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Optical Spectrum Analysis
Application Note 1550-4
Optical Spectrum
Analysis Basics
Table of Contents Introduction
Page
3
Chapter 1
Types of optical spectrum analyzers 4
Interferometer-Based Optical Spectrum Analyzers 5
Diffraction-Grating-Based Optical Spectrum Analyzers 6
Chapter 2
Diffraction-grating-based optical spectrum analyzers 12
Wavelength Tuning and Repeatability 12
Wavelength Resolution Bandwidth 12
Dynamic Range 13
Sensitivity 14
Tuning Speed 15
Polarization Insensitivity 17
Input Coupling 19
Appendix
Optical and microwave spectrum analyzers compared 20
2
Introduction This application note is intended to provide the reader with a basic
understanding of optical spectrum analyzers, their technologies,
specifications, and applications. Chapter 1 describes interfero-
meter-based and diffraction-grating-based optical spectrum analyzers.
Chapter 2 defines many of the specified performance parameters of
diffraction-g rating-based optical spectrum analyzers and discusses the
relative merits of the single monochromator, double monochromator,
and double-pass-monochromator-based optical spectrum analyzers.
For readers familiar with electrical spectrum analyzers, some of the
same terms are used, but with different definitions.
Optical spectrum analysis
Optical spectrum analysis is the measurement of optical power as
a function of wavelength. Applications include testing laser and LED
light sources for spectral purity and power distribution, as well as
testing transmission characteristics of optical devices.
The spectral width of a light source is an important parameter in
fiber-optic communication systems due to chromatic dispersion,
which occurs in the fiber and limits the 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 systems.
Figure 1 shows the spectrum of a Fabry-Perot laser. The laser is not
purely monochromatic; it consists of a series of evenly spaced
coherent spectral lines with an amplitude profile determined by the
characteristics of the gain media.
Optical spectrum analyzers can be divided into three categories:
diffraction-grating-based and two interferometer-based architectures,
the Fabry-Perot and Michelson interferometer-based optical spectrum
analyzers. Diffraction-grating-based optical spectrum analyzers are
capable of measuring spectra of lasers and LEDs. The resolution of
these instruments is variable, typically ranging from 0.1 nm to 5 or
10 nm. Fabry-Perot-interferometer-based optical spectrum analyzers
have a fixed, narrow resolution, typically specified in frequency,
between 100 MHz and 10 GHz. This narrow resolution allows them to
be used for measuring laser chirp, but can limit their measurement
Figure 1. Optical spans much more than the diffraction-grating-based optical spectrum
spectrum analyzer analyzers. Michelson interferometer-based optical spectrum analyzers,
measurement of a used for direct coherence-length measurements, display the spectrum
Fabry-Perot laser.
by calculating the Fourier transform of a measured interference
pattern.
3
Basic block diagram
Chapter I A simplified optical spectrum analyzer block diagram is shown in
Types of optical figure 2. The incoming light passes through a wavelength-tunable
spectrum analyzers optical filter (monochromator or interferometer) which resolves the
individual spectral components. The photodetector then converts
the optical signal to an electrical current proportional to the incident
optical power. An exception to this description is the Michelson
interferometer, which is not actually an optical filter.
The current from the photodetector is converted to a voltage by the
transimpedance amplifier and then digitized. Any remaining signal
processing, such as applying correction factors, is performed digitally.
The signal is then applied to the display as the vertical, or amplitude,
data. A ramp generator determines the horizontal location of the trace
as it sweeps from left to right. The ramp also tunes the optical filter so
that its resonant wavelength is proportional to the horizontal position.
A trace of optical power versus wavelength results. The displayed
width of each mode of the laser is a function of the spectral resolution
of the wavelength-tunable optical filter.
Figure 2.
Simplified optical
spectrum analyzer
block diagram.
4
Interferometer-based Fabry-Perot interferometers
The Fabry-Perot interferometer, shown in figure 3, consists of two
optical spectrum highly reflective, parallel mirrors that act as a resonant cavity which
analyzers filters the incoming light. The resolution of Fabry-Perot interferometer-
based optical spectrum analyzers, dependent on the reflection
coefficient of the mirrors and their spacing, is typically fixed, and the
wavelength is varied by changing the spacing between the mirrors by
a very small amount.
The advantage of the Fabry-Perot interferometer is its very narrow
spectral resolution, which allows it to measure laser chirp. The
major disadvantage is that at any one position multiple wavelengths
will be passed by the filter. (The spacing between these responses is
called the free spectral range.) This problem can be solved by placing
a monochromator in cascade with the Fabry-Perot interferometer to
filter out all power outside the interfer-ometer's free spectral range
about the wavelength of interest.
Figure 3. Fabry-Perot-interferometer-based optical spectrum analyzer.
