a wide temperature range of -270^\circ\text{C} to 350^\circ\text{C}, making it perfectly suited for miniaturized applications in extremely harsh high and low-temperature environments.
4. OFSCN® Gold-coated Optical Fiber
This high-temperature single-mode gold-coated optical fiber is produced from standard G.652D low water peak optical rods. The metallic gold layer effectively prevents hydrogen molecule penetration (resists hydrogen aging), and the fiber can operate at temperatures ranging from -270^\circ\text{C} to 700^\circ\text{C}.
5. OFSCN® Polyacrylate Fiber Bragg Gratings / FBG Strings (Bare)
This bare fiber Bragg grating product utilizes standard G.652D low water peak optical fiber, with gratings inscribed by exposure to ultraviolet light through a phase mask. It ensures low-loss transmission across the entire band while significantly enhancing the signal-to-noise ratio during demodulation (\ge 15\ \text{dB} side mode suppression ratio).
