FREQUENCY-CONVERSION OF AN OPTICAL FREQUENCY COMB

Abstract

The invention relates to a method for generating frequency converted laser radiation. It is an object of the invention to provide a method that enables the generation of a frequency converted optical frequency comb and that is easy to implement at low cost. It is a further object to enable adjustment of the center frequency and the mode spacing in a frequency converted optical frequency comb. The method of the invention comprises the following steps: generating pump laser radiation with a spectrum containing a plurality of equidistant spectral lines; passing the pump laser radiation through the non-linear medium of a singly resonant, single-frequency optical parametric oscillator, wherein the pump laser radiation is continuous wave or pulsed, wherein the pulse duration in the latter case is longer than the time the optical parametric oscillation requires to reach its steady state; and coupling out the non-resonant idler or signal laser radiation from the optical parametric oscillator as usable frequency converted laser radiation. Moreover, the invention relates to a laser device for carrying out the method of the invention.

Claims

1. A method for generating frequency converted laser radiation, comprising the steps of: generating pump laser radiation with a spectrum containing a plurality of equidistant spectral lines; passing the pump laser radiation through the non-linear medium of a singly resonant, single-frequency optical parametric oscillator, wherein the pump laser radiation is continuous wave or pulsed, wherein the pulse duration in the latter case is longer than the time the optical parametric oscillation requires to reach its steady state; and coupling out the non-resonant idler or signal laser radiation from the optical parametric oscillator as usable frequency converted laser radiation.

2. Method of claim 1, wherein the spectrum of the usable frequency converted laser radiation coupled out from the optical parametric oscillator is a frequency converted replica of the spectrum of the pump laser radiation.

3. Method of claim 1, wherein the resonant linewidth of the singly resonant optical parametric oscillator is smaller than the linewidths of the equidistant spectral lines contained in the pump laser radiation.

4. Method of claim 1, wherein the frequency of the usable frequency converted laser radiation is adjusted by changing the phase matching conditions of the non-linear medium and/or changing the resonant frequency of the singly resonant, single-frequency optical parametric oscillator and/or changing the resonant mode of the singly resonant, single-frequency optical parametric oscillator and/or tuning the center frequency of the pump laser radiation.

5. Method of claim 1, wherein the optical spectrum of the usable frequency converted laser radiation is actively stabilized by electronic feedback to the spectrum of the pump laser radiation and/or to the resonant mode of the singly resonant optical parametric oscillator.

6. Method of claim 1, wherein the pump laser radiation is generated by frequency modulation of the laser radiation emitted by a continuous wave laser source.

7. Method of claim 6, wherein the spacing of equidistant spectral lines contained in the usable frequency converted laser radiation coupled out from the optical parametric resonator is changed by adjusting the period of the frequency modulation.

8. A laser device comprising: a pump laser source configured to generate pump laser radiation with a spectrum containing a plurality of equidistant spectral lines; a singly resonant, single frequency optical parametric oscillator comprising a non-linear medium located in an optical cavity, with the pump laser radiation passing through the non-linear medium, wherein the optical cavity is configured to be resonant at only a single cavity mode, wherein the pump laser radiation is continuous wave or pulsed, wherein the pulse duration in the latter case is longer than the time the optical parametric oscillator requires to reach its steady state; and an arrangement of one or more optical components configured to couple out the non-resonant idler or signal laser radiation from the optical parametric oscillator as usable frequency converted laser radiation.

9. Laser device of claim 8, wherein the pulse duration of the pump laser radiation is longer than a multiple of the round-trip time of the resonant signal or idler radiation in the optical cavity.

10. Laser device of claim 8, wherein the optical cavity is a bow-tie cavity.

11. Laser device of claim 8, wherein an etalon is located within the optical cavity in the beam path of the resonant signal or idler laser radiation and outside the beam paths of the pump laser radiation and the non-resonant idler or signal laser radiation.

12. Laser device of claim 8, wherein the non-linear medium is a periodically poled non-linear crystal.

13. Laser device of claim 8, wherein the pump laser source comprises a continuous wave laser and a frequency modulator configured to modulate the frequency of the laser radiation emitted by the continuous wave laser.

14. Laser device of claim 13, wherein the frequency modulator comprises an electro-optic modulator and a radio frequency source driving the electro-optic modulator, wherein the radio frequency source is configured to deliver a periodically chirped radio frequency signal to the electro-optic modulator.

