Laser Frequency Response with Optisystems for a self-modulating laser...

1. National Institute of Technology Srinagar
Report On
Lab Project
Laser Frequency Response
Submitted By-:
Amber Deep Singh
En Roll – 22/10
Lab- Optical Fiber Comm.

2. Contents
1. Introduction
2.Laser
3.Laser Characteristics
4.Circuit Diagram
5.Components
6.Frequency response of our directly modulated
laser
7.Conclusion
8.References

3. Introduction
Frequency response is the quantitative measure of the
output spectrum of a system or device in response to a stimulus, and is
used to characterize the dynamics of the system. It is a measure of
magnitude and phase of the output as a function of frequency, in
comparison to the input. In simplest terms, if a sine wave is injected into
a system at a given frequency, a linear system will respond at that same
frequency with a certain magnitude and a certain phase angle relative to
the input. Also for a linear system, doubling the amplitude of the input
will double the amplitude of the output. In addition, if the system is
time-invariant, then the frequency response also will not vary with time.
Two applications of frequency response analysis are related but have
different objectives. For an audio system, the objective may be to
reproduce the input signal with no distortion. That would require a
uniform (flat) magnitude of response up to the bandwidth limitation of
the system, with the signal delayed by precisely the same amount of time
at all frequencies. That amount of time could be seconds, or weeks or
months in the case of recorded media. In contrast, for a feedback
apparatus used to control a dynamic system, the objective is to give the
closed-loop system improved response as compared to the
uncompensated system. The feedback generally needs to respond to
system dynamics within a very small number of cycles of oscillation
(usually less than one full cycle), and with a definite phase angle relative
to the commanded control input. For feedback of sufficient
amplification, getting the phase angle wrong can lead to instability for
an open-loop stable system, or failure to stabilize a system that is open-
loop unstable. Digital filters may be used for both audio systems and
feedback control systems, but since the objectives are different,
generally the phase characteristics of the filters will be significantly
different for the two applications.

4. LASER (Light Amplification by Stimulated Emission of
Radiation)
A laser is a device that emits light through a process of optical
amplification based on the stimulated emission of electromagnetic
radiation. The term "laser" originated as an acronym for "light
amplification by stimulated emission of radiation". A laser differs from
other sources of light because it emits light coherently. Spatial
coherence allows a laser to be focused to a tight spot, enabling
applications like laser cutting and lithography. Spatial coherence also
allows a laser beam to stay narrow over long distances (collimation),
enabling applications such as laser pointers. Lasers can also have high
temporal coherence which allows them to have a very narrow spectrum,
i.e., they only emit a single color of light. Temporal coherence can be
used to produce pulses of light—as short as a femtosecond.
Laser Characteristics
1) Size and configuration is compatible with launching light into an
optical fiber. Ideally, the light output should be highly directional.
2) It accurately tracks the electrical input signal to minimize
distortion and noise. Ideally, the source should be linear.
3) It emits light at wavelengths where the fiber has low losses and
low dispersion and where the detectors are efficient.
4) It is capable of simple signal modulation over a wide bandwidth
extending from audio frequencies to beyond the gigahertz range.
5) It couples sufficient optical power to overcome attenuation in the
fiber plus additional connector losses and leave adequate power to
drive the detector.
6) It has a very narrow spectral bandwidth (linewidth) in order to
minimize dispersion in the fiber.

5. 7) It is capable of maintaining a stable optical output which is largely
unaffected by changes in ambient conditions (e.g. temperature).
8) It is essential that the source is comparatively cheap and highly
reliable in order to compete with conventional transmission
techniques.
Circuit Diagram
As shown we use a carrier generator, which creates 298 channels with 25
MHz frequency separations, starting at 50 MHz. This initial signal can
be seen in the figure below:

6. 298 channels with 25 MHz frequency separations
This signal is applied to the laser diode. The RF spectrum analyzer is
used after the PIN in order to display the laser frequency response. Noise
and phase noise of laser rate equations, and noise sources in our PIN
were disabled.
In this project, three different values of the amplitude of the carrier
generator were used: 0.001, 0.01, and 0.8.
For only the first value, we drive the laser without generating laser
nonlinearities. This is the way to obtain the correct laser frequency
response.

7. For a laser driver without nonlinearities (iteration 1, the amplitude of the
carrier generator = 0.001), the observed frequency response of our
directly modulated laser is shown in the next figure.
Components
1.Carrier Generator
This component generates a user-defined number of carriers. The output
is a sum of sinusoidal electrical signals with constant amplitude. The
phase can be constant or random. This component generates a sum of
sinusoidal carriers with the same zero peak amplitude. The phase can be
defined as random, or user-defined. The user-defined phase is the same
for all the carriers. The user can remove carriers from the sum by
selecting Disable channels parameter and providing the list of channels
or carriers.
2. RF Spectrum Analyzer (RFSA)
This visualizer allows the user to calculate and display electrical signals
in the frequency domain. It can display the signal intensity, power
spectral density and phase.
3. Oscilloscope Visualizer
This visualizer allows the user to calculate and display electrical signals
in the time domain. It can display the signal amplitude and
autocorrelation.
4.CW Laser

8. Utilizes the rate equations to simulate the modulation dynamics of
a laser.
5.Optical Spectrum Analyzer(OSA)
This visualizer allows the user to calculate and display optical
signals in the frequency domain. It can display the signal intensity,
power spectral density, phase, group delay and dispersion for
polarizations X and Y.
Input of device is optical. Main parameters are resolution
bandwidth, filter type an bandwidth.

9. Frequency response of our directly modulated laser
As can be seen, for the default values of the physical parameters of our
laser rate equation model, the relaxation frequency is approximately 2
GHz.
By increasing the values of the amplitude of the carrier generator, the
nonlinearities of the laser are triggered. As a result, the observed
frequency response changes dramatically. The obtained results for the
displayed output for the next two iterations can be seen in the next two
figures.

11. Conclusion:
As we are increasing the amplitude of the carrier from 0.01-0.8 units, it is
seen that non-linearites kick in and cause the formation of kinks in the
gain curve. As a result, the observed frequency response changes
dramatically. Also, at higher amplitudes there is spreading in the spectrum
and our source is no longer a monochromatic one.

12. References:
 Optical Fiber Communications Principles and Practice
by John M. Senior
 www.optiwave.com (Optiwave PDFs)
 Optical fiber communication by Gerd Keiser
 www.wikipedia.org