Laser Frequency Response with Optisystems
Download as DOCX, PDF0 likes2,306 views
This report discusses the frequency response of a directly modulated laser. It describes how a carrier generator was used to create 298 channels with 25 MHz separations from 50 MHz as input to a laser diode. The laser's frequency response was then displayed using an RF spectrum analyzer. Nonlinearities in the laser were observed at higher amplitudes of the carrier generator input, changing the frequency response from a flat magnitude up to 2 GHz to one with kinks. Components of the simulation like the carrier generator, RFSA, laser and OSA are also outlined.
Laser Frequency Response with Optisystems
- 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