The tasks of the SICEP project have been:
(a) Development of a nonlinear interferometer, called f-2f interferometer.
(b) Development of a digital system device able to acquire CEP data at a repetition rate < 200 kHz.
(c) Building a prototype of an analog system able to acquire CEP data at high repetition rates > 200 kHz.
(d) Comparison of the two techniques and benchmarking.
We report progress in all of these tasks (see Fig. 1).
(a) f-2f interferometer
f-2f interferometers, consisting of a white-light generation stage to obtain an octave-spanning spectrum and second harmonic generation of the red side of the spectrum in order to obtain spectral interference with the blue side, is an integral part of many CEP detectors. It is well-known that intensity fluctuations impact the white-light generation step and result in an intensity-dependent phase shift. We have performed experiments that allow us to quantify the strength of the intensity-to-phase coupling.
We also successfully implemented an f-2f interferometer with a standard configuration, which is now an integral part of our final CEP tag digital product.
(b) CEP tag digital
We have developed a commercial product using a fast-line camera. It was tested to measure CEP at 100 kHz over more than 4 hours with no measurements lost. This device requires a high-performance computer to acquire the data.
We have also developed a low-cost prototype based on a fast linear sensor and field-programmable gate array (FPGA) programming for real-time Fourier analysis (milestone 2). The FPGA acquires a synchronization electric signal from the laser system, triggers the fast detector acquisition, reads out the data from the detector, and numerically calculates the Fourier transform. The resulting CEP and measurement time-stamp information is transferred to the computer using a standard Ethernet connection while simultaneously displaying video-rate CEP information on the laptop user interface. In contrast to the commercial product, this implementation only requires a standard laptop to visualize and save the data.
In both commercial product and low-cost prototype, we demonstrated single-shot CEP measurement at > 117 kHz pulse repetition rate, using the Pharos laser at Sphere Ultrafast Photonics in Porto. Both detectors were benchmarked against each other.
(c) CEP tag analog
In the CEP tag analog technique, the phase of f-2f spectra is found via optical Fourier transform. We filter the f-2f spectrum with two spectrally periodic, optical filters and detect each filtered signal with photodiodes. The CEP then is the inverse tangent (arctan) of the two signals. As a result, the CEP is obtained from sampling just two photodiode signals. Unlike other detection schemes, this can be done in principle at a repetition rate of many MHz. Our approach addresses a range of laser repetition rates that have become more common with the latest generation of industrial lasers.
A laboratory prototype was developed and the technique was experimentally demonstrated at the Lund Laser Centre at a laser repetition rate of 200 kHz and recently even at 586 kHz. To our knowledge, this is the fastest CEP measurement ever performed. An electronics implementation based on FPGA for real-time CEP detection was developed, where real-time refers to obtaining the result of the CEP measurement before the arrival of the next pulse, which is critical for feedback and stabilization applications. The spectrally periodic filters were implemented in a very compact way via a polarization-manipulated Michelson interferometer.
(d) Comparison of the two techniques and benchmarking
We compared both detectors in a measurement campaign at the Sphere facilities in Porto. Fig. 2 summarizes the most important results: The left shows a comparison between CEP measurements performed with the analog and digital detectors (commercial product) in parallel for a non-CEP-stabilized laser in single-shot at 117 kHz repetition rate. The agreement is remarkable and underlines the quality of our detectors.
