Opto-optical modulation of a saturable absorber for high bandwidth CEO stabilization of a femtosecond laser frequency comb

Abstract

A laser source that generates an optical frequency comb, comprising a pumped laser medium placed inside an optical cavity that incorporates at least one optically-controlled modulator, a detector generating an error signal, and a modulation optical source that is controlled by the error signal and whose radiation is directed onto said optically-controlled modulator thereby stabilizing the Carrier-Envelope Offset (CEO) frequency and/or the CEO phase and/or the repetition rate of said source.

Claims

1. A laser source that generates an optical frequency comb, comprising a pumped laser medium placed inside an optical system or cavity that incorporates at least one optically-controlled modulator, a detector generating an error signal, and a modulation optical source that is controlled by the error signal and whose radiation is directed onto said optically-controlled modulator thereby stabilizing a Carrier-Envelope Offset (CEO) frequency and/or a CEO phase and/or a repetition rate of said source, wherein said optically-controlled modulator has a variable gain or loss coefficient.

2. The source of claim 1, wherein said error signal is representative of the CEO frequency and/or phase and/or repetition rate of the laser radiation source.

3. The source of claim 1, wherein said optically-controlled modulator includes a SESAM (Semiconductor Saturable Absorber Mirror), and/or a SBR (Saturable Bragg Reflector), a transmissive semiconductor absorber, and/or a VECSEL (Vertical External Cavity Surface Emitting Laser), and/or a SDL (Semiconductor Disk Laser) and/or a carbon-nanotube based loss modulator, and/or a dye-based absorber, and/or a graphene-based loss modulator.

4. The source of claim 1, wherein said optically-controlled modulator is based on one or several optically-pumped quantum wells or quantum dots.

5. The source of claim 1, wherein said optically-controlled modulator is integrated in a reflective structure.

6. The source of claim 5, wherein said optically-controlled modulator is arranged to modelock the laser source.

7. The laser source of claim 6, wherein the detector comprises a supercontinuum-generating nonlinear medium and an f-to-2f interferometer.

8. The laser source of claim 1, wherein said pumped laser medium comprises one of: Er:Yb:glass, Yb:KYW; Yb:YAG; Yb:CALGO; Yb:Lu2O3, Yb:LuScO, Yb:ScO or Cr:forsterite DPSSLs, or a glass material comprising Er, Yb, or Tm, or the material Ti: Sapphire.

9. The laser source of claim 1, wherein said optically-controlled modulator comprises a stack of GaAs and AlAs layers or a stack comprising InP layers, and one or several embedded quantum well(s).

10. The laser source of claim 1, wherein the pumped laser medium is a thin disk or a fibre laser.

11. The laser source of claim 1, wherein the optical cavity comprises an element for dispersion control.

12. The laser source of claim 1, wherein the laser medium and/or the optically-controlled modulator are optically pumped by one or more laser sources.

13. A coherent radiation source that generates an optical frequency comb, comprising a pumped nonlinear medium or a pumped laser medium placed inside an optical cavity that incorporates at least one extra-cavity optically-controlled modulator, a detector generating an error signal, and a modulation optical source that is controlled by the error signal and whose radiation is directed onto said optically-controlled modulator thereby stabilizing a CEO frequency and/or a CEO phase and/or a repetition rate of the laser output of said source.

14. A laser source that generates an optical frequency comb, comprising: a laser resonator, a pumped gain medium inside said laser resonator, an optically-controllable modulator inside said laser resonator, a detector arranged to generate an error signal representative of a Carrier-Envelope Offset frequency of the laser source, a modulation optical source whose radiation is directed onto said optically-controllable modulator, and a controller circuit receiving said error signal and arranged for controlling said modulation optical source to stabilize the Carrier-Envelope Offset frequency of said laser source.

15. A laser source that generates an optical frequency comb, comprising a pumped laser medium placed inside an optical system or cavity that incorporates at least one optically-controlled modulator, a detector generating an error signal, and a modulation optical source that is controlled by the error signal and whose radiation is directed onto said optically-controlled modulator thereby stabilizing the Carrier-Envelope Offset (CEO) frequency and/or the CEO phase and/or the repetition rate of said source, wherein said optically-controlled modulator has a variable optical length.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood with the aid of the description of an embodiment given by way of example and illustrated by the figures, in which:

(2) FIG. 1 represents schematically an exemplary but not limiting mode of realization of the present invention.

