Course Duration: 50–65 minutes
Learn to set up, configure, and run the Wanlutech MT5500’s single-mode OTDR; test fibers with loopback and bidirectional methods; and read and document the results.
How This Course Works
- Six sections, each ending with a knowledge check. You get two attempts, and a first miss gives you a hint rather than the answer. Answering unlocks the next section.
- Then a 15-question final assessment, different from the knowledge checks. Score 80% (12 of 15) or higher to pass and earn your certificate.
- When you submit, your results are emailed automatically to the training administrator and to you, whether or not you pass.
What is an OTDR?
An Optical Time Domain Reflectometer (OTDR) is a test instrument used to characterize optical fiber cables. It injects short laser pulses into a fiber and measures the light that comes back to the instrument. The time delay between sending a pulse and receiving its return is used to calculate the distance to every event in the fiber — connectors, splices, bends, and breaks.
Two physical phenomena drive OTDR measurements: Rayleigh backscattering, light scattered continuously along the fiber that lets the OTDR measure attenuation, and Fresnel reflection, which occurs at abrupt changes in refractive index such as connectors and breaks and produces sharp reflection spikes on the trace.
The Wanlutech MT5500
The Wanlutech MT-5500 is a handheld, touchscreen multi-function tester built around a single-mode OTDR. Besides the OTDR, it includes a visual fault locator, optical power meter and light source, an IP camera (CCTV) tester, PoE output, an RJ45 cable TDR, and network tools — a lot of capability in one field instrument for low-voltage technicians.
| Parameter | Specification (base 1310/1550 nm model) |
|---|---|
| OTDR wavelengths | 1310 nm / 1550 nm — single-mode only (no 850/1300 nm multimode OTDR) |
| Dynamic range | 28 dB (1310 nm) / 26 dB (1550 nm); higher-range variants are sold |
| Distance ranges | 0.5, 1, 2, 5, 10, 25, 50, 100, 200 km; test distance under 150 km |
| Pulse widths | 5 ns to 20 µs (16 steps) |
| Event dead zone | ≤ 1.6 m |
| Attenuation dead zone | ≤ 8 m |
| Measurement time | 5 s to 3 min |
| Display | 5.4-inch IPS touchscreen |
| Other optical functions | 650 nm VFL; optical power meter (850–1625 nm); light source |
| Export formats | .SOR trace files, PDF and Excel reports |
| Live fiber | Not supported on the 1310/1550 nm model |
Common Applications
- New installation verification — confirm splice and connector quality
- Fault localization — pinpoint breaks, bends, and bad connectors
- Length measurement — document cable length
- Troubleshooting and documentation — the trace shows where loss occurs, which a power meter can’t
- Preventive maintenance — compare new traces with baseline traces to spot degradation
⚠ Laser Safety
The OTDR emits invisible infrared laser light, and the VFL emits a bright visible red laser. Never look into the test port or any connector end while a source is active, and never view a fiber end through a microscope unless you know the fiber is dark — use a video inspection scope. Keep ports capped when not in use.Which fiber can the MT5500's built-in OTDR test, and at which wavelengths?
Required Equipment
- MT5500 — charged, or on its DC power supply
- Launch cable (launch fiber, often in a ring or box) — single-mode, matched to the fiber under test
- Receive cable — for measuring the far-end connector
- Fiber cleaning kit — one-click cleaners, lint-free wipes, and fiber-grade cleaning fluid
- Inspection scope — to check every end-face before mating
Why You Need a Launch Cable
Every OTDR has a dead zone right after its output port, where the strong reflection from the front connector saturates the receiver and nearby events can’t be resolved. A launch cable moves the start of the fiber under test (FUT) past that dead zone, so the first connector of the actual cable shows up on the trace and can be measured.
How Long a Launch Cable?
The dead zone grows with pulse width, so the launch cable must be longer for wider pulses. EXFO’s rule of thumb: divide the pulse width in nanoseconds by 10 to get meters, then add 20%. VIAVI suggests 10–20 m for very short pulses (3–30 ns), 100–200 m for medium pulses (100–300 ns), and 1–2 km for long pulses (microseconds). A few hundred meters covers most premises and campus work.Why You Need a Receive Cable
To measure the loss of the last connector on the FUT, the OTDR needs fiber beyond it to compare backscatter before and after. A receive cable at the far end provides that — and confirms continuity of the fiber under test. Make it at least as long as the launch cable.
Step-by-Step Setup
- Confirm the fiber is dark — disconnected from all active equipment at both ends.
- Power on the MT5500 and open the OTDR function.
- Inspect the OTDR test port with an inspection scope. Clean it if it’s contaminated.
- Inspect and clean both ends of the launch cable, then connect it to the OTDR test port.
- Inspect and clean the first connector of the FUT, and connect the far end of the launch cable to it.
- At the far end, inspect, clean, and connect the receive cable to the FUT.
⚠ Never Connect the OTDR to a Live Fiber
The MT5500’s 1310/1550 nm OTDR doesn’t support live-fiber testing. Traffic light from active equipment can damage the OTDR’s sensitive receiver, and the OTDR’s pulses can disrupt the network. Disconnect the fiber from equipment at both ends before testing.⚠ Clean and Inspect Every Connection
Contaminated connectors are the most common cause of bad OTDR readings and damaged end-faces. Inspect — and clean if needed — every connector, every time, before mating. A dirty OTDR port contaminates every connector you plug into it.Connector Type Reference
| Connector | Common Use | Key Feature |
|---|---|---|
| SC/APC | SM outside plant, FTTx, RF video | Green; 8° angled polish — very low reflectance |
| SC/UPC | SM premises and data centers | Blue; flat (ultra) polish — higher reflectance than APC |
| LC/UPC | SM high-density panels | Small form factor; blue |
| FC/UPC | Test equipment, telecom | Threaded coupling |
Never mate an APC connector with a UPC connector — the angled and flat end-faces don’t meet properly, causing high loss and possible damage. Use a hybrid (UPC-to-APC) launch cable or patch cord when the FUT is APC. The MT5500 kit includes adapters and an FC/UPC-to-FC/APC patch cord for this.
Why do you connect a receive cable at the far end of the fiber under test?
Accessing Test Parameters
Open the OTDR function and go to the parameter settings. Automatic settings are handy for a quick look at an unknown fiber; for acceptance testing and troubleshooting, set the parameters yourself so every fiber in a cable is tested the same way.
Key Test Parameters
| Parameter | Purpose | Typical Setting |
|---|---|---|
| Wavelength | Selects the test laser; each wavelength shows loss differently | 1310 nm and 1550 nm — test both; 1550 nm is more sensitive to bends |
| Distance Range | Sets how far the OTDR looks | About 1.5–2× the expected length, rounded up to the next available range |
| Pulse Width | Laser pulse duration; trades dynamic range against dead zone | Short pulses for short premises links; wider pulses for long runs |
| Measurement (averaging) time | Longer averaging reduces noise | MT5500 offers 5 s to 3 min; use longer times for noisy traces |
| Refractive Index (IOR) | Converts time to distance; must match the fiber | Use the fiber datasheet value — e.g., Corning SMF-28 Ultra: 1.4676 at 1310 nm, 1.4682 at 1550 nm |
Pulse Width Trade-offs
Pulse width is one of the most important settings to understand. A narrow pulse (for example 5–100 ns) produces a short dead zone and better resolution for closely spaced events, but carries less energy, so its dynamic range is limited. A wide pulse (microseconds) carries more energy, extending dynamic range and reach, but creates a longer dead zone that can hide nearby events.
Narrow Pulse
Better near-end resolution. Shorter dead zones. Ideal for short cables and closely spaced events. Limited dynamic range.Wide Pulse
Greater dynamic range and reach. Suitable for long fiber runs. Longer dead zones — nearby events may be masked. Needs a longer launch cable.Refractive Index (IOR)
The refractive index tells the OTDR how fast light travels in the fiber being tested. An incorrect IOR causes distance errors — the OTDR reports events at the wrong location. Always use the value from the fiber manufacturer’s datasheet, for the wavelength you’re testing.
Why Test at Both Wavelengths?
Macrobends lose much more light at 1550 nm than at 1310 nm. An event that’s small at 1310 nm but noticeably larger at 1550 nm points to a bend or stress point rather than a splice — a useful troubleshooting clue.
You expect a link to be about 2 km long. The MT5500's distance ranges include 0.5, 1, 2, 5, and 10 km. Which should you choose?
What is OTDR Loopback Testing?
In a loopback test, a loopback jumper at the far end joins two fibers of the cable (for example fibers 1 and 2). The OTDR’s pulse travels out on fiber 1, through the loopback jumper, and back on fiber 2 — so a single trace from the near end shows both fibers. The OTDR uses its one test port; there is no separate receive port. A receive cable is connected to the end of the return fiber and left unterminated.
