Optical Power Measurement
Optical power measurement is the basis of fiber optic metrology. An optical power detector is found in nearly every lightwave test instrument.Basically there are two types of power measurements: absolute power measurement and relative power measurement.
Relative power measurements are used for testing of attenuation, gain and return loss, etc. Absolute power measurement is needed in conjunction with optical sources, detectors and receivers. For example, the absolute power of an optical transmitter or optical amplifier is important for the power margin of a communication system.
There are two main types of optical power meters: power meters with thermal detectors, in which the temperature rise caused by optical radiation is measured, and photodetectors, in which the incident photons generate electron-hole pairs.
Photodetector type power meters have a relatively small wavelength coverage and need absolute calibration, however, they are the most popular types because of their high sensitivity.
Optical power meters with thermal detectors have wide and flat wavelength characteristics which makes them sometimes preferred in calibration laboratories.
Power Meters with Thermal Detectors
Power Meters with Photodetectors
|Wavelength Dependence||1. wavelength independent
2. wide wavelength range
wavelength range: 2:1
|Sensitivity||very low (typically 10uW)||very high (down to less than 1pW)|
|Accuracy||+/-1% depending on calibration method||+/-2% depending on calibration method|
Fiber optic light source is a fiber optic test equipment to measure the fiber optic loss for both single mode fiber cable and multimode fiber cables; usually the fiber optic light source is used with the fiber optic power meters.
This fiber optical light source can provide wavelength output according to the specific requirements including the 650nm red source, 1310nm/1550nm wavelength for the single mode fiber and 850nm/1300nm wavelength for the multimode fiber.
Optical sources are one of the most researched areas in fiber optic communication. The following types of fiber optic light sources are used in telecommunications and data communications.
A. Fabry-Perot Lasers
Fabry-Perot lasers (FP lasers) is the most widely used light source for lightwave telecommunication systems. It is named for the French scientists Charles Fabry and Alfred Perot.
In a Fabry-Perot (FP) laser, light is reflected and re-reflected between two “mirrors” at either end of a semiconductor material that has been biased electrically. The material and two mirrors form a resonant cavity that roughly determines the wavelength of the light produced.
One of the mirror is only partly reflective, allowing some portion of the light to “leak” out into an external fiber, whereas most is internally reflected. This is directly analogous to a “high-Q” resonant L-C tuned circuit where the circulating energy is much higher than that coupled into a load.
The following figure illustrates a section view of an FP laser parallel to the direction of light emission. The laser has two parts: a semiconductor optical amplifier to provide gain and mirrors to form a resonator around the amplifier.
B. Distributed Feedback Lasers (DFBs)
A Distributed Feedback laser (DFB) is a type of laser diode, quantum cascade laser or optical fiber laser where the active region of the device is periodically structured as a diffraction grating. The structure builds a one dimensional interference grating (Bragg scattering) and the grating provides optical feedback for the laser.
The DFB laser is designed to overcome the spectral shortcomings of the FP laser. The following figure shows the structure of a DFB laser.
It is very similar to an FP laser with the addition of a Bragg reflector structure located near the light-emitting active region. The Bragg reflector grating provides a periodic change in the index of refraction in the waveguide.
C. Vertical Cavity Surface-Emitting Laser (VCSEL)
The vertical cavity surface-emitting laser (VCSEL) was originally developed as a low cost alternative to FP and DFB lasers. The first commercial applications of these lasers is in the area of high-speed data communication links replacing LEDs.
Vertical cavity lasers emit perpendicular to the top plane of a semiconductor wafer. It uses a multilayer dielectric mirror that is grown directly on the semiconductor surface as shown below.
This mirror consists of alternations of high and low index of refraction layers to form a Bragg reflector. The distinguishing feature of this structure is its extremely short optical amplifier length (on the order of 100nm). This length is compared to the 300um length typical of an FP and DFB laser.
This short amplifier length limits the available gain from the amplifier to a very small value.
D. Surface-Emitting LEDs
The most common multimode source is the surface-emitting LED (SLED) shown below.
The SLED has low-bandgap semiconductor materials sandwiched between high-bandgap materials as is found in semiconductor lasers. The major difference in an LED is that there are no mirrors to provide feedback.