(3) FIG. 2 represents schematically the principle of the invention implemented in a modelocked laser.

(4) FIG. 3 is an exemplary plot of the CEO transfer functions obtained using the present invention in comparison with known stabilization methods.

(5) FIG. 4 represents the RF spectra of the CEO beat obtained with the actuator of the present invention where the SESAM is optically modulated and with the standard method of pump current modulation of the gain medium for comparison.

(6) FIG. 5 plots the frequency-noise power spectral density (PSD) obtained with the present invention and with conventional stabilization methods, as well as the corresponding integrated phase-noise.

(7) FIG. 6 plots the Allan deviation of the CEO frequency achieved with the present invention.

(8) FIGS. 7a and 7b show an example of the dynamic response of the repetition rate in an Er:Yb:glass DPSSL obtained with the present invention in comparison with former known methods.

DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS OF THE INVENTION

(9) The exemplary embodiment of the invention represented in FIG. 1, comprises a SESAM-modelocked Er:Yb:glass laser oscillator referred to as ERGO, which emits a 15-nm wide spectrum (FWHM) at 1558 nm. It must be understood, however, that the invention is also applicable to other modelocked laser systems. The ERGO laser consists of a standing-wave resonator with a 2-mm thick Er:Yb:glass gain medium 120, a plurality of Gires-Tournois interferometer (GTI-) mirrors 150 with a group-delay dispersion (GDD) of 100 fs.sup.2 each for solution formation, an output coupler 140 with a transmission of 1.7%, and a SESAM to obtain self-starting and reliable modelocking. The SESAM 170 consists of a standard anti-resonant design in this particular case, but the invention also applies to all other types of SESAM structures or any other type of optically-controlled modulator.

(10) Moreover, the laser source of the invention could include, in the optical cavity, in alternative or in combination with the GTI-type dispersive mirrors, any kind of dispersive mirror or mirrors, for example chirped mirrors.

(11) The gain medium of the Er:Yb:glass laser is pumped by a telecom-grade single-mode diode laser 180 providing a continuous wave (cw) optical power of roughly 600 mW at a wavelength of 976 nm. The laser oscillator has a repetition rate of 75 MHz and emits 170-fs transform-limited solution pulses at an average output power of 110 mW. The self-referencing of this frequency comb was previously achieved using the standard method of feedback to the current of the laser diode that pumps the Er:Yb:glass gain material. In this case, a tight lock of the CEO beat is achieved with a modest feedback bandwidth of about 5.5 kHz.

(12) The present invention comprises a new optically-controlled modulator for CEO frequency control realized by focusing a suitable modulated light source 190 onto the SESAM inside the fs-laser resonator. In the embodiment of the invention represented by FIG. 1, the additional light source 190 comprises an 812-nm, multimode fibre-coupled laser diode. Due to geometrical constraints of the fs-laser cavity, the pump beam was focused onto the SESAM with an incidence angle of about 5 deg. The obtained pump spot was spatially overlapped with the intra-cavity fs-pulses on the SESAM. However, it was roughly twice in size, because of the multimode fibre output of the 812-nm pump laser.

(13) The CEO beat can be generated and detected by any known means; in the presented example this is achieved by means of a standard f-to-2f interferometer 130 after supercontinuum spectrum generation in a polarization-maintaining highly-nonlinear fibre (PM-HNLF), coupled by a suitable optical output coupler 140. However, other types of non-linear media can be used as well for supercontinuum generation, such as photonic crystal fibres (PCF) or waveguides. For the results shown here, the CEO beat frequency was set to roughly 20 MHz by slightly adjusting the pump laser current of the gain medium, but this is not a limiting feature of the invention.

(14) FIG. 2 represents a generalization of the detailed embodiment of FIG. 1. The invention includes a laser gain medium 120 that is pumped by a suitable pump source 180 such as to induce emission of laser radiation. The output coupler 140 samples the laser radiation and sends it to the detector 130 that generates an error signal 135, which is used to modulate a light source 190. The radiation of the light source 190 is directed to the optically-controlled optical modulator (00M) 170 that is incorporated in the laser cavity. It will be readily understood that these elements of the invention can be embodied by the specific elements represented by the same reference numbers in the embodiment of FIG. 1, or by any other suitable means.

