One Fiber Strand, 100G: Is That Cheaper Than Pulling a Second Fiber?
Most 100G optical links traditionally expect two strands — one transmitting in each direction. A new class of coherent optics can instead send bidirectional 100G across a single working fiber, potentially turning a fiber-exhausted route into a 100G route without acquiring another strand. The technology is impressive. The more interesting question is whether the expensive optic actually costs less than the alternative.
Fiber shortages do not always mean there is no cable in the ground. Sometimes there is plenty of cable but no unused strand available on the exact route a customer needs. That problem can occur on metro rings, cable networks, wireless backhaul routes, campuses and older enterprise connections built when capacity requirements were much smaller. Until recently, upgrading a long-distance link to 100G often meant securing a conventional fiber pair. Coherent’s new single-working-fiber 100G module creates another option: use the strand already available in both directions.
Can 100G Really Run Both Directions on One Strand?
Yes. Coherent now commercially offers a QSFP28 digital coherent optic specifically designed for bidirectional 100G transmission over one working fiber.
The module integrates two independently tunable lasers — one used for transmission and one as the coherent receiver’s local oscillator — inside a compact QSFP28 module. The opposing directions are optically engineered so both can share the same physical strand.
Coherent states that the product can support links up to 300 km and deliver as much as a tenfold capacity increase when replacing legacy 10G bidirectional infrastructure. Actual reach remains dependent on the optical path and line-system design.
Why Ordinary Fiber Links Usually Use Two Strands
Conventional Duplex 100G
One strand carries traffic in one direction and a second strand carries traffic in the opposite direction.
Single-Fiber 100G
Both directions share one physical working fiber using a bidirectional coherent optical design.
But “We Need Another Fiber” Can Mean Three Completely Different Things
A Spare Strand Already Exists
The cable already contains another unused strand that the organization owns. The incremental expense may be little more than splicing, testing and optics. Single-fiber 100G may offer little economic advantage here.
A Second Strand Must Be Leased
Another strand exists but belongs to a carrier, utility or network owner. The buyer may face a recurring strand-mile charge for the entire term. This is where single-fiber economics become much more interesting.
No Spare Fiber Exists
The route is physically exhausted. Another cable, new conduit, aerial attachment or civil construction may be required. In this case the specialized optics can potentially avoid a very large infrastructure project.
First Check: Is There Already a Spare Strand?
Before buying specialized optics, determine what actually exists inside the cable.
Commercial fiber cables commonly contain many strands. A company may be using only two strands while the same cable contains dozens more.
If the organization owns those unused strands, activating another one can be far cheaper than redesigning the optical link.
If You Must Lease the Second Strand, the Math Changes Fast
Dark-fiber prices are extremely route-specific, but public utility rate cards illustrate why a specialized optic can sometimes make economic sense.
| Published Example | Single-Strand Rate | 10-Mile Cost | 5-Year Gross Cost |
|---|---|---|---|
| Riverside Public Utilities | $125 / strand-mile / month for first 10 | $1,250/mo | $75,000 |
| City of Eugene Airport | $65.82 / strand-mile / month | $658.20/mo | $39,492 |
| La Plata Electric Association | $546.36 / fiber-mile / year | About $455/mo equivalent | $27,318 |
These public rates are examples only and are not directly comparable commercial products. They demonstrate how dramatically dark-fiber pricing varies by network, location, term and owner. Endpoint, lateral, splicing and other charges may be additional.
Then Compare That With Construction
If another strand cannot simply be leased, the comparison can move from thousands of dollars to tens or hundreds of thousands.
Those figures use the Fiber Broadband Association / Cartesian national deployment benchmark and are not estimates for a specific enterprise extension.
They illustrate the underlying economic principle: once acquiring another strand requires actual outside-plant construction, avoiding that build can justify a very substantial optics premium.
Construction Is Not Really “Pulling One More Strand”
Fiber construction economics are slightly counterintuitive.
Contractors generally do not dig a trench several miles simply to place one additional glass strand. New fiber cable normally contains many strands because the incremental cost of placing additional fibers inside the cable is small compared with mobilizing crews, obtaining permits, boring under roads and installing conduit.
This is why a fiber-exhausted route can be expensive to expand even though the additional amount of glass the customer actually needs is tiny.
Single-Fiber 100G Is Not the Same as Ordinary 100G ZR
| Characteristic | Conventional 100G ZR DCO | Single-Working-Fiber 100G DCO |
|---|---|---|
| Working fibers | Typically 2 | 1 |
| Capacity | 100G | 100G |
| Coherent transmission | Yes | Yes |
| Tunable DWDM technology | Yes | Yes |
| Best use case | Normal duplex fiber routes | Fiber-constrained routes |
| Primary economic advantage | Mature 100G coherent transport | Avoids consuming second strand |
The Reach Number Needs One Important Qualification
Coherent advertises the single-working-fiber module for distances up to 300 km.
That does not mean every 300 km strand can simply have two modules plugged in and immediately pass 100G.