Current is passed through the active region to create hole and electron pairs in the low-bandgap active region. The electrons from the conduction band lose energy spontaneously and emit photons in all directions. A fraction of the generated light is coupled into the multimode fiber.
E. Edge-Emitting LEDs
Edge-Emitting LED (EELED) is very similar to an FP laser without mirrors. The following diagram shows its structure.
This EELED configuration shows two segments. One segment is forward biased to produce gain in a semiconductor optical amplifier. The other segment is reverse biased to produce an optical absorber.
The absorber prevents the optical amplifier from becoming a FP laser. The output of the semiconductor optical amplifier is also anti-reflection-coated to further prevent a mirror from forming.
The EELED optical amplifier produces ASE. Spontaneously emitted light at the input of the amplifier produces ASE at the amplifier output.
The fiber optic power meter is a special light meter that measures how much light is coming out of the end of the fiber optic cable. The power meter needs to be able to measure the light from the fiber optic cable at the proper wavelength and over the appropriate power range.
Most power meters used in datacom networks are designed to work at 850nm and 1300nm, the wavelengths used in datacom networks. Power levels are modest, in the range of –15 to –35dBm for multimode link, 0 to –40dBm for single mode links. Power meters generally can be adapted to a number of different connector styles for convenience in testing.
Fiber optic power meters, like DMMs, come in a variety of types. The measurement uncertainty of practically all fiber optic power meters is the same, limited by the physical constraints of transferring standards with optical connections. Most meters have an uncertainty of +/-5% or approximately 0.2dB, no matter what the resolution of the display may be. Lower cost meters or those for field use usually have a resolution of 0.1dB, laboratory meters display 0.01dB, and a resolution of 0.001dB is available on a few specialized meters.
The light that transmits data over fiber optic cable is invisible to the naked eye, making it difficult to ensure without a formal test that installers have made the proper connection. A visual fault locator (sometimes called a cable tracer) is a quick and dirty way to test the continuity of a fiber cable connection by sending visible light over a fiber optic cable.
A typical fault locator is essentially a flashlight that applies its LED, incandescent light source or laser to one of a cable, which is visible from the other end. A fault locator enables you to find a specific cable out of bundle and ensure that a connection has been established.
Most powerful units that use laser light sources can actually make points of high loss – such as breaks, kinks, and bad splices – visible to the naked eye, as long as the cable sheath is not completely opaque.
For example, the yellow or orange colored sheaths commonly used on single mode and multimode fibers usually admit enough of the light energy lost by major cable faults to make the losses detectable from outside.
In a world of complex and costly testing tools, visual fault locators are one of the simplest and most inexpensive items in a fiber optic toolkit.
The fiber identifier is a piece of test equipment that allows the fiber optic technician to see through the jacket, strength member, and buffer of the fiber optic cable. It is designed to place a macrobend in the fiber optic cable under test. Photodiodes in the fiber identifier detect light penetrating through the fiber optic cable. The electronics in the fiber identifier measure the detected light energy and display that direction of light travel through the optical fiber.
The fiber identifier is used very much like a visual fault locator (VFL) when it comes to troubleshooting. One key difference is that the fiber identifier replaces your eyes. Another difference is that fiber optic cable under test typically does not have to be disconnected from an active circuit – it can remain plugged into the transmitter and receiver.
The infrared light travelling through the optical fiber during normal operation is often enough to perform most tests. However, sometimes an additional infrared light source is required to adequately troubleshoot.
A fiber optic talk set is sometimes used when technicians need to be at different positions along the same link. The talk set includes a transmitter and receiver that can be attached directly to nearby fiber cables.
Fiber optic talk sets are an inexpensive solution to meet your communication needs when testing multimode or single mode fiber optic cables. Designed for voice communication over spare fibers, they provide full duplex, hands-free operation.
Optical spectrum analysis is the measurement of optical power as a function of wavelength. The spectrum of a light source is an important parameter in fiber optic communication systems. For example, chromatic dispersion can occur in the fiber and limit the achievable modulation bandwidth of the system. The effect of chromatic dispersion can be seen in the time domain as pulse broadening of a digital waveform. Since chromatic dispersion is a function of the spectral width of the light source, narrow spectral widths are desirable for high-speed communication system.