(15) A fundamental parameter for an actuator to be used for CEO frequency stabilization is its modulation bandwidth, i.e., how fast it is able to act on the CEO frequency f.sub.CEO. A large enough bandwidth is needed to correct for fast frequency fluctuations contributing to the linewidth of the CEO beat, if a tight lock is targeted. The bandwidth of the new proposed SESAM-pumped modulator was determined in the implementation example of the invention by measuring the CEO-beat transfer function, i.e., the amplitude and phase response of f.sub.CEO to a modulation of the SESAM pump laser. The upper panel of FIG. 3 shows the CEO dynamic tuning range achieved with the present invention in comparison to the standard method of gain pump modulation, whereas the lower panel displays the corresponding phase shift.

(16) Generally, an actuator that acts on the CEO frequency f.sub.CEO also has an effect on the repetition rate f.sub.rep of the comb, which results from the existing coupling between the control of f.sub.CEO and f.sub.rep. As a consequence, the present invention can also be used for high bandwidth control and stabilization of the comb repetition rate, e.g., by phase-locking to an optical frequency standard.

(17) Example of CEO Stabilization Performance Achieved with the Invention

(18) The high bandwidth of the SESAM-pumped modulator enables a straightforward tight lock of the CEO beat to be achieved. The stabilization is realized in one possible embodiment of the invention by feeding the amplified and low-pass filtered output signal from an f-to-2f interferometer into a digital phase detector, where it is compared to a reference signal. Any sufficiently stable reference signal can be used here; the presented example relies on a frequency synthesizer which is stabilized to an H-maser. The phase-detector output, which serves as a phase error signal, is then fed to a suitable controller circuit, for example a Proportional-Integrator-Derivative (PID) servo-controller used as modulation source of the current driver that controls the SESAM pump laser.

(19) An example of the radio-frequency (RF) spectrum obtained for the stabilized CEO beat stabilized with the present invention is shown in FIG. 4, where it is compared to the standard stabilization with feedback to the pump laser of the gain material. An improvement by 20 dB is obtained in the signal-to-noise ratio using the present invention in comparison to the result obtained with pump current modulation of the gain material. For a more quantitative comparison of the noise performance of the two stabilization methods, FIG. 5 shows in the top panel the corresponding frequency noise power spectral density (PSD) of the stabilized CEO beat as a function of the Fourier frequency obtained with the present invention and with the standard method of feedback to the pump laser current in the same laser.

(20) The stabilized CEO signal presents a significant improvement with the new SESAM-pumped modulator over the standard gain modulation at all Fourier frequencies owing to the larger achievable control bandwidth. In the particular case of the ERGO laser exemplified here, this results in a small residual integrated phase noise of 63 mrad (1 Hz-100 kHz), which is better by more than an order of magnitude than the value of 720 mrad obtained for gain pump modulation. The lower panel of FIG. 5 shows the corresponding integrated phase-noise for the two stabilization methods.

(21) The frequency stability of the CEO beat locked using the new SESAM-pumping method and measured using a dead-time-free counter shows an improvement of about a factor of 4 in the Allan deviation in comparison to the standard pump current stabilization as shown in FIG. 6 that plots the Allan deviation of the CEO frequency achieved in an Er:Yb:glass DPSSL with the present invention of SESAM-modulation and with the standard gain-modulation via the pump current for comparison.

(22) FIGS. 7a and 7b show an example of the dynamic response of the repetition rate in an Er:Yb:glass DPSSL obtained with the present invention of SESAM optical pumping and using the standard method of gain medium modulation via the pump current for comparison. The upper panel shows the dynamic tuning response and the lower panel displays the phase shift. This figure illustrates the potential of the present invention for high bandwidth stabilization of the repetition rate of a frequency comb.

(23) The newly proposed optically-controlled modulator to control and stabilize the CEO beat of a modelocked laser makes use of optical pumping of the SESAM that is already incorporated in the laser cavity for modelocking in one possible embodiment of the invention. This embodiment does not imply any change in the laser resonator. Alternatively, one or several additional optically-controlled modulators can be added in the resonator for control of the CEO frequency or the repetition rate of the frequency comb in other embodiments of the invention.

