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Although various aspects have already been mentioned above, the most important performance figures are briefly described in the following:
Wavelength range
Every spectrum analyzer is limited to a certain range of optical wavelengths. In some cases, one cannot access the full wavelength range in a single spectrum, since a diffraction grating or the photodetector needs to be replaced, for example.
Wavelength resolution and filter shape
The wavelength resolution can be a fixed value for simple spectrum analyzers, while for other instruments it can be modified. Universal optical spectrum analyzers (normally based on gratings) often offer wavelength resolutions e.G. Between 0.1 nm and 5 nm. High-performance devices even reach the order of 0.01 nm (= 10 pm) resolution. Measurements with low wavelength resolution can be sensible for increased acquisition speed, for example. Not only the wavelength resolution in terms of a full width at half maximum is important, but also the filter shape – in particular, how steeply and how far the filter function drops with increasing wavelength offset. That may also influence the dynamic range.Specialized high-resolution spectrum analyzers, for example those based on fabry–perot interferometers, may offer much higher wavelength resolution, but only in a very restricted wavelength range because of their small free spectral range.
Wavelength accuracy
Wavelength resolution should not be confused with wavelength accuracy. For example, an instrument which has long been used in a rough environment may have lost its wavelength calibration, so that wavelength determinations are consistently wrong by some amount. Some instruments can be recalibrated even by the user, if suitable optical frequency standards are available.
Dynamic range and sensitivity
Instruments have a limited dynamic range, i.E., a limited range of optical powers or power spectral densities. That limit is not necessarily set by the used photodetector. It can also arise from the problem that an intense wavelength component can influence the readings at other wavelengths, e.G. By scattering of light within a grating monochromator or via noise issues in an interferometric spectrum analyzer. It may simply indicate the minimum amount of optical input power which can be reasonably well detected (e.G. A couple of decibels above the noise level). It may indicate how much weaker a detectable weak signal may be compared with another strong signal. That quantity may also depend on the wavelength separation between the two signals.
There is also a maximum optical input power which the device can tolerate without being damaged. Some spectrum analyzers have an optional input attenuator, which one can use for operation at higher power levels.
Power accuracy
As mentioned above, optical spectrum analyzers are often not particularly accurate for measurements of optical power. Some instruments, however, can be expected to have a reasonable calibration – in particular, fiber-coupled devices for use with a single-mode fibers. For example, in an optical fiber communications network one may need to accurately determine channel powers.
Acquisition speed
Particularly for scanning instruments, acquisition speed can be an important factor. It often substantially depends on the chosen device settings concerning wavelength range, wavelength resolution and sensitivity. For a fair comparison between different devices, such settings need to be considered.
Wavelength range
It is the maximum wavelength range the optical analyzer can cover while guaranteeing the specified performance. Although a large wavelength range is desired, practical limitation comes from the applicable wavelength window of optical filters, photodetectors, and other optical devices. The typical wavelength range covered by a commercially available grating-based optical analyzer is from 400 to 1700 nm.
Wavelength accuracy
Specifies how accurately the optical analyzer measures the wavelength. Most commercial optical analyzers separately specify absolute wavelength accuracy and relative wavelength accuracy. Absolute wavelength accuracy specifies how accurate the measured absolute wavelength value is, which is often affected by the wavelength calibration. Relative wavelength accuracy tells how accurate the measured wavelength separation between two optical signals is, which is mainly determined by the nonlinearity of the optical filters. In typical optical analyzers, wavelength accuracy of less than 0.1 nm can be achieved.
Resolution bandwidth
It defines how fine an optical analyzer slices the signal optical spectrum during the measurement. As an optical analyzer measures signal optical spectral density, which is the total optical power within a specified bandwidth, a smaller-resolution bandwidth means a more detailed characterization of the optical signal. However, the minimum resolution bandwidth of an optical analyzer is usually limited by the narrowest bandwidth of the optical system the optical analyzer can provide and limited by the lowest detectable optical power of the receiver. The finest optical resolution bandwidth of a grating-based commercial optical analyzer ranges from 0.1 to 0.01 nm.