The prevalence of wavelength division multiplexed (WDM) systems has stimulated significant activity in the measurement of optical spectra. WDM has also made optical spectrum analysis a key measurement capability that must be embedded inside telecommunication network elements.
Figure 1 shows an example measurement made by an optical spectrum analyzer (OSA). It shows the power versus wavelength for a Fabry-Perot (FP) laser. The FP laser shows a series of longitudinal modes that have significant energy over a 20nm span. The plot shows that the measurement has been made using an instrument filter bandwidth of 0.2nm with an instrument sensitivity setting of –55 dBm. From the spacing of the modes and power distribution, the laser length and coherence properties of the laser can be determined.
:: Types of Optical Spectrum Analyzer (OSA)
A) Fabry-Perot Interferometers Based OSA
The FP interferometer, shown in figure 2 below, consists of two highly reflective, parallel mirrors that act as a resonant cavity which filters the incoming light. The resolution of FP-inteferometer-based OSA depends on the reflection coefficient of the mirrors and the mirror spacing. Wavelength tuning of the FP interferometer is accomplished by adjusting the mirror spacing or by rotating the interferometer with respect tot he input beam.
The advantages of the FP interferometer is its potential for very narrow spectral resolution and its simplicity of construction. The added resolution allows measurements such as laser chirp to be performed.
The major disadvantage is that the filters have repeated passbands. The spacing between these passbands is called the free spectral range. If the mirrors are spaced very widely apart, very high resolution can be obtained, but the free spectral range is small. This problem can be solved by placing a second filter in cascade with the FP interferometer to filter out power outside the interferometer’s free spectral range.
B) Michelson Interferometers Based OSA
Another type of spectrum analyzer is based on the Michelson interferometer as shown in figure 3 below.
The input signal is split into two paths. One path is fixed in length and one is variable. The Michelson interferometer creates an interference pattern between the signal and a delayed version of itself at the detector. The resulting waveform is the autocorrelation function of the input signal and is often referred to as an interferogram.
Michelson-interferometer-based spectrum analyzer make direct measurements of coherence length. Other types of OSAs cannot make direct coherence-length measurements.
If the period of the zero crossings in the interferogram are accurately measured by comparison to a wavelength standard, the wavelength of the unknown signal can be determined with high accuracy. It is the potential for high wavelength accuracy that distinguishes this instrument. A state of the art wavelength meter can measure wavelength to less than 1 part per million. A 1550nm laser could be measured to +/-0.0015nm.
The Michelson interferometer can also provide displays of power versus wavelength. To determine the power spectra of the input signal, a Fourier transform is performed on the interferogram. The resolution of the instrument is determined by the path-length delay that is used to create the interferogram.
Because this instrument does not depend on a tunable bandpass filter for wavelength identification, Michelson-interferometer-based designs cannot be used in applications where a true bandpass filter is required. This type of analyzer also tends to have less dynamic range than diffraction-grating-based OSAs due to the shot noise that is always present in the optical receiver for large input signals.
C) Diffraction-Grating-Based OSA
The most common OSAs for fiber optic applications use diffraction gratings as the basis for a tunable optical filter. Figure 4 below shows what a diffraction-grating-based OSA might look like.
In the monochromator, a diffraction grating (a mirror with finely spaced corrugated lines on the surface) separates the different wavelengths of light. The diffracted light comes off at an angle proportional to wavelength. The result is similar to the rainbow produced by visible light passing through a prism.
In the infrared, prisms do not work well because the dispersion (in other words, change of refractive index versus wavelength) of glass in the 1 to 2um wavelength range is small. Diffraction gratings are used instead. They provide a greater separation of wavelengths allowing for better wavelength resolution.
A diffraction grating is made up of an array of equidistant parallel slits (in the case of a transmissive grating) or reflectors (in the case of a reflective grating). The spacing of the slits or reflectors is on the order of the wavelength of the light for which the grating is intended to be used. The grating separates the different wavelengths of light because the grating lines cause the reflected rays to undergo constructive interference only in very specific directions. Only the wavelength that passes through the aperture reaches the photodetector to be measured.
The angle of the grating determines the wavelength to which the OSA is tuned. The size of the input and output apertures together with the size of the beam on the diffraction grating determines the spectral width of the optical filter.