When to Use Loopback
Loopback cuts the number of OTDR shots in half and keeps the instrument at one location, which helps when both ends are hard to staff or when many fibers need testing. It still requires someone at the far end to install and move the loopback jumpers.Loopback Test Setup
- Confirm both fibers are dark at both ends.
- Connect the launch cable to the MT5500’s test port and to fiber 1 at the near end.
- At the far end, connect a short loopback jumper between fiber 1 and fiber 2.
- At the near end, connect a receive cable to fiber 2. Leave its far end open.
- Set the distance range to cover the launch cable, both fibers, the loopback jumper, and the receive cable.
- Run the test. The trace shows fiber 1, the loopback, then fiber 2.
⚠ Range and Reach in Loopback
The pulse travels through two fibers, so the trace is roughly twice the cable length plus your launch and receive cables. Set the range accordingly. Because the light passes through both fibers, the longest link you can loopback-test is about half the link length you could test one fiber at a time.Interpreting a Loopback Trace
- Near-end connector of fiber 1 — the first event after the launch cable
- Fiber 1 slope — its attenuation (dB/km), with any splices or connectors along it
- Loopback jumper — a pair of connector events near the middle of the trace (the far end of the cable)
- Fiber 2 events — fiber 2 seen from the far end back toward you
- Near-end connector of fiber 2 — measured using the receive cable
- End of the receive cable — the final reflection
No loopback event or trace beyond it? If the trace drops into noise before the midpoint, fiber 1 has a break, severe bend, or connector failure. Use the distance cursor to locate it, then test fiber 2 separately.
Getting Bidirectional Results with Loopback
Each loopback trace sees fiber 1 in one direction and fiber 2 in the other. Swap the launch and receive connections at the near end and test again: now each fiber is seen from the opposite direction. Averaging the two traces gives bidirectional results for both fibers — the technique Section 5 explains.
In an OTDR loopback test, how is the return fiber (fiber 2) finished at the near end?
What is Bidirectional Testing?
Bidirectional (dual-ended) OTDR testing measures each fiber from both ends: one trace from End A toward End B, and a second from End B toward End A. Averaging the two gives the most accurate loss for every splice and connector.
Why Test Both Directions? — “Gainers”
An OTDR measures loss by comparing backscatter before and after an event. If the two spliced fibers have slightly different backscatter (for example, different mode field diameters), a splice can look like a gain from one direction and an exaggerated loss from the other. Averaging the two directions cancels this error and gives the true splice loss.Equipment Required
- MT5500 — one unit moved to each end in turn, or one at each end
- Launch and receive cables — swapped between ends with the OTDR
- Radios or phones — for coordinating with your partner
- Fiber map / labeling scheme — so every trace is tied to the right fiber
Bidirectional Test Procedure
- At End A, connect the launch cable to fiber 1; at End B, connect the receive cable to fiber 1.
- Confirm with your partner which fiber is under test.
- Run the A→B test and save the trace with the fiber number and direction in the filename.
- Repeat for every fiber in the cable.
- Move the OTDR and launch cable to End B and the receive cable to End A.
- Run the B→A test for every fiber, using the same settings (wavelength, pulse width, range, IOR), and save each trace.
- Average the A→B and B→A results in the OTDR or trace-analysis software.
⚠ Fiber Identification
Never assume which fiber you’re connected to. Confirm the fiber with your partner before every test. Mislabeled traces produce wrong averages and wrong documentation.Bidirectional Splice Loss Calculation
Formula
True Splice Loss = (LossA→B + LossB→A) ÷ 2Example: −0.08 dB (a “gainer”) one way and 0.24 dB the other: (−0.08 + 0.24) ÷ 2 = 0.08 dB.
Where OTDR Testing Fits in Certification
Under TIA-568.3-D, Tier 1 (basic) certification uses an optical loss test set — light source and power meter — to measure link loss, length, and polarity, and it’s required. Tier 2 (extended) testing adds an OTDR trace. It’s valuable for documenting and troubleshooting, but it’s optional and does not replace the Tier 1 loss test.
