The overall goal of this procedure is to store and delay information encoded in optical data packets by exploiting time frequency coherence. This is accomplished by first injecting an optical data packet into the Quantum Light Storage QLS system for a single packet. The spectral representation is continuous, which means that the difference between two adjacent frequencies of the spectrum is zero.
The second step is to multiply the spectrum with a frequency comb inside the QLS system by stimulated Bria wall scattering. This multiplication in the frequency domain corresponds to an extraction of single EQU distant frequencies out of the packet spectrum. The multiplication in the frequency domain equals a convolution with the pulse in the time domain.
This result in a train of copies of the original signal. The final step is to extract one of these copies by a rectangular read signal. The delayed data signal appears at the output of the QLS system.
Ultimately, an oscilloscope is used to show and measure the delay of the optical data signals. The main advantage of this method compared to other techniques like slow light, where just one bit can be stored is that with quasi light storage, several thousand bits can be delayed and stored. Generally, individuals which are new to this method will struggle because time frequency coherence is used, which is not common knowledge to everyone.
VDIs method can provide an insight into the storage of amplitude modulated signals. It can also be applied to the storage of higher order modulation formats, which use a combination of phase and amplitude modulation. The experiment takes place on an optical bench with auxiliary equipment mounted on racks.
The bench setup is shown in this diagram. Light in the experiment follows two main paths. Connect the modulator with the fiber and the other end of the fiber with port two of the circulator, connect the modulator to port two of the circulator.
The second path is for frequency comb generation. Again, mount a laser diode, connect it to a phase modulator with a polarization controller. From there, have a fiber.
Go to an optical amplifier, take its output to port one of the circulator, connect each of the laser diodes to temperature and current controllers. Also input the signal from a waveform generator that is passed through an electrical amplifier into each modulator. To detect phase modulated signals, add additional components beyond the circulator.
Connect the output of the circulator to a 50 50 coupler. Then connect a local oscillator to the coupler. After this, connect a third modulator for extracting delayed copies to the output of the 50 50 coupler.
Next, connect a 90 10 coupler to the modulator output. To complete the setup, apply a bias voltage to the modulator and synchronize it with a rectangular signal. From the output port of the waveform generator, attach an oscilloscope to the 90%port of the coupler and an optical spectrum analyzer to the 10%port program.
The waveform generator for the data packet, the frequency comb, and the retrieval signal. With the system prepared and the diode lasers operating. Begin measurements by turning on the output for the data signal.
At the waveform generator, adjust the bias on the modulator at the power supply to achieve a good quality signal on the oscilloscope. Turn off the waveform generator. Next, employ heterodyne detection to adjust the quality of the frequency comb, disconnect the output of the comb modulator from the optical amplifier, and input it into a 50 50 coupler.
Connect a fiber laser as a local oscillator to the other input, and set the difference between the signal and the oscillator to around eight gigahertz. Once this is done, connect the output of the coupler to a photo diode and an electrical spectrum analyzer. Return to the comb modulator to adjust the applied bias.
Change the bias until a flat frequency comb is achieved. When there is a good quality comb, reconnect the output of the comb modulator to the optical amplifier fire. Ensure that the waveform generator is off and adjust the frequency difference between the two laser diodes to the Bria wall shift.
Turn on the optical amplifier and use the optical spectrum analyzer to set its power to value below the threshold of stimulated bria wall scattering. Now shift the wavelength of the data laser DDE into the gain region of the co modulator. Check that the signal is amplified.
Adjust the polarization on the data modulator to maximize the intensity of the data signal. Turn on both the data and comb outputs of the waveform generator and increase the power output of the optical amplifier. The oscilloscope should the different copies generated by the quasi light storage system.
Extract a copy by using one of the marker signals of the waveform generator to set up a rectangular pulse with the length of the packet. Turn on the bias for the extraction modulator and change it until the extracted signal is maximized and all other copies are suppressed. Shift the rectangular pulse to the desired version of the stored pattern.
The stored pattern can be saved with the oscilloscope. Shown here in black is the original phase modulated signal with a data rate of one gigabit per second. The colored lines represent the extracted copies at different storage times using the stimulated brios, scattering based quasi light storage.
The storage versions of the signal are almost distortion free. The quality and number of copies depends on the pump power, the flatness of the comb and the polarization in this case due to limitations of the equipment, the maximum storage time was 60 nanoseconds Following this procedure. Other modulation formats like ture amplitude modulation, or face shift can be stored and delayed as well.
After watching this video, you should have a good understanding of how the Quai Light Storage method works and how it can be done in your lab as well.
