# US 2019/0170588 A1 · Ultrashort laser pulse characterization and compression method

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- Number: US 2019/0170588 A1
- Document ID: US20190170588A1
- Status: Published application
- Kind: A1
- Application number: 16/324,713
- Filed: 2017-08-09
- Published: 2019-06-06
- Earliest claimed: 2016-08-10
- Assignees: Sphere Ultrafast Photonics, S.A.
- Applicants: Sphere Ultrafast Photonics, S.A.
- Inventors: Francisco Jose MAIA DA SILVA; Helder Manuel PAIVA REBELO CEREJO CRESPO; Paulo Tiago FERRAZ DE MEIRA GUERREIRO; Rosa María ROMERO MUÑIZ
- Web page: https://patentgrep.com/patent/US20190170588A1

## CPC

- G01J 11/00: Measuring the characteristics of individual optical pulses or of optical pulse trains
- H01S 3/0014: {Monitoring arrangements not otherwise provided for (photometry G01J1/00, e.g. G01J1/4257; radiation pyrometry G01J5/00; measuring coherence of light G01J9/00; measuring wavelength of light G01J9/00, e.g. G01J9/0246; measuring optical pulses G01J11/00; calorimetrically measuring power of laser beams G01K17/003)}
- H01S 3/0057: {Temporal shaping, e.g. pulse compression, frequency chirping (soliton generation and propagation G02F1/3513, H01S3/063 and H01S3/108)}
- H01S 3/0092: {Nonlinear frequency conversion, e.g. second harmonic generation [SHG] or sum- or difference-frequency generation outside the laser cavity (nonlinear frequency conversion per se G02F1/35)}
- G01J: MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY (light sources F21, H01J, H01K, H05B; investigating properties of materials by optical means G01N)
- H01S: DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL

## Abstract

The present disclosure generally relates to laser systems and laser pulse characterization methods and respective systems. An embodiment of the method comprises generating two collinear replicas of the pulse being characterized; applying two different predetermined spectral phases to each replica, so as to simultaneously scan delay and dispersion; applying a nonlinear process to the pulse to be characterized; measuring the resulting signal from the application of the predetermined spectral phases and nonlinear process; applying a numerical iterative algorithm to the measured signal to retrieve the spectral phase of the pulse to be characterized; such process being done as a scanning procedure or in parallel utilizing a single laser shot. The two time-delayed replicas may be generated using a birefringent etalon; a controllable amount of dispersion and delay between replicas may be provided by a pair of birefringent wedges, one of them being translated for phase control; dispersion and delay between replicas can also be imparted in parallel by using a single birefringent wedge, which encodes these along a spatial dimension; opposite dispersion to that of the wedges may be introduced by an appropriate optical element, such as chirped mirrors or a dispersive geometric arrangement; the frequency spectrum may be measured as well as the dispersion and delay dependent second-harmonic of the signal being phase modulated.

## Claims

### Claim 1

A method for characterizing the spectral phase of laser pulses, the method comprising:
generating two replicas of each pulse to be characterized;
applying predetermined nonlinear spectral phases to the two replicas so as to scan a dispersion range by one or both said replicas;
optically combining the two signals of the preceding step;
applying a nonlinear optical process to the combined signal of the preceding step;
for each of the applied spectral phases, measuring the amplitude spectrum of the resulting signal of the preceding step; and
calculating the spectral phase of the pulse to be characterized, from the measured amplitude spectrum, for each of the applied spectral phases, and from the linear spectrum of the pulse to be characterized.

### Claim 2 (depends on 1)

The method according to claim 1, wherein the predetermined nonlinear spectral phases are applied to the two replicas so as to scan a delay range between the two replicas.

### Claim 3 (depends on 1)

The method according to claim 1, wherein the predetermined nonlinear spectral phases are applied to the two replicas so as to scan a range of differential dispersion between the two replicas.

### Claim 4 (depends on 1)

The method according to claim 1, wherein the linear spectrum of the pulse to be characterized is measured from said pulse.

### Claim 5 (depends on 1)

The method according to claim 1, further comprising:
calculating the linear spectrum of the pulse to be characterized with the calculation of the spectral phase of the pulse to be characterized.

### Claim 6 (depends on 1)

The method according to claim 1, wherein optically combining the two signals includes combining coherently the two signals.

### Claim 7 (depends on 1)

The method according to claim 1, wherein the applied nonlinear optical process, and consequently the measured resulting signal, comprises a nonlinear effect that effects the fundamental spectrum of the pulse, in particular second-harmonic generation, sum-frequency generation, difference-frequency generation, the optical Kerr effect, third-harmonic generation, or combinations thereof.

### Claim 8 (depends on 1)

The method according to claim 1, wherein calculating the spectral phase of the pulse to be characterized includes calculating a numerical iterative algorithm for iteratively approximating the spectral phase of the pulse to be characterized.

