Why Do You Need a PON Power Meter

PON power meters are essential for field technicians to install or maintain any type of PON network. The PON power meter can simultaneously test the upstream and downstream wavelengths of 1490nm, 1550nm and 1310nm through optical fiber, as well as estimate the signals of voice, data and video streams.

The requirements for testing fiber optic networks will vary according to the specific type of network and the overall testing requirements of the network designer. Regardless of the type, two basic or general-purpose optical test equipment will be used; an optical time domain reflector or OTDR, and a pair of optical test equipment called a power meter and a light source. These tests are usually in units of "dB". The term dB is the expression of attenuation or power loss when the fiber is transmitted from the terminal to a point along the fiber path. After connecting the optical fiber to the active device, all tests are in dBm. Active devices will transmit actual or real optical power at a specific wavelength, and use 1mW as a reference.

Definition of PON

The term PON stands for "passive optical network". PON is a fiber optic telecommunication network that provides broadband to transmit data to customer premises via fiber optic cables. Its architecture realizes the point-to-point or point-to-multipoint node arrangement in the communication network. Point-to-multipoint networks use a single optical fiber to provide services to multiple endpoints through the use of passive or passive optical splitters. Splitters are used to distribute fiber bandwidth among multiple access points. Passive optical networks are often referred to as the "last mile" between the Internet Service Provider (ISP) and its customers.

Optical Power

The most basic fiber optic measurement is optical power from the end of a fiber. This measurement is the basis for loss measurements as well as the power from a source or presented at a receiver. Typically both transmitters and receivers have receptacles for fiber optic connectors, so measuring the power of a transmitter is done by attaching a test cable to the source and measuring the power at the other end. For receivers, one disconnects the cable attached to the receiver receptacle and measures the output with the meter.

While optical power meters are the primary power measurement instrument, optical loss test sets (OLTSs) and optical time domain reflectometers (OTDRs) also measure power in testing loss. TIA standard test FOTP-95 covers the measurement of optical power.

Optical power is based on the heating power of the light, and some optical lab instruments actually measure the heat when light is absorbed in a detector. While this may work for high power lasers, these detectors are not sensitive enough for the low power levels typical for fiber optic communication systems (Table 1).

Optical power meters typically use semiconductor detectors since they are sensitive to light in the wavelengths and power levels common to fiber optics. Most fiber optic power meters are available with a choice of 3 different detectors, silicon (Si), Germanium (Ge), or Indium-Gallium-Arsenide (InGaAs).

Table 1. Optical power levels typical of fiber optic communication systems

Fiber optic power meters have inputs for attaching fiber optic connectors and detectors designed to capture all the light coming out of the fiber. Power meters generally have modular adapters that allow connecting to various types of connectors. This connection is considered a "no loss" connection. In reality, we do not capture all the light from the fiber because there is a glass window on the detector and that window and the detector are slightly reflective. However the coupling is very consistent and when we calibrated the meter, we calibrate with a fiber optic cable under the same conditions. Thus, what we measure of the light by presenting a connector to the power meter is both consistent and calibrated as long as you choose the proper calibration wavelength.

Silicon photodiodes are sensitive to light in the range of 400 to 1000 nm and germanium and indium-gallium-arsenide photodiodes are sensitive to light in the range of 800 to 1600 nm.

Silicon detectors are very low noise detectors sensitive to light at approximately 400 to 1100 nm wavelength, depending on the exact method of fabrication. Thus, they are useful for standard datacom links using 820 nm LEDs and glass fiber or 665 nm LEDs and plastic fiber. They can also be used with older telecom systems that used 850 nm lasers.

Silicon detectors have inherently low noise, low leakage currents and therefore very low noise floors when used with transimpedance amplifiers in power meters. Typical noise floors on fiber optic instruments using Si detectors is -70 to -90 dBm, or about 1 to 100 picowatts.

Germanium detectors are sensitive to light in the 800 to 1800 nm wavelength, making them useful for all systems using glass fiber, including 1300 and 1550 nm single mode systems. Ge detectors are noisier however, creating a higher noise floor for low level measurements. This noise is proportional to detector area, so by using a smaller detector, one obtains a lower noise floor. However, smaller detectors require positioning the fiber end very close to the window of the detector and centered accurately over the detector sensitive area. The noise of a 2 mm Ge detector is typically 10 to 50 times lower than room temperature 5 mm Ge detectors.

Some manufacturers of fiber optic power meters have chosen to cool these large Ge detectors to reduce the noise and get lower measurement limits. This leads to more sensitive measurements but with a penalty of increased circuit complexity, .instrument weight and short battery life, since one must provide up to 1 amp current to the thermoelectric cooler in the Ge detector package.

Another solution for extremely low level measurements at 1300 and 1550 nm is to utilize InGaAs detector technology, , which has been developed for the receivers of high speed long wavelength communication systems. InGaAs detectors have the same sensitivity range as Ge, but are much less noisy. With InGaAs detectors, measurements can be made to -65 dBm (less than 0.5 nW) with ease. However, InGaAs detectors are very expensive, limiting their usage to only the most expensive instruments.

