Wavemeter (WM) or Optical Spectrum Analyzer (OSA)?

Telecom Equipment Manufacturer Test engineers must often include a wavelength characterization procedure to qualify a product under test.  Such test can be performed by different equipment ranging from a tunable filter with an adjunct power meter, an Optical Spectrum Analyzer (OSA) or a Wavemeter.  An OSA is indeed an assembly of a Diffraction Grating based tunable filter section, a chopping motor and slits, then a power detector (see our article for more in depth info on OSAs).  OSAs being optimized for the specific application of wavelength versus power measurement at a specific resolution, there is not much value for a test engineer to build his own tunable filter based measurement system and write all the software required.  The other test product common on the market is the Wavelength meter.  All Telecom Wavelength Meters found on the market are based on a Michelson interferometer.  Although both the OSA and the Wavemeter will deliver wavelength and power readings for a given WDM or other light signal, the attributes of both products must be considered and the selection must be made based on the measurement specifications needed.

The main differences between the OSA and the Wavemeter are the power reading dynamic range, the wavelength selectivity, the wavelength range, the acquisition speed, the feature richness and the price.  In most cases, a Wavemeter are cheaper than OSAs.  The reason is quite simple: the construction of a Michelson based Wavemeter is not as complex as the construction of an OSA monochromator (the filtering part).  But some Multi-Wavelength Meters (MWM) are now priced higher than OSAs; mostly because of their feature richness. When involved in determining both a wavelength and its associated power level, the OSA is the best general purpose optical measurement tool you can buy.  OSAs cover a very large wavelength range sometimes from 350 to 1700nm making them a great choice for R&D groups.  Throughout that range, the reading accuracy will vary as the internal error correction algorithms to compensate for linearity issues, wavelength dependencies and gain blocs.  In the case of a Wavemeter, the interferometer is a single mechanical sweeping mechanism that travels through the entire span whatever the range you query; only the values displayed will change.  There is only one detector.  Most Wavemeter have a InGaAs detector well suited to cover the traditional O,S,C and L bands used in Telecom.  Multi Wavelength Meters behave much like an OSA and can display many peeks of wavelength in one sweep.  Such Wavemeters still operate on Michelson’s interferometry but include Fourier transforms algorithms and much more signal processing hence their higher price.

Lets talk specs:

The best Wavelength Meters (WM) offer wavelength measurement accuracy of ±0.2 parts per million (ppm) which, at 1550 nano-meter (nm), translates to ±0.3 pico-meter (pm).  The Ando AQ6317B OSA offers ±20pm at 1550 and the Agilent 86142B is at ±10pm.  Lets be clear; OSAs can guaranty such level of accuracy after a calibration using their internal calibration source (if equipped) and only using a high resolution mode.  Wavelength Meters will deliver their accuracy across all wavelength range.

Where the OSA shines compared to the WM is with dynamic range.  A Burleigh WM will typically read a signal down to -35dBm then only see noise.  An Agilent OSA will read down to -90dBm. In general, WM and MWM saturate if the incoming signal is above +10dBm. Most of the OSA can handle powers up to +23dBm. In many telecom applications, if the source goes through an amplifier before hitting the network (or the test station) and although the selection of a WM might be best to read wavelengths accurately, the addition of an attenuator, a splitter and power meter before the WM will increase the station cost largely and the OSA might then become the best choice.

Although the WM has better capability to resolve and display on a wavelength or many wavelengths in the case of a MWM, it has a limitation that cannot be overlooked. This limitation comes with the spec named Selectivity. Typically, if 2 peaks are closer than 50GHz with power levels differences of 25dB or more, the WM might not be able to resolve the difference between those 2 peaks correctly and treat them as one “blob of light”.  The measurement will then be flawed if your use of a WM was to look at each individual wavelength and not sum them as one.

Neither of the OSA or the MWM offers accurate power reading in the C band.  Specs will vary from a typical accuracy of 0.2 to 0.5dB.  The OSA could be considered more accurate on power measurement as it is less sensitive to Polarization.

Then which one should I choose?:

We are really in the heart of test engineering here where the measurement requirements rule on which test product to select.  The first question to answer is if the data to be acquired is a single wavelength or multiple.  If many, the choice becomes limited to either a Multi Wavelength Meter or an OSA.

Then, it becomes a question of application.  Are you testing systems or characterizing components?  If working on components, what aspect of the said component is critical?  If you are working on a power equalizer dedicated to ensure that all light channels are transmitted at equal power then the power reading accuracy might be more important than the wavelength measurement accuracy.  In that case, an OSA seems more suitable. If many wavelengths are to be found and displayed, what are the criteria’s for a pass or fail test?  If you need to display and qualify that two wavelength peeks are in line with the DWDM ITU grid then the MWM will give you the most accurate wavelength measurement.  Instead, if you are qualifying incoming wavelengths in adjacent channels at very different power levels then you might face the wavelength selectivity issue that we exemplify in the “what is wavelength selectivity?” video that you can view here then the OSA will be your safe choice.

