Why are optical fibers typically used with 1550 nm light? Can this light be seen by the human eye?
I. Can We See 1550 nm Light With the Naked Eye?
Absolutely not.
The human eye’s sensitivity range to electromagnetic waves (the visible light spectrum) is typically between ** 380\ \text{nm} \sim 780\ \text{nm} **.
- ** 1550\ \text{nm} ** falls within the Near-Infrared (NIR) band, which has a wavelength significantly longer than red light (approx. 700\ \text{nm}). Its photons have lower energy, and the photoreceptor cells in the human eye have no physiological response to it.
- Safety Notice: Although invisible to the naked eye, high-power 1550 nm lasers can still focus and cause thermal damage to eye tissues (particularly the cornea and lens). Therefore, when operating fiber optic sensing and demodulation equipment, never look directly at the fiber end face or output port.
II. Why Is the 1550 nm Wavelength Commonly Used for Fiber Optic Sensing?
Fiber optic sensing (including Fiber Bragg Grating (FBG) sensing, Distributed Fiber Optic Sensing, etc.) widely adopts the 1550 nm window primarily due to the following physical and engineering reasons:
1. Extremely Low Transmission Loss in Quartz Fiber (Lowest Attenuation Window)
The loss in standard silica ( \text{SiO}_2 ) single-mode fiber at different wavelengths is mainly determined by two factors:
- Rayleigh Scattering: Inversely proportional to the fourth power of the wavelength (Loss \propto \lambda^{-4} ). Longer wavelengths result in lower scattering loss.
- Infrared Multiphonon Absorption: Increases sharply for wavelengths exceeding 1600\ \text{nm}.
The intersection of these two phenomena creates a lowest attenuation window for silica fiber around ** 1550\ \text{nm} (the C-band)**, with transmission losses typically as low as ** 0.18 \sim 0.20\ \text{dB/km} **. This low-loss characteristic ensures that the sensing optical signal can be transmitted over long distances, from kilometers to tens of kilometers, with minimal attenuation.
2. Mature Fiber Optic Communication Industry Chain and High-Quality Optical Components
1550 nm is the mainstream operating wavelength for global fiber optic communication backbone networks. Consequently, this wavelength benefits from the world’s most mature and highest-performance optoelectronic components:
- Light Sources and Detectors: High-stability narrow-linewidth lasers (e.g., DFB lasers), high-responsivity InGaAs (Indium Gallium Arsenide) photodetectors.
- Optical Amplification and Passive Components: Erbium-Doped Fiber Amplifiers (EDFAs), fiber optic circulators, couplers, and other components are extremely stable, cost-effective, and highly mature in their manufacturing processes.
3. Suitability for Wavelength Division Multiplexing (WDM) Large-Capacity Networking
Around the 1550 nm range (specifically 1525\ \text{nm} \sim 1565\ \text{nm}, a bandwidth of 40\ \text{nm}), there is sufficient spectral space. Fiber Bragg Grating (FBG) sensors can utilize Wavelength Division Multiplexing (WDM) technology to transmit dozens of sensors with different center wavelengths ( \lambda_B ) in series on a single-mode fiber, enabling quasi-distributed, multi-point synchronous monitoring.
III. OFSCN® Official Product Wavelength Standards
Within the Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) fiber Bragg grating product line, the standard wavelength specifications for sensors and fiber Bragg gratings are as follows:
- Default Standard Wavelength Range: ** 1525\ \text{nm} \sim 1565\ \text{nm} ** (Standard Telecom C-band, compatible with mainstream 40 nm range FBG demodulators on the market).
- Customizable Wavelength Range: ** 1510\ \text{nm} \sim 1605\ \text{nm} ** (Covers a broader S+C+L band to meet the demand for high-density multiplexing of measurement points).