Table 3.2. Characteristics of detectors used in Fiber Optic Power Meters

Point-to-point or P-T-P optical network

A P-T-P network is a network with two terminal points and no content in between. As with all fiber optic networks, when constructing a fiber optic network, the fiber must be terminated for any testing. Therefore, terminate one end of the network and perform an OTDR test on each fiber to ensure that there is no problem with the termination and the excess fiber length. The test result will be stored for future needs and expressed in "dB". If the network needs fusion splicing, after the fiber fusion splicing, use the OTDR again to ensure that the splicing and the increased fiber length meet the requirements again. The test using OTDR continues and is completed after the end of each fiber is terminated. At this point, another set of tests is needed, usually called end-to-end testing. The test requires the use of a light source and a power meter, and all test results will be stored again. The optical power meter will be set to "dB" and refer to the light source commonly referred to as "zeroing". Then move these units to opposite ends, and the field technician will send and receive the wavelength specified by the designer. Again, this is a measure of the designer's overall link loss budget. The P-T-P network refers to its terminals as "A" and "B" and will transmit and receive at least two unique wavelengths through each fiber. This usually requires ensuring that any wavelength used by the transmitter can be used between the two specified wavelengths. The network designer will define these wavelengths and provide labels for these ends. The technician performing the test will reference these tags in any report returned to the designer.

Point-to-multipoint PON type network

Now, when building a point-to-multipoint network (such as a passive optical network or PON), many tests and test equipment remain the same, but some special functions are required. The OTDR test during the construction period remains unchanged, and the test is performed every time the fiber is terminated or spliced. Again, this will continue to the far end of the fiber after the fiber is terminated. After terminating all the fibers, they will be tested again with a power meter and light source. The activation phase of the PON network requires special equipment. This is the similarity between P-T-P and P-T-Multipoint.

The PON network activation stage first connects the power meter to an active device called an optical line terminal or OLT and sets it to an appropriate wavelength, and sets the unit to "dBm", which becomes the reference power.

Note: There are several generations of PON network OLTs that use different wavelengths, so the optical power meter must be able to set these wavelengths. GPON is 1490nm, XGPON is 1577nm, NGPON has multiple wavelengths from 1596nm to 1602nm.

Measurement Units: "dB" and "dBm"

Whenever tests are performed on fiber optic networks, the results are displayed on a meter readout in “dB.” Optical loss is measured in “dB” while optical power is measured in “dBm.” Loss is a negative number (like –3.2 dB) as are most power measurements. Confused? Many fiber optic techs are too. Let’s see if we can clear up some of the confusion. 
When we make fiber optic measurements, we are measuring the power in the light we measure. The standards we use for power measurements, maintained by NIST (the US National Institute of Standards and Technology,) are actually determined by the heating effect of the light as it is absorbed in a detector. Every fiber optic power meter sold is calibrated traceable to the NIST standard so different meters should measure the same power, within the limits of calibration uncertainty.

Optical power in fiber optics is similar to the heating power of a light bulb, just at much lower power levels. While a light bulb may put out 100 watts, most fiber optic sources are in the milliwatt to microwatt range (0.001 to 0.000001 watts), so you won’t feel the power coming out of a fiber and it’s generally not harmful.

In the early days of fiber optics, source output power was usually measured in milliwatts and loss was measured in dB or deciBels. Over the years, all measurements migrated to dB for convenience. This was when the confusion began.

Loss measurements were generally measured in dB since dB is a ratio of two power levels, one of which is considered the reference value. The dB is a logarithmic scale (remember “logs” from high school math?) where each 10 dB represents a ratio of 10 times.

The actual equation used to calculate dB is

dB = 10 log (measured power / reference power).

Here is an Excel spreadsheet that calculates dB/power ratio and dBm/milliwatts.

So 10 dB is a ratio of 10 times (either 10 times as much or one-tenth as much), 20 dB is a ratio of 100, 30 dB is a ratio of 1000, etc. When the two optical powers compared are equal, dB = 0, a result of the log scale used in dB but a convenient value that’s easily remembered.

If we have loss in a fiber optic system, the measured power is less than the reference power, so the ratio of measured power to reference power is less than 1 and the log is negative, making dB a negative number. When we set the reference value, the meter reads “0 dB” because the reference value we set and the value the meter is measuring is the same. Then when we measure loss, the power measured is less, so the meter will read “ – 3.0 dB” for example, if the tested power is half the reference value. Although meters measure a negative number for loss, convention has us saying the loss is a positive number, so we say the loss is 3.0 dB when the meter reads  – 3.0 dB.

Measurements of optical power are expressed in units of dBm. The “m” in dBm refers to the reference power which is 1 milliwatt. Thus a source with a power level of 0 dBm has a power of 1 milliwatt. Likewise, -10 dBm is 0.1 milliwatt and +10 dBm is 10 milliwatts.