### Claim 9 (depends on 8)

The method according to claim 8, wherein the numerical iterative algorithm comprises minimizing an error function defined between the measured linear spectrum of the pulse to be characterized and a calculated linear spectrum of the pulse to be characterized, by an iterative approximation of the spectral phase of the pulse to be characterized.

### Claim 10 (depends on 8)

The method according to claim 8, wherein the numerical iterative algorithm comprises minimizing an error function defined between the highest frequency component along the scan axis of the measured signal and the highest frequency component along the scan axis of the calculated signal, by an iterative approximation of the spectral phase of the pulse to be characterized.

### Claim 11 (depends on 8)

The method according to claim 8, wherein the numerical iterative algorithm comprises minimizing an error function defined between the second highest frequency component along the scan axis of the measured signal and the second highest frequency component along the scan axis of the calculated signal by an iterative approximation of the spectral phase of the pulse to be characterized.

### Claim 12 (depends on 8)

The method according to claim 8, wherein the numerical iterative algorithm comprises minimizing an error function calculated:
using the property that the integral of the measured signal over dispersion for a given wavelength is independent of the phase of the pulse to be characterized; or
locally, for each wavelength, with an overall error being a weighted function of all the local errors.

### Claim 13 (depends on 1)

The method according to claim 1, wherein applying spectral phases so as to perform a scan along a range is performed between a lower limit and an upper limit of said range; continuously along said range or in a stepwise fashion along said range.

### Claim 14 (depends on 1)

The method according to claim 1, wherein applying spectral phases so as to perform a scan is performed with either (a) the duration of the pulse to be characterized reaching a minimum during said scan, or (b) said replicas not being synchronized during said scan.

### Claim 15

(canceled)

### Claim 16 (depends on 1)

The method according to claim 1, wherein applying spectral phases so as to perform a scan, applying said nonlinear optical process, and measuring said amplitude spectrum of the resulting signal is performed in a parallel for a plurality of applied spectral phases.

### Claim 17 (depends on 8)

The method according to claim 8, wherein applying a numerical iterative algorithm comprises defining the function of the spectral phase of the pulse to be characterized as a discrete sampling function, a Taylor expansion function, or a Fourier series function.

### Claim 18 (depends on 17)

The method according to claim 17, wherein the function of the spectral phase of the pulse to be characterized is represented by the consecutive derivatives of said function with respect to frequency.

### Claim 19 (depends on 1)

The method according to claim 1, further comprising correcting for a non-flat spectral detection response by:
simulating a trace for the approximation of the pulse to be characterized at the current iteration;
comparing the simulated scan marginal to the measured scan marginal in order to calculate the spectral response to be compensated such that the numerical integral of the trace over the dispersion scan parameter, i.e. the frequency marginal, does not depend on the original spectral phase of the pulse to be characterized; and
compensating for the calculated spectral response either by dividing the experimental trace by it or by including it in the retrieval process, by multiplying it by the ideal simulated trace, in each iteration.

### Claim 20 (depends on 9)

The method according to claim 9, further comprising compensating for a non-flat spectral detection response by minimizing the error function for each wavelength, with an overall error being a weighted function of all these errors.

### Claim 21 (depends on 7)

The method according to claim 7, wherein the measured resulting signal comprises a nonlinear effect that effects, a second-harmonic generation, a sum-frequency generation, or both, and wherein the measured second-harmonic generation and sum-frequency generation (“SHG/SFG”) signal are calculated by:
applying an inverse Fourier transform to the spectrum of the pulse to be characterized in order to obtain the corresponding electric field in the time domain;
squaring the time-dependent electric field so as to perform the SHG/SFG calculation; and
applying a Fourier transform to obtain the SHG/SFG spectrum.

### Claim 22 (depends on 21)

The method according to claim 21, wherein for the SHG/SFG calculation the spectrum is multiplied by an adequate spectral filter.

### Claim 23 (depends on 1)

The method according to claim 1, wherein the laser pulses are pulses comprised in a range of 300 cycles to sub-cycle laser pulses.

### Claim 24

A system for characterizing laser pulses, comprising:
a replica generator for generating two replicas of each pulse to be characterized;
two nonlinear spectral phase shifters for applying predetermined nonlinear spectral phases to the two replicas in order to scan a dispersion range by one or both said replicas;
an optical adder for combining the two signals from said shifter;
a nonlinear optical element for applying a nonlinear optical process to the combined signal;
a sensor for measuring the amplitude spectrum of the resulting nonlinear signal for each of the applied spectral phases; and
an electronic data processor configured to calculate the spectral phase of the pulse to be characterized, from the measured amplitude spectrum, for each of the applied spectral phases, and from the linear spectrum of the pulse to be characterized.

### Claim 25

(canceled)

### Claim 26

(canceled)

### Claim 27

(canceled)

### Claim 28

(canceled)

### Claim 29

(canceled)

### Claim 30

(canceled)

### Claim 31

(canceled)

### Claim 32

(canceled)