Cause why the VOA embedded on an OA board on the main optical path of a coherent system must be configured on the OUT port of the board

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In a coherent system, the VOA embedded on an OA board must be configured on the OUT port of the board. This position change helps adjust the incident optical power and also helps adjust the input optical power of the receive-end OA board in the downstream direction. This requirement decreases non-linear effect and helps improve the system performance.

Other related questions:
Cause why VOAs for main optical paths of different directions cannot be configured on the same VA4 board in a coherent system
This rule prevents signals of multiple optical paths from being concurrently interrupted in case of a VA4 board fault.

Reason why a coherent system uses fewer OA boards than a non-coherent system
OA boards are used to compensate for insertion loss. Only one OA board is required in case that Gmax (maximum gain) �?Fiber loss + DCM loss; otherwise, two OA boards are required. The coherent system does not require DCMs, and a single OA board can compensate for larger link loss. When the link loss is within the permitted range, the coherent and non-coherent systems require the same number of OA boards. When the fiber loss is out of the permitted range, the coherent system, however, requires fewer OA boards. For example, in a system with a 60 km span, 18 dB fiber loss, and 5 dB DCM loss: The gain required by a coherent system is calculated as follows: Gain = Fiber loss + DCM loss = 18 dB + 0 dB (no DCM) = 18 dB < Gmax. Therefore, one OA board (OAU101) is required. The gain required by a non-coherent system is calculated as follows: Gain = Fiber loss + DCM loss = 18 dB + 5 dB = 23 dB < Gmax. Therefore, one OA board (OAU101) is required. In a system with a 100 km span, 28 dB fiber loss, and 9 dB DCM loss: The gain required by a coherent system is calculated as follows: Gain = Fiber loss + DCM loss = 28 dB + 0 dB (no DCM) = 28 dB < Gmax. Therefore, one OA board (OAU101) is required. The gain required by a non-coherent system is calculated as follows: Gain = Fiber loss + DCM loss = 28 dB + 9 dB = 37 dB > Gmax (36 dB for an EDFA board). Therefore, two OA boards (OAU101 and OBU101) are required.

Cause why TD20/TM20 boards are used at an ROADM site in a coherent system
The OSN 8800 uses the frequency selection technology of coherent boards and works with ROADM boards such as TM20 and TD20 to implement the ROADM solution with a simple structure, greatly reducing the costs for constructing an optical-layer ASON network. For example, when the TN11TM201 and TN12TD201 boards are used, up to 20 colorless wavelengths can be added and dropped, greatly saving board and slot resources.

Optical power difference between the IN port of the D40 board and the OUT/MON port of the M40 board
The MON port of M40 board has 10 dB lower optical power than the IN port of the D40 board and the OUT port of the M40V board. The split ratio between the MON port and OUT port and that between the MON port and IN port are both 10:90. In other words, the optical power of the MON port is 10 dB lower than that of the IN port. The calculation formula is as follows: Optical power (dBm) of the IN port �?Optical power (dBm) of the MON port = 10 x lg(90/10) = 10 dB.

Calculation of the total input optical power of an OA board
The total input optical power of an OA board can be calculated using the following formula: Total input optical power = Single-wavelength nominal input optical power + 10lgN (N: number of in-service wavelengths).

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