Instruments that measure in dB can be either optical power meters or optical loss test sets (OLTS). The optical power meter usually reads in dBm for power measurements or dB with respect to a user-set reference value for loss. While most power meters have ranges of +3 to –50 dBm, most sources are in the range of 0 to –10 dBm for lasers and –10 to –20 dBm for LEDs. Only lasers used in CATV or long-haul telephone systems have powers high enough to be really dangerous, up to +20 dBm – that’s 100 milliwatts or a tenth of a watt!

The OLTS or the power meter on the dB scale measures relative power or loss with respect to the reference level set by the user. The range they measure will be determined by the output power of the source in the unit and the sensitivity of the detector. For multimode fiber, an OLTS using a LED source will usually measure over a range of 0-30 dB, more than adequate for most multimode cable plants which are under 10 dB loss. Singlemode networks use lasers and may have loss ranges of up to 50 dB for long-haul telecom systems, but campus cabling using singlemode may only have 1-3 dB loss. Thus a singlemode OLTS may be different for short and long systems.

If you remember that dB is for measuring loss, dBm is for measuring power and the more negative a number is, the higher the loss, it’s hard to go wrong. Set your zero before measuring loss and check it occasionally while making measurements.

Calibration

Calibrating fiber optic power measurement equipment requires setting up a reference standard traceable to a national standards like like the US National Institute of Standards and Technology (NIST, Boulder, CO) for comparison purposes while calibrating every power meter or other instrument. The NIST standard for all power measurements is an ECPR, or electrically calibrated pyroelectric radiometer, which measures optical power by comparing the heating power of the light to the well-known heating power of a resistor. Calibration is done at 850, 1300 and 1550 nm. Sometimes, 1310 nm is used as the calibrated wavelength on a power meter, a holdover from the early 1980s when the telcos and AT&T used 1310 nm as a standard, but the standard for power meter calibration is 1300 nm. To conveniently transfer this measurement to fiber optic power meter manufacturers calibration laboratories, NIST currently uses a laboratory optical power meter which is sent around to labs to use as a transfer standard.

To transfer from this transfer standard to production instruments, power meter manufacturers use calibrated detectors or power meters which are regularly checked against one another to detect any one detector's variability, and all are periodically recalibrated to NIST's transfer standards.

In order to transfer the calibration, one needs a source of known characteristics. Typically laser sources at 850, 1300 and 1550 nm pigtailed to single mode fibers are used . The laser sources have very narrow spectral width to allow accurate wavelength calibration, and the single mode fiber controls the output beam presented to the detector of the instrument. Each of these sources is checked for wavelength regularly to insure that no drift has occurred. The output power of these lasers is precisely controlled by an optical feedback circuit to insure stability. Even the temperature of the laser is often controlled precisely to insure no drift in output power or wavelength occurs during the calibration process.

Using the sources described above, one measures the output of one of the lasers on a transfer standard meter or detector and record the value. The instrument under test is then adjusted to read the same value as the transfer standard detector and a single point calibration is done.

Here is the calibration over wavelength for a commercial fiber optic power meter.You can see the wavelength sensitivity of the detector used in the meter. It varies almost 7dB over the meter's wavelength range.

For all power meters, especially those with autoranging, one must calibrate on every range, double checking to insure that the meters have a smooth transition between ranges to prevent calibration discontinuities. Calibration is therefore checked at several points near the top and bottom of the range for every meter.

Meters calibrated in this manner have an uncertainty of calibration of about +/- 5%, compared to the NIST primary standards. Limitations in the uncertainty are the inherent inconsistencies in optical coupling, about 1% at every transfer, and slight variations in wavelength calibration. NIST is working continuously with instrument manufacturers and private calibration labs to try to reduce the uncertainty of these calibrations.

Recalibration of instruments should be done annually, however experience has shown that the accuracy of meters rarely changes significantly during that period, as long as the electronics of the meter do not fail. Unfortunately, the calibration of fiber optic power meters requires considerable investment in capital equipment and continual updating of the transfer standards, so very few private calibration labs exist today. Most meters must be returned to the original manufacturer for calibration.

Conclusion

When the link is connected to the far end, the technician will repeat the test and make sure there is no problem. This kind of test continues to the far end, in the PON network there is an active device called an optical network terminal or ONT or sometimes called an optical network unit or ONU. In any case, the device receives the light from the OLT transmitter and communicates with the OLT through its own transmitter. The ONT cannot communicate with the OLT without first receiving the OLT's transmitter wavelength. At this time, it is absolutely necessary to use a special optical power meter to measure the power of the OLT, and allow the power to pass through and provide the signal to the ONT/ONU so that it can send back the signal.

The PON meter has two test ports; one is named DROP and the other is named ONT or ONU. The technician will connect the branch that is connected to the OLT through the optical fiber to the port named DROP, and then connect the optical fiber pigtail of the ONT/ONU connector to the ONT/ONU port. The PON meter is now connected in series between the OLT and the ONT/ONU, and allows the OLT to communicate with the ONT/ONU. The technician will observe the input OLT power level and the output ONT/ONU power level again. If everything is normal, the slave station is connected to the ONT/ONU, and the service activation task can continue.