(24) Absorber recombination timescales in the picosecond regime set the limit for the bandwidth of the SESAM-based modulator, which should enable feedback-loop bandwidths far beyond 1 MHz. This makes this novel stabilization technique particularly attractive to self-reference frequency combs from lasers that were not possible to stabilize so far due to a lack a bandwidth, e.g., in the case of high repetition rate lasers with broad CEO linewidth. Moreover, this CEO beat actuator is compatible with high-power lasers such as thin disk lasers for instance.

(25) The optically-controlled SESAM that is used in the present invention constitutes an optically-controlled modulator that has a variable loss coefficient, dependent on the intensity of the SESAM-pumping laser radiation, since the latter optical source is controlled by the error signal that is representative of the phase error of the modelocked laser radiation, the device of the invention achieves a stabilization of the comb CEO frequency or repetition rate.

(26) According to a different embodiment of the present invention the optically-controlled modulator could include an optically active region that is pumped by a suitable optical modulation source, and exhibits a variable gain coefficient. In this variant also, the optical modulator could be controlled such as to stabilize the comb CEO frequency.

(27) In another variant, the optically-controlled modulator could introduce also a variable optical length and, therefore, be used as control modulator to stabilize the repetition rate of the laser source.

(28) Preferably, the recovery dynamics of the optically-controlled modulator is optimized for a lifetime of at least 5% of the cavity roundtrip time, such that efficient bleaching of the absorption with cw-light can be achieved.

(29) The change in optical absorption (or gain) of the optical modulator used in the invention does not need to reach very high values. In typical SESAM-based realization, the optically-controlled modulator is driven such that its optical absorption changes by less than 20%. The figures relative to variable-gain modulators are similar.

(30) In the example presented above, the optical modulator is conveniently integrated in one SESAM of the laser cavity. This is not however a necessary feature, and the optical modulator could be positioned in any suitable location in the laser cavity. In a different embodiment, the laser source of the invention could also include more than one optically-controlled SESAM or more than one optically-controlled modulator. In this manner, among other things, independent stabilization of the CEO and repetition rate could be achieved. The repetition rate of the optical cavity and the output power can vary according to the circumstances and the need, but importantly the present invention allows production of laser light at a cavity repetition rate in excess of 1 GHz, and average power levels above 10 W.

(31) The present invention could use Er:Yb:glass, Yb:KYW; Yb:YAG; Yb:CALGO; Yb:Lu.sub.2O.sub.3, Yb:KYW or Cr:forsterite, or an optical fibre material comprising Er, Yb, or Tm as laser medium, but is not limited to such active media. It could be extended to any kind of suitable laser material. The geometry of the laser material is also not essential, and the invention could be employed to stabilize frequency combs in which the laser medium is a thin disk, or a fibre, or is in any suitable configuration.

(32) When the optically-controlled modulator is based on a SESAM device, this could comprise a stack of GaAl and AlAs layers or a stack comprising InP layers, and an embedded quantum well, but could also be realized with any other suitable semiconductor structure, including for example, saturable absorbers based on embedded quantum dots, arranged or not in layered structures, or any suitable nonlinear optical device. The quantum well(s) or quantum dot layer(s) may be integrated in a transmissive structure.

(33) Further, the duration of light pulses generated in the source of the invention can span between 1 fs and 1000 fs, preferably lower than 300 fs.

(34) Finally, the optically-controlled modulator needs not necessarily be optically-pumped by a laser source, but other suitable light sources can be considered.

(35) In the above text, the optically-controlled modulator can include a device comprising any one of: an optically-pumped semiconductor (OPS) or a Vertical External Cavity Surface Emitting Laser (VECSEL) or a Semiconductor Disk Laser (SDL) or parts of said device (such as the gain chip), a graphene-based loss modulator, a carbon-nanotube based loss modulator, or a dye-based absorber. Moreover, the acronym SESAM can include any one of the following expressions: Semiconductor Saturable Absorber Mirror, or Saturable Absorber Mirror (SAM), or Saturable Absorber (SA), or Saturable Mirror (SM), or Saturable Bragg Reflector (SBR).