Periodic Reporting for period 1 - SICEP (Single-shot, high repetition rate detection of the Carrier-Envelope-Phase of ultrashort laser pulses)
Okres sprawozdawczy: 2023-08-01 do 2025-01-31
We have investigated a novel, all-optical method to measure the carrier-envelope-phase (CEP) of ultrashort laser pulses in single-shot for every laser shot at a high repetition rate. The CEP describes the offset between the oscillating electric field and the pulse envelope. In the so-called few-cycle regime, i.e. when there are only a few oscillations under the envelope, varying the CEP can generate distinct asymmetries in the waveform of the pulse. Such asymmetry can be utilized to, e.g. steer carriers in a semiconductor into a particular spatial direction, an effect that requires CEP measurement and control and is not present for longer pulses. Few-cycle pulses of only a few femtoseconds (1 fs = 10-15 s) duration are today available from an increasing number of lasers as technology rapidly advances, and have important applications in a number of fields, such as high precision spectroscopy, attosecond science, petahertz electronics, and time-domain spectroscopy. The project has been performed as a collaboration between the attosecond research group at Lund University and the deep-tech company Sphere Ultrafast Photonics, based in Porto, Portugal.
The method combines nonlinear interferometry, to encode the CEP in the form of spectral fringes, with an ultrafast detection of those fringes and thus of the CEP. Our technique avoids averaging and can instead determine the CEP in single shot for millions of individual pulses every second. We thus address a range of laser repetition rates that have become more common with the latest generation of industrial lasers.
The method combines nonlinear interferometry, to encode the CEP in the form of spectral fringes, with an ultrafast detection of those fringes and thus of the CEP. Our technique avoids averaging and can instead determine the CEP in single shot for millions of individual pulses every second. We thus address a range of laser repetition rates that have become more common with the latest generation of industrial lasers.
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.
(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.
The impact of our CEP detector is the possibility to measure the CEP for every pulse, even at the full high repetition rate of the laser, where conventional techniques today can only average over many pulses. This opens up novel ways to stabilize the CEP of a laser via feedback control or to exploit CEP effects with lasers that cannot be stabilized, simply by recording the CEP of every laser shot, i.e. CEP tagging. Measuring, controlling, and exploiting the CEP is an emerging technique that is today established in ultrafast laser science, with high potential for industrial and medical applications.
A patent application has been submitted to both the US and the European patent offices and an article is now published.
We have successfully sold three digital systems—two in Germany and one in China. Additionally, we have been receiving inquiries from other universities in the US and Europe. We are actively promoting the CEP tag system (commercial product) on our website, and we hope to reach more clients in the coming months.
Below, we summarize the results achieved during this POC project.
1. Successful correlation measurements between the two different systems (analog and digital)
2. Direct CEP stabilization of an ultrafast titanium-sapphire-front-end OPCPA system by a single feedback loop using the analog detector
3. Successful implementation of a low-cost prototype digital detector, based on a linear sensor and FPGA for recording and analyzing
4. Successful implementation of a Digital commercial product (Milestone 1)
5. Successful implementation of the nonlinear interferometer, included in the digital CEP tag commercial product) (Milestone 2)
6. Successful implementation of an analog prototype working at 586 kHz (Milestone 3)
7. Market report for CEP tag systems (Milestone 4)
A patent application has been submitted to both the US and the European patent offices and an article is now published.
We have successfully sold three digital systems—two in Germany and one in China. Additionally, we have been receiving inquiries from other universities in the US and Europe. We are actively promoting the CEP tag system (commercial product) on our website, and we hope to reach more clients in the coming months.
Below, we summarize the results achieved during this POC project.
1. Successful correlation measurements between the two different systems (analog and digital)
2. Direct CEP stabilization of an ultrafast titanium-sapphire-front-end OPCPA system by a single feedback loop using the analog detector
3. Successful implementation of a low-cost prototype digital detector, based on a linear sensor and FPGA for recording and analyzing
4. Successful implementation of a Digital commercial product (Milestone 1)
5. Successful implementation of the nonlinear interferometer, included in the digital CEP tag commercial product) (Milestone 2)
6. Successful implementation of an analog prototype working at 586 kHz (Milestone 3)
7. Market report for CEP tag systems (Milestone 4)