Wavelength meters can measure amplitude versus wavelength, as do grating-based optical spectrum analyzer (OSA). Wavelength meters distinguish themselves by making very accurate measurements of wavelength.
A grating-based OSA can measure a 1550nm signal with +/-0.1nm absolute-wavelength accuracy, assuming that the instrument has had a recent user calibration. A wavelength meter can make this measurement with better than +/-0.001nm accuracy. This is a 100 times improvement.
What is it important to measure a 1550nm signal to +/-0.001nm accuracy? Here are two examples.
A) A researcher may want to study long-term wavelength drift of a distributed feedback (DFB) laser for use in a wavelength division multiplexed (WDM) communication system. WDM lasers must be proven to drift less than 0.1nm over a 25 year lifetime with accelerated aging studies.
B) When making chromatic dispersion measurements, it is necessary to measure the slope of the group delay versus wavelength function. Manufacturers need to know the zero-dispersion wavelength for components like dispersion compensators to tenths of a nanometer. Dispersion measurement algorithms require a derivative of the group delay with respect to wavelength. In order to get accurate dispersion values, the relative wavelength steps must be measured very accurately.
:: Types of Accurate Wavelength Measurement
There are several approaches used to accurately measure wavelength.
A. Optical bandpass filter techniques. These include optical spectrum analysis using a grating-based filter, Fabry-Perot- filters.
B. Interferometric fringe-counting techniques are commonly used for high accuracy applications.
C. Wavelength discriminator techniques. A sloping insertion loss versus wavelength function can be used to determine optical wavelength.
:: Michelson Interferometer Wavelength Meter
The following figure shows a block diagram of a Michelson interferometer.
Light from a fiber optic input is collimated and directed to the input of the interferometer. The input signal is split into two paths with a beam splitter. Both beams are then incident on 100% reflecting mirrors that bounce the light back toward the beam splitter. These mirrors are most often constructed as retroreflectors so that the beams are reflected back at nearly the same angle as they are sent into the mirror.
Part of the light reflected from the two arms of the interferometer goes back toward the input beam. The other portion of the light is incident on a photodetector. Since there is no loss assumed in the interferometer, all of the light is directed to either the photodetector or the input port.
If the variable length interferometer mirror is moved, the amount of light reaching the photodetector will oscillate up and down because of constructive and destructive interference effects between the two paths of the interferometer. Through the analysis of these interference patterns, the wavelength of light can be calculated.
There are two alternate but equivalent viewpoints useful in analyzing the interferometer operation.
The two beams can be analyzed in terms of light interfering as the path length in the interferometer changes. This will be referred to as the fringe-counting description of wavelength meter operation.
Alternately, if one arm of the interferometer is moved at a constant rate, the frequency of the light in the moving arm is Doppler-frequency shifted. The detector then mixes the light from Doppler-shifted and unshifted arms. The beat frequency between these two signals will be used to calculate the unknown frequency of the input signal.
The optical time domain reflectometer (OTDR) is both the best known and least understood fiber optic test equipment. The OTDR does not measure loss, but instead implies it by looking at the backscatter signature of the fiber.
The OTDR works like radar or sonar, sending out a pulse of light from a very powerful laser, which is scattered by the glass in the core of the fiber. About one millionth of the light is scattered back up the fiber to the OTDR, where it is captured, averaged to improve the signal to noise ratio, and analyzed.
OTDRs launch short duration light pulses into a fiber and then measure, as a function of time after the launch, the optical signal returned to the instrument.
As the optical pulses propagate along the fiber, they encounter reflecting and scattering sites resulting in a fraction of the signal being reflected back in the opposite direction. Rayleigh scattering and Fresnel reflections are physical causes for this behavior. By measuring the arrival time of the returning light, the locations and magnitudes of faults can be determined and the fiber link can be characterized.
The OTDR suffers from several uncertainties in measurements, and from physical limitations. The measurement uncertainties come primarily from the variations in backscatter of the fiber. The backscatter coefficient is a function of the material properties of the glass in the core and the diameter of the core.
Variations of the fiber materials or geometry can cause major changes in the backscattered light, making splice or connector measurements uncertain by as much as +/-0.4dB.
Within these limitations, OTDRs are used primarily to find faults in longer fiber runs, especially when they are remote, buried, or otherwise inaccessible.They should never be used to measure the cable plant loss for power budgeting.