Sensitivity
Specifies the minimum measurable signal optical power before it reaches the background noise floor. Therefore, the detection sensitivity is basically determined by the noise characteristic of the photodiode used inside the optical analyzer. For a short-wavelength optical analyzer, the detection sensitivity is generally better due to the use of silicon photodiode, which covers the wavelength from 400 to 1000 nm. For the wavelength from 1000 to 1700 nm, an InGaAs photodiode has to be used, and the noise level is generally high, and the detection sensitivity is therefore poor compared to the short-wavelength optical analyzers. Commercially available optical analyzers can provide a detection sensitivity of − 120 dBm in the 400–1700 nm wavelength range.
Maximum power
The maximum allowable signal optical power before the optical analyzer detection system is saturated. A typical optical analyzer can tolerate 20 dBm signal optical power or higher.
Calibration accuracy
Specifies how accurate the absolute optical power reading is in the measurement. Typically, a calibration accuracy of less than 0.5 dB can be achieved in a commercial optical analyzer.
Amplitude stability
Specifies the maximum allowable fluctuation of the power reading over time, when the actual input signal optical power is constant. Typical amplitude stability of a commercial optical analyzer is less than 0.01 dB per minute.
Dynamic range
The maximum distinguishable amplitude difference between two optical signals with their wavelengths a certain number of nanometers apart. This becomes a concern because in practical optical analyzers if a weak optical signal is in close vicinity to a strong optical signal, the weak signal may become unmeasurable because the receiver is overwhelmed by the strong signal; obviously, this effect depends on the wavelength separation between these two optical signals. The typical dynamic range of a commercial optical analyzer is about 60 dB for a 0.8-nm wavelength interval and 52 dB for a 0.2 nm wavelength interval.
Frequency sweep rate
Specifies the speed of an optical analyzer sweeping over the wavelength during the measurement. It depends on the measurement wavelength span and the resolution bandwidth used. Generally, the number of measurement samples an optical analyzer takes in each sweep is equal to the span width divided by the resolution bandwidth. In practical applications, choosing sweep speed also depends on the power levels of the optical signal. At low power levels, the average may have to be made in the detection, and that may slow down the sweep speed.
Polarization dependence
Specifies the maximum allowable fluctuations of the power reading while changing the state of polarization of the optical signal. Polarization dependence of an optical analyzer is usually caused by the polarization-dependent transmission of the optical system, such as gratings and optical filters used in the optical analyzer. Commercial optical analyzers usually have less than 0.1 dB polarization dependence.
To apply the spectrum analysis principles originally developed for radio to light waves, it is important to consider the behavior of light as a closely related form of electromagnetic radiation. An Optical Spectrum Analyzer must be able to select a specific wavelength for measurement, then a photodetector can be used to convert light energy into measurable electrical energy (power). There are different technologies that can be used to select individual wavelengths for measurement.
To determine the power for an individual wavelength, the diffraction grating method uses a rotating filter or “grating” inside the optical spectrum analyzer. As the grating in the monochromator moves different wavelengths are presented to the photodetector sequentially as the grating rotates. This method is known for accommodating a wide spectral range and producing accurate readings. The sensitive moving parts in this type of device make it important to protect the optical analyzer from drops or other types of shock.
The interferometer method filters incoming light using parallel mirrors to create a resonant cavity. To set the input wavelength, the spacing between the mirrors is tuned using piezo elements. The resolution of the Fabry-Perot optical spectrum analyzer is determined by the precision of this mirror spacing function. This method provides high wavelength measurement accuracy, although the dynamic range is limited. With no moving parts and the ability to detect closely spaced channels, Fabry-Perot lends itself well to monitoring and measurement applications in the lab or field.
