When designing an OTDR (Optical Time Domain Reflectometer), the fundamental reason for combining 1310 nm and 1550 nm into a single port while isolating 1625 nm (or 1650 nm) into a dedicated port is to distinguish between "dark fiber testing" (offline, unlit testing) and "live fiber testing" (online, lit testing), as well as to resolve internal physical hardware conflicts (filters).
Detailed breakdown of the core reasons
1310 nm and 1550 nm are the "Golden Duo" for Offline Testing
1310 nm and 1550 nm are the most standard operating wavelengths in communication networks. When we shoot light at these two wavelengths for testing, there must be absolutely no active traffic (no communication signals) in the fiber. This is known as "dark fiber testing."
Internal multiplexing is easy
Since both wavelengths are used in a "dark" environment, the OTDR internally only needs an optical coupler (WDM - Wavelength Division Multiplexer) to combine the 1310 nm and 1550 nm lasers onto the same output fiber.
Simple software switching
During testing, you select 1310 nm or 1550 nm via the instrument panel. The OTDR's internal circuitry controls the corresponding laser to emit light, and the receiver can capture the reflected light of both wavelengths without any interference. Therefore, sharing a physical port is the most economical and compact design.
1625 nm Takes on the Special Mission of "Online Maintenance (Live Testing)"
1625 nm (as well as 1650 nm) is known as the "maintenance wavelength." Its primary function is to conduct real-time troubleshooting while the fiber is actively transmitting live traffic data (e.g., active broadband connections, normally operating PON networks).
Active traffic typically runs in the 1310 nm, 1490 nm, and 1550 nm bands.
1625 nm intentionally avoids these communication bands. Thus, shooting a 1625 nm test signal into the fiber will not interfere with the users' normal communications.
Why Can't They Share the Same Port? — The Hard Conflict of Optical Filters
This is the most crucial hardware reason.
When performing live testing with 1625 nm, the fiber contains not only the 1625 nm reflected light but also extremely strong 1310/1490/1550 nm active traffic light (which could even be high-power lasers from base stations). If this intense traffic light enters the OTDR's highly sensitive receiver (APD - Avalanche Photodiode) directly, it will not only cause severe measurement noise but could also instantly burn out the OTDR's detector.
The 1625 nm port must be equipped with a "Filter".
To protect the detector, a highly isolated optical fiber filter is permanently built into the internal physical circuitry of the 1625 nm port. The sole purpose of this filter is to "only allow 1625 nm light to pass through while completely blocking 1310/1490/1550 nm light from the outside."
What happens if they are combined into one port?
If you were to connect the 1310 nm and 1550 nm lasers to this filtered port, any attempt to emit or receive 1310/1550 nm test light would be completely blocked dead by the internal filter, rendering it entirely non-functional.
The dual-port design of an OTDR is essentially a physical isolation for two different testing scenarios
Standard Port (1310/1550 nm)
Has no internal filter and offers a clear path. It is dedicated to standard light and used for testing offline, dark fibers with no active traffic.
Filtered Port (1625 nm)
Comes with a built-in "security guard" (filter) that shuts out strong traffic light. It is dedicated to testing live, actively communicating fibers.
Separating them physically guarantees the smooth execution of offline testing while protecting the instrument from being destroyed by strong light during live testing.
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