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Why Is Single-Reel Inspection of Communication Optical Cables Necessary?

Single-reel inspection of communication optical cables is an important quality control step prior to construction. Its purpose is to comprehensively verify the appearance condition, geometric parameters, and optical transmission performance of delivered optical cables before installation, so as to identify potential hidden defects arising from manufacturing, transportation, loading and unloading, or storage at an early stage, and to prevent such issues from being exposed after cable installation, which could otherwise result in rework, schedule delays, and economic losses.

Once communication optical cables have been installed-especially after aerial installation, direct burial, or duct installation-the cost of repair and replacement is extremely high. Therefore, single-reel inspection is a key measure to shift quality risk control forward and to define responsibility boundaries in advance.

Problems That May Arise If Single-Reel Inspection Is Not Performed

• Hidden fiber breaks or micro-cracks exposed only after installation

During transportation and handling, optical cables may be subjected to impact, compression, or bending, leading to stress concentration and the formation of micro-cracks in the fibers. Such defects are not easily detected by visual inspection alone, but may develop into fiber breaks under installation tension or long-term operation, resulting in communication interruption.

• Insufficient actual cable length, forcing changes to repeater locations or splicing schemes

If reel length is not verified during single-reel inspection, insufficient length may only be discovered during installation, which may require temporary additional splices or route adjustments. This not only affects system attenuation performance but may also compromise the rationality of the original design scheme.

• Abnormal OTDR traces with unclear responsibility attribution

If no single-reel OTDR test is conducted before installation, abnormalities such as excessive attenuation or fiber breaks discovered later make it difficult to determine whether the issue originates from manufacturing defects, construction damage, or operational environmental factors, resulting in unclear responsibility.

• Difficulty in claims after construction completion

Without original pre-installation test data as evidence, it is often difficult to assert quality responsibility or file claims against suppliers or construction parties.

General Requirements for Single-Reel Inspection

Appearance Inspection

Check whether the cable reel packaging is damaged, and open the reel to inspect whether the cable sheath shows any damage or loose strands, and whether the cable end sealing is intact. Any damage shall be documented in detail through photographs or video recordings.

Procedure

Review construction drawings, design documents, and contracts to understand technical specifications such as cable type and performance requirements.

Verify factory certificates of conformity, factory test report, and related documentation for each delivered reel.

Prepare necessary measuring instruments, test cords, and test record forms including proper fiber optic cable test equipment.

Use standardized inspection record forms to document on-site single-reel unpacking inspections, verify all delivered cable information, and clearly mark reel number, end designation, length, type, and intended installation section on the cable reel.

After passing inspection, promptly restore packaging, reseal and secure cable ends, reattach reel protective boards, and place the reel in a safe storage location.

Precautions

If quality issues or non-compliance with design or contract requirements are identified during inspection, relevant records shall be made promptly through photographs and written documentation. Such cables shall not be used in the project and the supplier shall be contacted for handling.

Single-Reel Optical Attenuation Measurement

General Requirements

Single-reel attenuation test fiber optic cable is conducted to verify optical fiber length, attenuation, and other transmission characteristics of delivered optical cables.

Measuring instruments with high performance and accuracy shall be selected and properly calibrated.

Testing methods shall be standardized. For on-site single-reel testing, non-destructive methods are recommended, generally adopting the backscattering measurement method recommended by ITU-T (International Telecommunication Union – Telecommunication Standardization Sector).

A technical supervisor with practical experience and analytical capability shall be assigned to guide the work and analyze and resolve issues. Testing personnel shall possess optical fiber cable testing skills.

Measurement Method

Single-reel optical attenuation testing generally adopts the backscattering method. The backscattering method, also known as the OTDR (Optical Time Domain Reflectometer) method, uses backscattered light to measure fiber attenuation. It is a non-destructive, single-ended measurement method that offers high accuracy, convenient operation, and high efficiency, making it suitable for field testing.

During testing, a coupler is used to connect the fiber core of the single-reel optical cable to the OTDR. In single-reel attenuation testing of power communication optical cables, the wavelength setting is generally 1550 nm. The measurement setup is shown in Figures 2-4.

Figure 2-4 Schematic diagram of the backscattering method for single-reel optical cable testing

Due to the existence of measurement dead zones in OTDR instruments, event points may be missed or attenuation values may appear excessively high. Therefore, a 1–2 km launch fiber may be inserted between the instrument and the fiber under test via a coupler to obtain more accurate measurement results.

Analysis of Measurement Results

The backscattering method provides backscattering trace curves, fiber length, and fiber attenuation results simultaneously. When reviewing test fiber optic cable results, attention shall be paid to cursor positioning. The start point of the tested reel length should be set at the end of the launch fiber step, and the end point should be set before the terminal reflection peak, as shown in Figure 2-6.

Figure 2-6:Results of the backward measurement method

When fiber quality is good, the backscattering curve is uniform and the slope is gentle. The test curve should show no step-like drops, excessively steep slopes, non-Fresnel reflection points (fiber micro-cracks), or non-terminal reflection peaks (break points). Test results shall be recorded using standardized record forms.

Determination of "Excessive Slope"

Under a wavelength of 1550 nm, the average attenuation of single-mode optical fiber generally should not exceed 0.22–0.25 dB/km. When the measured result is significantly higher than this range, and improper test parameter settings have been ruled out, the fiber core may be preliminarily judged to have quality abnormalities.

Immediate Rejection or Retesting?

When test results are close to the limit or suspicious abnormal events are observed, retesting shall be conducted first. If necessary, pulse width, averaging time, or the launch fiber should be adjusted before retesting.

Fibers that still fail to meet the specified requirements after retesting shall be classified as non-conforming fibers.

Whole-Reel Rejection or Single-Fiber Handling?

When abnormalities are limited to individual fiber cores, the "single-fiber rejection" principle may be applied, and the remaining fibers may continue to be used after confirmation of conformity.

When multiple fibers within the same reel exhibit abnormalities, or when abnormalities show a systematic distribution, the entire reel shall undergo focused re-evaluation, and if necessary, be judged as non-conforming.