Michelson interferometers
The Michelson interferometer, shown in figure 4, is based on creating
an interference pattern between the signal and a delayed version of
itself. The power of this interference pattern is measured for a range
of delay values. The resulting waveform is the autocorrelation function
of the input signal. This enables the Michelson interferometer-based
spectrum analyzer to make direct measurements of coherence length,
as well as very accurate wavelength measurements. Other types of
optical spectrum analyzers cannot make direct coherence-length
measurements.
To determine the power spectra of the input signal, a Fourier transform
is performed on the autocorrelation waveform. Because no real
filtering occurs, Michelson interferometer-based optical spectrum
analyzers cannot be put in a span of zero nanometers, which would
be useful for viewing the power at a given wavelength as a function
of time. This type of analyzer also tends to have less dynamic range
than diffraction-grating-based optical spectrum analyzers.
5
Figure 4.
Michelson-interferometer-based
optical spectrum analyzer.
Diffraction-grating-based The most common optical spectrum analyzers use monochromators
as the tunable optical filter. In the monochromator, a diffraction
optical spectrum grating (a mirror with finely spaced corrugated lines on the surface)
analyzers separates the different wavelengths of light. The result is similar to
that achieved with a prism. Figure 5 shows what a prism-based optical
spectrum analyzer might look like. The prism separates the different
wavelengths of light, and only the wavelength that passes through the
aperture reaches the photodetector. The angle of the prism determines
the wavelength to which the optical spectrum analyzer is tuned, and
the size of the aperture determines the wavelength resolution.
Figure 5.
Concept of prism-based optical spectrum analyzer.
Diffraction gratings are used instead of prisms because
diffraction gratings provide greater separation among
wavelengths of light.
Diffraction gratings are used instead of prisms because they provide a
greater separation of wavelengths, with less attenuation. This allows for
better wavelength resolution.
A diffraction grating is a mirror with grooves on its surface, as shown
in figure 6. The spacing between grooves is extremely narrow,
approximately equal to the wavelengths of interest. When a parallel
light beam strikes the diffraction grating, the light is reflected in a
number of directions.
6
The first reflection is called the zero-order beam (m=O), and it reflects
in the same direction as it would if the diffraction grating were replaced
by a plane mirror. This beam is not separated into different wavelengths
and is not used by the optical spectrum analyzer.
The first-order beam (m=l) is created by the constructive interference
of reflections off each groove. For constructive interference to occur,
the path-length difference between reflections from adjacent grooves,
must equal one wavelength. If the input light contains more than one
wavelength component, the beam will have some angular dispersion;
that is, the reflection angle for each wavelength must be different in
order to satisfy the requirement that the path-length difference off
adjacent grooves is equal to one wavelength. Thus, the optical spectrum
analyzer separates different wavelengths of light.
Figure 6.
The diffraction grating separates the
input beam into a number of output
beams. Within each output beam,
except the zero order beam, different
wavelengths are separated.
For the second-order beam (m=2), the path-length difference from
adjacent grooves equals two wavelengths. A three wavelength
difference defines the third-order beam, and so on.
Optical spectrum analyzers utilize multiple-order beams to cover their
full wavelength range with narrow resolution.
Figure 7 shows the operation of a diffraction-grating-based optical
spectrum analyzer. As with the prism-based analyzer, the diffracted light
passes through an aperture to the photodetector. As the diffraction
grating rotates, the instrument sweeps a range of wavelengths, allowing
the diffracted light -- the particular wavelength depends on the position
of the diffraction grating -- to pass through to the aperture. This
technique allows the coverage of a wide wavelength range.
7
Figure 7.
Diffraction-grating-based
optical spectrum analyzer.
Single Monochromator
Diffraction-grating-based optical spectrum analyzers contain either a
single monochromator, a double monochromator, or a double-pass
monochromator. Figure 8 shows a single monochromator-based
instrument. In these instruments, a diffraction grating is used to
separate the different wavelengths of light. The second concave mirror
focuses the desired wavelength of light at the aperture. The aperture
width is variable and is used to determine the wavelength resolution
of the instrument.
Figure 8.
Single-monochromator-based
optical spectrum analyzer.
8
Double Monochromator
Double monochromators, such as shown in figure 9, are sometimes
used to improve on the dynamic range of single monochromator
systems. Double monochromators are equivalent to a pair of sweeping
filters. While this technique improves dynamic range, double
monochromators typically have reduced span widths due to the
limitations of monochromator-to-monochromator tuning match;
double monochromators also have degraded sensitivity due to losses
in the monochromators.
Figure 9.
Double-monochromator-based
optical spectrum analyzer.
9
Double-Pass Monochromator
Agilent 71450B/1B/2B optical spectrum analyzers use a unique
wavelength-selection scheme