15. Laser device of claim 14, wherein the period of the chirping is variable.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The enclosed drawings disclose preferred embodiments of the present invention. It should be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention. In the drawings:

[0031] FIG. 1 schematically shows a laser device according to an embodiment of the invention as a block diagram;

[0032] FIG. 2 is a more detailed illustration of the laser device of FIG. 1;

[0033] FIG. 3 schematically shows the pump laser source of the laser device illustrated in FIGS. 1 and 2.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0034] FIG. 1 schematically illustrates a laser device according to an embodiment of the invention. The laser device comprises a pump laser source 11. The pump laser source 11 generates at its output pump laser radiation with a spectrum containing a plurality of equidistant spectral lines. The spectrum of the pump laser source 11 may be an optical frequency comb, a combination of two or more different optical frequency combs, or a combination of continuous wave laser radiation with one or more optical frequency combs. The pump laser radiation is provided to a singly resonant, single-frequency optical parametric oscillator 12. In the depicted embodiment, the optical parametric oscillator 12 has three outputs, namely a residual pump output 13, a resonant output 15 and a non-resonant output 14. The non-resonant (idler or signal) laser radiation is coupled out from the optical parametric resonator 12 as usable frequency converted laser radiation via output 14. The (idler or signal) laser radiation at the non-resonant output 14 has a spectrum which is the convolution of the spectrum of the pump laser radiation and the spectrum of the single resonant cavity mode of the optical parametric oscillator 12. The single cavity mode spectrum is much narrower than the spacing between the spectral lines in the pump spectrum, thus the laser radiation at the non-resonant output 14 also contains a plurality of equidistant spectral lines with the same spectral shape and spectral feature spacing as in the pump laser radiation.

[0035] As can be seen in FIG. 2, a non-linear medium 21 (e.g. a periodically poled non-linear crystal) is placed inside a bow-tie cavity 23 that is resonant for either the signal or idler laser radiation. An etalon 22 is placed in the cavity 23 outside the beam path of the pump laser radiation and the non-resonant (idler respectively signal laser radiation). The cavity 23 is designed to be resonant at only one single cavity mode which matches the spectrum of the signal or idler laser radiation. The residual pump laser radiation and the non-resonant output beam paths (for coupling out the usable frequency converted laser radiation) are split using a dichroic filter 25. The residual pump 13 and resonant output 15 may be accessible to the user of the device as well. According to the invention, the pump laser radiation can be continuous wave or pulsed. In the case of pulsed pump laser radiation the pulse duration is longer than the time the optical parametric oscillator requires to reach its steady state, i.e. long enough for the stored energy in the resonant cavity mode to reach its steady state. This typically takes several times the cavity round-trip time.

[0036] The optical frequency comb contained in the spectrum of the usable laser radiation at the non-resonant output 14 can be frequency tuned by adjusting the phase matching conditions in the non-linear medium 21, for example by heating, rotating, or shifting the medium. The optical frequency comb contained in the radiation at the non-resonant output 14 can also be tuned by adjusting the length of the cavity, for example by using a piezoelectric transducer 24 carrying one of the cavity mirrors and/or by rotating the etalon 22. The optical frequency comb at the non-resonant output 14 can further be tuned by selecting a different cavity mode for oscillation, for example by rotating the etalon 22 to such a degree that a cavity mode hop occurs. Finally, the optical frequency comb at the output 14 can be tuned by wavelength tuning of the pump laser source 11. When the pump laser source 11 is wavelength tuned, the resonant mode of the cavity 23 does not change, so energy conservation causes the radiation at the non-resonant output 14 to change by the same amount of energy as the pump laser radiation.

[0037] FIG. 3 depicts a preferred embodiment of the pump laser source 11. A single frequency continuous wave laser 31, e.g. a DFB diode laser or an ECDL, is fiber coupled, and the output is connected to an EOM 32. The EOM is driven by a radio frequency source 33 that delivers a periodically frequency modulated, i.e. chirped radio frequency signal. The radiation at the fiber-coupled output of the EOM 32 consists of an optical frequency comb with mode spacing equal to the repetition rate of the periodic chirp, and with spectral extent equal to twice the maximum frequency of the chirp. The period of the chirp is variable for adjusting the mode spacing of the optical frequency comb. The linewidth of the continuous wave laser 31 should be much narrower than the mode spacing. The fiber coupled optical frequency comb is input to a rare earth-doped fiber amplifier 34 that is designed to amplify narrow source spectra to high power without spectral broadening or shifting, e.g. due to Raman or Brillouin scattering.