Conventional 100G ZR coherent modules illustrate why. Cisco’s current 100G ZR specification supports up to about 80 km directly over dark fiber and up to 300 km over an amplified DWDM system. Coherent optical links remain dependent on fiber loss, receive power, OSNR, amplification and other line conditions.
There Is Also a Resilience Tradeoff
Using one strand instead of two improves fiber utilization. It also concentrates both transmission directions onto one physical strand.
If that strand fails, the entire bidirectional connection is lost.
A conventional duplex pair has two fibers, but that does not automatically provide true redundancy either. If both strands share the same cable, conduit and physical route, one backhoe can still remove both directions simultaneously.
Single-Fiber Advantage
- Consumes half as many working strands.
- Can unlock fiber-exhausted routes.
- May avoid recurring dark-fiber lease expense.
- May avoid costly outside-plant construction.
- Can preserve another strand for a different service.
Resilience Questions
- Both directions depend on one strand.
- One strand is not route diversity.
- Protection still requires a separate physical path.
- Amplifiers and passive components may create other failure points.
- Spares for specialized optics should be planned.
When Single-Fiber 100G Has the Strongest Business Case
Fiber-Exhausted Route
Only one usable strand remains and adding another requires construction or a new cable.
Expensive Strand Lease
The network owner charges a recurring per-strand, per-mile rate and the route is long enough for that charge to become significant.
Existing 10G BiDi
A legacy single-fiber bidirectional route needs a major capacity upgrade without rebuilding the underlying outside plant.
Wireless Transport
Tower and aggregation routes frequently face fiber scarcity, limited conduit capacity and long distances.
Remote Campus Link
One usable strand already connects sites and obtaining a second involves third-party rights-of-way or expensive construction.
Preserve Fiber Inventory
A network owner wants to double the number of bidirectional services possible from a limited strand count.
When It Probably Does Not Make Sense
| Situation | Likely Better Approach | Reason |
|---|---|---|
| Owned spare strand already available | Use conventional duplex optics | There may be little economic reason to pay a single-fiber optics premium. |
| Very short campus link | Compare simpler BiDi alternatives | Long-reach coherent functionality may be unnecessary. |
| Host platform not qualified | Change platform or retain current design | QSFP28 form factor alone does not guarantee software, thermal or optical compatibility. |
| Need for true route redundancy | Build or lease diverse route | Single-fiber transmission increases strand efficiency but does not create diversity. |
| Second strand costs almost nothing | Lease or activate second strand | The economics favor standard duplex architecture. |
The Most Important Number Is the Optics Premium
There is no useful universal retail price for this calculation.
Coherent sells the module through carrier, OEM and optical-networking channels, and enterprise pricing will depend on volume, platform qualification, support and supply relationship.
Instead, obtain two actual quotes:
Quote B: complete pair of conventional duplex 100G modules plus the cost of acquiring the second strand.
The difference between those optics packages is the premium the single-fiber architecture must recover.
The Break-Even Formula Is Surprisingly Simple
Single-fiber optics premium + additional single-fiber line equipment
<
second-strand lease payments + strand activation costs + construction avoided
On an existing route where another strand is leased monthly, term length becomes extremely important.
A $1,000 monthly strand charge costs $36,000 over three years, $60,000 over five years and $120,000 over ten years before escalators.
On a route requiring new construction, the break-even can occur immediately because the avoided civil project can be larger than the entire optical electronics budget.
Eight Questions Before Choosing One Strand
| Question | Why It Matters |
|---|---|
| Do we truly have only one usable strand? | Check fiber records before paying for a specialized solution. |
| What would another strand actually cost? | Separate owned spare fiber, leased fiber and new construction. |
| Does our router support the module? | Confirm software, thermal, power and management compatibility. |
| What is the measured route loss? | Distance by itself does not determine optical feasibility. |
| Is amplification required? | Longer routes may need additional active line equipment. |
| What spare strategy is required? | Specialized optics may not be something an enterprise wants to source only after a failure. |
| Will we need more than 100G later? | Avoid solving today’s strand problem with an architecture that immediately creates tomorrow’s capacity problem. |
| Do we need route diversity? | If availability is mission-critical, a second physical route may be more valuable than maximizing one strand. |
Public dark-fiber tariffs are included only as illustrations of how strand leasing can be priced. Commercial carrier pricing varies substantially by route, market, distance, strand count, lateral requirements, contract term and competition. Published construction benchmarks describe network deployment costs and should not be interpreted as a quotation for a specific enterprise build.
Single-Fiber 100G vs Second Strand vs Construction Calculator
Compare the incremental cost of upgrading the existing single strand with specialized coherent optics against leasing another dark-fiber strand or constructing additional fiber. Enter actual optical quotes whenever possible; the equipment-price fields intentionally default to zero.
| Option | Optical Equipment | Fiber / Construction | Term Cost | Incremental TCO |
|---|
Planning model only. It compares incremental costs and assumes the existing single working fiber is already available under all options. It does not include financing, taxes, maintenance, cross-connects, colocation, router upgrades, amplifiers not entered above, outage costs or route-diversity benefits. Construction benchmarks are not carrier quotes. Confirm actual strand availability and optical feasibility before procurement.
