Fiber optic lines transmit data through optical fiber cables, converting electrical signals to light. This enables dramatically faster, higher-capacity, lower-latency communication compared to copper (metal) lines. NTT East/West's "Flets Hikari" is Japan's largest provider, offering speeds from 1Gbps to 10Gbps.
For telephony, "Hikari Denwa" (fiber phone) is significant. While traditional analog phone lines cost about 1,870 yen/month, Hikari Denwa costs about 550 yen/month with nationwide flat-rate calls at 8.8 yen/3 minutes. Voice quality matches or exceeds analog, with services like Number Display available.
Fiber's weakness is power dependency. Analog lines worked during outages via exchange-supplied power, but fiber requires power for the home ONU (Optical Network Unit), disabling both phone and internet during blackouts. Keep mobile phones and public phones as backup for disaster preparedness.
NTT's "PSTN Migration" from 2024 is transitioning all legacy exchanges to IP networks, eventually making all landlines IP-based. Users' numbers and phones remain unchanged. See Hikari Denwa migration guide and landline cancellation guide for details.