When working on a Test Station with only one wavelength to be measured at a time, with power between +10dBm and -40dBm, the decision becomes an easy one: a cheaper and more precise Wavemeter.  In many instances, technology as old as the Agilent/HP 86120B will do just fine, at the fraction of the price of today’s newest units.

If your test strategy involves the use of pre-programmed test features such as EDFA noise measurement then the choice might be limited to the OSA.  Before selecting a model from another, you must then verify which features are offered with each Optical Spectrum Analyzer model.

If one of your measurements involves the reading of a broad spectrum light source then again, the OSA is the only choice being the only unit designed to filter and display a large spectra of light at a time.


If you want to read in more depth about Michelson interferometer, I suggest http://en.wikipedia.org/wiki/Michelson_interferometer

Let’s demystify the OSA – Part 3

The detection section:

AQ6315After the input, the light goes through the filtering section which acts like a rainbow and separates colors. In order to get valuable wavelength readings, the goal is to only send one color to the detector.  The reality is that no filter can be so precise. The high end laboratory grade units can offer resolution down to 15pm.

There are many aspects to account for in order to get the best possible reading at the detector.  One of them is noise.  All optical test systems are driven by electronic circuitry.  The amount of light left after the filtering section is very low thus the conversion of light into current then into voltage delivers low level signals thus signals very sensitive to noise. To shelter the circuits from the electrical noise, shielding is used around most detection parts.  On top of shielding, a chopper motor is used to “slice” the optical signal to a 50 percent duty cycle.  The detection circuitry is synchronized with the chopping motor and expects complete darkness then light then darkness and so on.  When no signal is expected but some is found, it is known to be noise and can be removed from the displayed results.

Part of the complexity leading to inaccuracy of the OSA is the signal amplification and data processing to cover such large wavelength range.  The detector, most times an InGaAs is the last part of the detection section. In the case of an OSA which includes visible light measurement such as the Ando AQ6315E , an InGaAs detector can’t read the 350-700nm range and a detector switching mechanism is included after the monochromator.  When reading the low wavelength range, a Silicon (Si) detector is used instead of the InGaAs.  Such OSA is not used in telecom measurements as they are expensive, old and the visible range is of little value in such application.

Continue reading “Let’s demystify the OSA – Part 3”

Let’s demystify the OSA – Part 2

post-2-picThe signal input section:

There are two very different inputs found in OSAs; free space inputs and fibered inputs.  The most known free space (in air) input OSA is the Ando AQ6317B.  That OSA is likely the most spread OSA worldwide and was sold in mass volume during the late 1990 and early 2000 which were the boom years of CDWM and Fiber optics based telecom systems.  It is fit with an FC front adaptor and can accept either FCAPC or FCPC connector holding any kind of fiber so multimode or singlemode although the fiber used will create some uncertainty in the power readings.  The use of free space for the input section offers the great attribute of fiber independence but forces the topology of the monochromator to have the filtering section directly behind the input adaptor. The fibered input OSAs do not have this limitation.

Other OSAs such as the Agilent/HP 86140A and the rest of the Agilent OSA family or the smaller Anritsu MS9710C and previous Anritsu OSA generation have a fibered input which means that the front mating sleeve is directly tied to a fiber right behind it or that the connector in the front of the unit has glass in it and, when so, there is normally a collimator or grin lens right behind it.  For best results, the fiber used should be matched with the signal input section fiber.  Most of those OSAs were built with singlemode fiber and 62.5um Multimode fibered OSAs such as the Agilent 86141B are rare find on the used market.

So if you are buying an OSA for measurement of “yet to be defined” input signals, you might want to favor a free space OSA while if your measurement is aimed at clearly known signals, a fibered OSA matching the fiber you intend to use will do you just as well as free air.  Note that the price between the two input technologies is similar so the selection of one or the other will likely not be based on price.

Continue reading “Let’s demystify the OSA – Part 2”

Let’s demystify the OSA – Part 1

The Optical Spectrum Analyzer (OSA) is likely the most common piece of test equipment after the optical power meter that is fouAnritsu MS9710Cnd in telecom equipment test labs and production test stations.  The OSA is chosen mostly because of its extreme versatility in reading various optical signals both in power and in wavelength. There are two types of OSA; one is based on Michelson’s interferometry and most time referred to as a Multi Wavelength Meter.  We will not expand on that type as we assimilate it to a Wave Meter which will be discussed separately.  The other one is based on Diffraction Grating mirror and we’ll focus on that technology.

The OSA is the common name for a spectrometer used specifically in the telecom test field.  The main difference between a “plain” spectrometer is the size, its wavelength range and the numerous automated features that are found in an OSA and not programmed in a Spectrometer.  We mostly find spectrometer in laboratories dedicated to the study of optical components and behavior’s; not in telecom.  The common name for the light filtering section of an OSA is a monochromator which can be split in two parts “mono” for single and “chromator” for light so the part dedicated to filtering out, as much as possible, just the one wavelength from all the light coming through the input.The Optical Spectrum Analyzer (OSA) is likely the most common piece of test equipment after the optical power meter that is found in telecom equipment test labs and production test stations.  The OSA is chosen mostly because of its extreme versatility in reading various optical signals both in power and in wavelength.


Continue reading “Let’s demystify the OSA – Part 1”