The important selection figure of an optical spectrum analyzers is its operating frequency, wavelength range and resolution. Even though there are many performance figures that need to consider in the selection of optical analyzer, a few of important are briefly described below.
Operating frequency (ghz)
The operating frequency of the optical spectrum analyzer is its fundamental frequency range over which it will functions. Need to select as per the test requirement.
Wavelength range
Optical spectrum analyzers are always designed to a certain optical wavelengths range due to the wavelength limitation of a diffraction grating or the photodetector. For different wavelengths, a photo-detector needs to be replaced, to match the requirement. Optical analyzer need to select as per the required signal wavelength.
Wavelength resolution and filter shape
The wavelength resolution of most of the optical spectrum analyzers will be a fixed value in the range of about 0.1 nm and 5 nm. For very high-performance optical analyzer can reach in the order of 0.01 nm (= 10 pm) resolution. Higher the resolution better will be measurements. Inbuilt optical filters help to tune the optical signal for better measurements in their spectral range.
Dynamic range and sensitivity
The dynamic range of the optical spectrum analyzer is based on the internal photodetector sensitivity. The sensitivity of the analyzer indicates the minimum amount of power level required for the optical input signal to detect by the photo-detector. In simple terms, it indicates how much weaker signal optical analyzer can measure above the noise level. As the photodetector also has a maximum optical input level it can tolerate without failure, there is an upper limit to the optical analyzer. The dynamic range of the optical spectrum analyzer indicates the lower and the upper measurement limit of the instrument.
Wavelength accuracy of optical spectrum analyzer
Wavelength accuracy is the measurement accuracy of the optical spectrum analyzer. It is also based on the calibration of the instrument. After a long use in a rough environment, the wavelength calibration may be lost and it may lead to consistently wrong measurements. Higher the accuracy better for the measurements and increase the reliability.
Power accuracy of optical spectrum analyzer
Power accuracy of an optical spectrum analyzers indicate the accuracy in the measurement of the input signal power level, it is also based on the accuracy of the instrument calibration. For an optical fibre communications network, it is ideal to use high power accuracy optical spectrum analyzer or a dedicated optical power meter for the measurements to determine channel powers.
Acquisition speed
The acquisition speed of an optical spectrum analyzer is substantially based on other features like wavelength range, resolution, dynamic range and sensitivity. Higher the acquisition and measurement speed better for the applications.
An optical analyzer is a precision and high accurate instrument for measuring and displaying the distributed power levels of an optical signal from a source in a specified wavelength span for r&d and manufacturing applications. The application of the optical spectrum analyzer includes consumer electronics, telecommunications, healthcare, r&d in science and medical industry, security, environmental monitoring etc.
Optical Input of Optical Analyzer
The input light may be injected into different ways, depending on the source to be analyzed: Many optical spectrum analyzers have a fiber-optic input, usually with some kind of fiber connector (e.g. PC/PC, SC or ST) for attaching a fiber patch cable. Depending on the device, one may use cables with single-mode or multimode fibers, sometimes even with fiber bundles.
In other cases, there is a free-space optical input. One typically needs to focus the input light to an optical slit, with the beam direction roughly perpendicular to the slit surface. Fiber inputs are often the more convenient solution. The spectrum analyzer may then not occupy valuable space on an optical table; it may be placed in some shelf or in a mobile rack. This type of instrument is particularly appropriate in application areas where the light is already sent through fibers. But, there are also cases where one can conveniently collect light with a fiber bundle, e.g. with a large circular input aperture, where the output fibers in the analyzer may be rearranged e.g. along a line which fits to the input slit of the monochromator.
Even for free-space optical setups, it may be convenient to launch the light into a suitable fiber in order to transport it to the spectrum analyzer. Problems may arise when the coupling efficiency cannot be considered to be constant over the relevant spectral range because that can obviously lead to distortions of the recorded optical spectra. Note, however, that the same kind of problem can also occur to some extent with free-space beams.
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