Comparison: Loopback vs. Two-End Testing
| Feature | Loopback (from one end) | Two-end testing |
|---|---|---|
| OTDR location | Stays at one end | Moves to (or sits at) each end |
| People | Usually 2 — someone places loopbacks at the far end | Usually 2 |
| Fibers per trace | 2 | 1 |
| Maximum link length | About half (light travels two fibers) | Full OTDR reach |
| Bidirectional results | Yes — by swapping launch/receive and re-testing | Yes |
A splice measures −0.08 dB (a gain) from End A and 0.24 dB from End B. What is the true splice loss?
Reading an OTDR Trace
An OTDR trace (sometimes called a reflectogram) plots power level (dB) on the vertical axis against distance on the horizontal axis. A healthy fiber shows a straight, steadily declining line, because every meter of fiber attenuates the signal slightly. Any deviation from that line is an event.
Event Types
| Trace Pattern | What It Means | Typical Cause |
|---|---|---|
| Reflection spike with step loss | Reflective event with loss | Connector, mechanical splice, air gap |
| Step loss only (no spike) | Non-reflective event — loss only | Fusion splice, macrobend, crush |
| Step loss much larger at 1550 nm than 1310 nm | Bend-sensitive loss | Macrobend, tight radius, stress |
| Large reflection then noise | Fiber end | End connector, cleaved end, or break |
| Trace drops into noise early | Signal lost before the expected end | Break, severe bend, failed connector |
| Reflection at a repeat distance with no loss | Possible “ghost” | Echo of a strong reflection; appears at a multiple of a real event’s distance |
Loss Limits (TIA-568.3-D)
- Splice (fusion or mechanical): ≤ 0.3 dB maximum
- Connector (mated pair): ≤ 0.75 dB maximum
- Return loss, SM connectors/splices: ≥ 26 dB (≥ 55 dB for CATV/analog video)
- SM fiber, inside plant: ≤ 1.0 dB/km at 1310 and 1550 nm
- SM fiber, outside plant: ≤ 0.5 dB/km at 1310 and 1550 nm
- Design targets are usually tighter — follow the project specification
Good fusion splices are typically well under 0.1 dB. Quality fiber attenuates far less than the TIA maximums — for example, Corning SMF-28 Ultra is specified at ≤ 0.32 dB/km at 1310 nm and ≤ 0.18 dB/km at 1550 nm.
Using the MT5500 Event Analysis
The MT5500 analyzes the trace automatically and shows an event list and event map with the distance and loss of each connector and splice, plus overall fiber length, loss, and attenuation. Review the events after each test, and investigate anything that exceeds your project’s limits before turning over documentation.
Saving and Exporting Results
- Save each trace with a descriptive name: cable ID, fiber number, wavelength, direction, and date.
- Traces are saved as .SOR files (the Bellcore/Telcordia standard OTDR format), which most trace-analysis software can open.
- The MT5500 can also export reports as PDF or Excel files for turnover packages.
File Naming Convention
Use a consistent scheme: CableID_FiberNum_Wavelength_Direction_YYYYMMDDExample: MDF-IDF2_F04_1310_AtoB_20260416.sor
An event shows a reflection spike together with a step down in the trace. What is it most likely?
Final Assessment
Answer all 15 questions, then submit. A score of 80% (12 of 15) or higher is required to pass. These questions are different from the section knowledge checks.
What does OTDR stand for?
Which two physical phenomena does an OTDR rely on?
What is the main purpose of a launch cable?
What must be done to every connector end-face before it's mated?
Why must the MT5500's OTDR never be connected to a fiber carrying live traffic?
What does the refractive index (IOR) setting directly affect?
Which pulse width gives the greatest dynamic range (reach)?
A trace event is small at 1310 nm but much larger at 1550 nm. What does that suggest?
In an OTDR loopback test, what is connected at the far end of the cable?
Why can loopback testing reach only about half the link length of single-fiber testing?
What does a trace that drops into noise before the expected end of the fiber most likely indicate?
What is the main advantage of bidirectional OTDR testing?
A step loss with no reflection spike most likely indicates:
Under TIA-568.3-D, what is the maximum allowed splice loss?
Under TIA-568.3-D, can an OTDR trace (Tier 2) replace the light source/power meter loss test (Tier 1) for certification?
Your Information
When you submit, your results are emailed automatically to the Five 9s Communications training administrator, and a copy is sent to the email address you enter here — whether or not you pass.
Assessment Results
Score: 0 / 0
Percentage: 0%
Status: —
Time in course: —
Passing completes the knowledge portion. Your lead still verifies hands-on OTDR testing in the field.