Download Characterisation of the Tunable Laser Source

Optoelectronics 1: Devices for Optical Communications
Characterization of a Tunable Laser Source Using an Optical Spectrum
Analyzer
Objective:
To analyse the spectrum of a Multi-Wavelength Tunable External Cavity Laser Source using
Optical Spectrum Analyser. To measure the Spectral Width (FWHW) as a function of the radiation
wavelength.
Equipment:
 Multi-Wavelength Tunable Source
 Optical Spectrum Analyser
Background:
See attached notes
Method:
Connect a patchlead to the source and to the optical spectrum analyser.
Enable the ECL optical output.
Set the ECL optical output power to 0 dBm.
Set the ECL Emission Wavelength to 1520 nm.
Note the readings for the FWHM and Centre Wavelength values taken from the OSA monitor.
Increase the ECL Emission wavelength in 10-20 nm steps and tabulate the results for the FWHM
and Centre Wavelength values.
Results:
Record all the results listed in the paragraphs above. Comment on your results.
Background information
Optical spectrum analyser
Optical spectrum analysers (OSA) can divide a lightwave signal into its constituent wavelengths. This means that it is
possible to see the spectral profile of the signal over a certain wavelength range. The profile is graphically displayed,
with wavelength on the horizontal axis and power on the vertical axis. In this way, the many signals combined on a
single fibre in a dense wavelength division multiplexing (DWDM) system can be taken apart to perform per-channel
analysis of the optical signal and its spectral interaction with the other wavelengths
Applications for optical spectrum analyzers include testing devices lasers and LED light sources for spectral
purity and power distribution, and testing other optical devices for transmission quality. Optical spectrum analysers are
ideal instruments for laser modes analysis, very high-resolution spectroscopic measurements, telecommunication device
and system tests and other applications.
Basic block diagram.
A simplified optical spectrum analyzer block diagram is shown in figure 1. The incoming light passes through a
wavelength-tunable 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 1. Simplified optical spectrum analyzer block diagram.
Fabry-Perot interferometers.
The Fabry-Perot interferometer, shown in figure 2, consists of two highly reflective, parallel mirrors that act as a
resonant cavity, which 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 interferometer's free spectral range about the wavelength of interest.
Figure 2. Fabry-Perot-interferometer-based optical spectrum analyzer.
Multi-Wavelength Tunable External Cavity Laser Source
If the laser cavity is much longer than the wavelength, which is usually the case, more than one wavelength will be
emitted. Looking at the spectrum (amplitude versus wavelength) of lasers, each individual spectral laser line is referred
to as one mode. This type of laser is known as a Multiple Longitudinal Mode laser (MLM). An example of a MLM type
laser is the Fabry-Perot (FP) laser. When there are multiple longitudinal modes, the source has a greater spectral width.
Some applications require a laser to emit light only at a single narrow wavelength range (e.g. Dense
Wavelength Division Multiplexing (DWDM) application). One approach to limit laser oscillation to a single
longitudinal mode, is the distributed feedback laser (DFB). DFB lasers contain a diffraction grating that scatters light
back into the active region. Feedback from the grating causes interference effects that allow oscillation only at the
wavelengths at which the interference is constructive, reinforcing the generated light.
The optical layout of the multi-wavelength tunable laser source is shown in figure 3. Double-pass reflection on
the grating provides maximum dispersion, while the very short cavity, which maximizes mode spacing, yields a
spectrally pure single-mode operation.
Fig. 3 . ECL optical design.
Typical parameters measured for optical sources
3 dB width
The 3 dB width is determined by finding the peak of the LED spectrum, and dropping down 3 dB on each side. The
spectral width of the LED is determined by the separation of these two points because each has a power spectral density
equal to one half the peak power spectral density.
Center wavelength
Center wavelength is the average of two wavelengths determined in the 3 dB width measurements. Typically the values
of mean wavelength and center wavelength are similar.
Full Width Half Max (FWHM)
FWHM describes the spectral width of the half power points of the optical source. Half power points are where power
spectral density is one half of the peak amplitude. FWHM value and the 3 dB width values are typically very close to
one another.
FWHM measurement assumes a Gaussian shaped trace as follow: