Stabilizing optical frequency combs

Abstract

A method for operating a laser device (1), wherein an optical frequency comb can be stabilized and the frequencies of the modes thereof are describable by the formula f.sub.m=mf.sub.rep+f.sub.0, where f.sub.rep is a mode spacing, f.sub.0 is an offset frequency and m is a natural number. At least one signal (S1, S2, S3, S4) is determined, which correlates with an actual value of a degree of freedom (F), wherein the degree of freedom (F) is a linear combination of the offset frequency f.sub.0 and the mode spacing f.sub.rep of the frequency comb. The actual value of the degree of freedom (F) is set in a predetermined capture range (F) of a second control unit (40) using a first control unit (10) on the basis of the signal. As soon as the capture range (F.sub.capture) of the second control unit (40) is reached, the second control unit (40) is activated and the actual value is regulated to an intended value (F.sub.intended) with the aid of the second control unit (40).

Claims

1. A method for operating a laser device (1) comprising the steps: a) providing an optical frequency comb having a plurality of modes, the frequencies of which are describable by the formula f.sub.m=mf.sub.rep+f.sub.0, wherein m is a natural number, f.sub.rep is a mode spacing and f.sub.0 is an offset frequency, b) determining at least one signal (S1, S2, S3, S4) that correlates with an actual value of a degree of freedom (F), wherein the degree of freedom (F) is a linear combination of the offset frequency f.sub.0 and the mode spacing f.sub.rep, c) transmitting, to a first control unit (10), at least one first input signal, the at least one first input signal being included in a set of the at least one signal (S1, S2, S3, S4), d) adjusting, by the first control unit (10), the actual value of the degree of freedom (F) to a predetermined capture range (F.sub.capture) of a second control unit (40) on the basis of the at least one first input signal, e) activating the second control unit (40), once the capture range (F.sub.capture) of the second control unit (40) is reached and, f) controlling the actual value to a target value (F.sub.intended) by means of the second control unit (40), characterized in that the second control unit (40) comprises a single second control loop (42) or a group of cascaded second control loops (42) that control, on the basis of a single second input signal (S4) included in the set of the at least one signal, one or more actuators (8a, 8b, 8c, 8d, 8e), wherein the second control loops (42) arranged downstream in the cascading are configured to keep respective second control loops (42) that are arranged upstream within their respective control ranges.

2. The method of claim 1, characterized in that the first control unit (10) comprises a plurality of first control loops (12) that are sequentially used so as to bring the actual value of the degree of freedom (F) into the capture range (F.sub.capture) of the second control unit (40).

3. The method of claim 2, characterized in that the sequential usage of the first control loops (12) is controlled by a state machine (30).

4. The method of claim 1, characterized in that the second input signal is derived from a beat signal and the sign of the beat frequency is determined prior to or during the activation of the second control unit (40).

5. The method of claim 1, characterized in that the at least one first input signal comprises the second input signal.

6. The method of claim 1, characterized in that the first control unit (10) separately selectively activates one or more second control loops (42).

7. The method of claim 1, characterized in that the first control unit (10) adjusts one or more of the actuators (8) controlled by the second control loops (42) in parallel or alternatively to the second control unit (40).

8. The method of claim 1, characterized by driving one or more actuators (8), which are independent of the second control unit (40), by means of the first control unit (10).

9. The method of claim 1, characterized in that the first control unit (10) processes at least one actuator signal that represents the control value of an actuator (8).

10. The method of claim 1, characterized in that, when the actual value is outside a set validity range (F.sub.validity), the first control unit (10) varies a control value of at least one actuator (8) in a stochastic or uniform manner until the validity range (F.sub.validity) is reached again.

11. The method of claim 10, characterized in that a verification determining whether the actual value is within the validity range (F.sub.validity) is performed by determining a power level of a beat signal.

12. The method of claim 1, comprising the step of driving at least one actuator (8) depending on one or more previous states of the first control unit (10).

13. The method of claim 1, characterized in that the offset frequency (f.sub.0) and the mode spacing (f.sub.rep) of the frequency comb are stabilized.

14. The method of claim 13, characterized by detecting a state of successful stabilizing and then displaying or forwarding a message indicating the state of successful stabilizing.

15. The method of claim 13, characterized by an algorithm for automatically returning to the stabilized operation, when the state of stabilizing has been left, in particular, by repeating the steps a) to f).

16. A method for operating a laser device (1) comprising the steps: a) providing an optical frequency comb having a plurality of modes, the frequencies of which are describable by the formula f.sub.m=mf.sub.rep+f.sub.0, wherein m is a natural number, f.sub.rep is a mode spacing and f.sub.0 is an offset frequency, b) determining at least one signal (S1, S2, S3, S4) that correlates with an actual value of a degree of freedom (F), wherein the degree of freedom (F) is a linear combination of the offset frequency f.sub.0 and the mode spacing f.sub.rep, c) transmitting, to a first control unit (10), at least one first input signal, the at least one first input signal being included in a set of the at least one signal (S1, S2, S3, S4), d) adjusting, by the first control unit (10), the actual value of the degree of freedom (F) to a predetermined capture range (F.sub.capture) of a second control unit (40) on the basis of the at least one first input signal, e) activating the second control unit (40), once the capture range (F.sub.capture) of the second control unit (40) is reached and, f) controlling the actual value to a target value (F.sub.intended) by means of the second control unit (40), characterized in that the first control unit (10) comprises a plurality of first control loops (12) that are sequentially used so as to bring the actual value of the degree of freedom (F) into the capture range (F.sub.capture) of the second control unit (40).

17. A method for operating a laser device (1) comprising the steps: a) providing an optical frequency comb having a plurality of modes, the frequencies of which are describable by the formula f.sub.m=mf.sub.rep+f.sub.0, wherein m is a natural number, f.sub.rep is a mode spacing and f.sub.0 is an offset frequency, b) determining at least one signal (S1, S2, S3, S4) that correlates with an actual value of a degree of freedom (F), wherein the degree of freedom (F) is a linear combination of the offset frequency f.sub.0 and the mode spacing f.sub.rep, c) transmitting, to a first control unit (10), at least one first input signal, the at least one first input signal being included in a set of the at least one signal (S1, S2, S3, S4), d) adjusting, by the first control unit (10), the actual value of the degree of freedom (F) to a predetermined capture range (F.sub.capture) of a second control unit (40) on the basis of the at least one first input signal, e) activating the second control unit (40), once the capture range (F.sub.capture) of the second control unit (40) is reached and, f) controlling the actual value to a target value (F.sub.intended) by means of the second control unit (40), characterized by driving one or more actuators (8), which are independent of the second control unit (40), by means of the first control unit (10).

18. A method for operating a laser device (1) comprising the steps: a) providing an optical frequency comb having a plurality of modes, the frequencies of which are describable by the formula f.sub.m=mf.sub.rep+f.sub.0, wherein m is a natural number, f.sub.rep is a mode spacing and f.sub.0 is an offset frequency, b) determining at least one signal (S1, S2, S3, S4) that correlates with an actual value of a degree of freedom (F), wherein the degree of freedom (F) is a linear combination of the offset frequency f.sub.0 and the mode spacing f.sub.rep, c) transmitting, to a first control unit (10), at least one first input signal, the at least one first input signal being included in a set of the at least one signal (S1, S2, S3, S4), d) adjusting, by the first control unit (10), the actual value of the degree of freedom (F) to a predetermined capture range (F.sub.capture) of a second control unit (40) on the basis of the at least one first input signal, e) activating the second control unit (40), once the capture range (F.sub.capture) of the second control unit (40) is reached and, f) controlling the actual value to a target value (F.sub.intended) by means of the second control unit (40), characterized in that, when the actual value is outside a set validity range (F.sub.validity), the first control unit (10) varies a control value of at least one actuator (8) in a stochastic or uniform manner until the validity range (F.sub.validity) is reached again.

19. A method for operating a laser device (1) comprising the steps: a) providing an optical frequency comb having a plurality of modes, the frequencies of which are describable by the formula f.sub.m=mf.sub.rep+f.sub.0, wherein m is a natural number, f.sub.rep is a mode spacing and f.sub.0 is an offset frequency, b) determining at least one signal (S1, S2, S3, S4) that correlates with an actual value of a degree of freedom (F), wherein the degree of freedom (F) is a linear combination of the offset frequency f.sub.0 and the mode spacing f.sub.rep, c) transmitting, to a first control unit (10), at least one first input signal, the at least one first input signal being included in a set of the at least one signal (S1, S2, S3, S4), d) adjusting, by the first control unit (10), the actual value of the degree of freedom (F) to a predetermined capture range (F.sub.capture) of a second control unit (40) on the basis of the at least one first input signal, e) activating the second control unit (40), once the capture range (F.sub.capture) of the second control unit (40) is reached and, f) controlling the actual value to a target value (F.sub.intended) by means of the second control unit (40), wherein when the actual value is outside a set validity range, the first control unit drives at least one actuator depending on one or more previous states of the first control unit.

20. The method of claim 17, characterized in that the first control unit (10) processes at least one actuator signal that represents the control value o an actuator (8).

21. The method of claim 18, comprising at least one actuator (8) that is driven depending on one or more previous states of the first control unit (10).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Hereinafter, the invention and its advantages will be explained in more detail by means of drawings. In the drawings:

(2) FIG. 1 illustrates a schematic representation of a portion of an exemplary laser device that may be operated by the inventive method,

(3) FIG. 2 illustrates a frequency domain representation of modes of an exemplary frequency comb with the frequency being plotted along the horizontal axis and the light intensity being plotted along the vertical axis,

(4) FIG. 3 illustrates a schematic representation of some relevant ranges of values for the inventive degree of freedom of a frequency comb, and

(5) FIG. 4 shows a schematic representation of at least a part of a signal processing system for implementing the inventive method according to one embodiment.

DETAILED DESCRIPTION OF THE INVENTION

(6) FIG. 1 illustrates an exemplary laser device 1 that may be operated by using the inventive method. It should be appreciated, however, that the inventive method may be used for operating other laser devices as long as the provision of an optical frequency comb is possible.

(7) In the laser device 1 of FIG. 1, a laser active medium 2 is provided on an optical axis 3 of a resonator 4. The laser active medium 2 may, for instance, be a Ti:Sa crystal. Other laser active media, in particular, laser crystals such as, for instance, Yb:YAG, Cr:LiSAF or Cr:Forsterite are possible. The laser active medium 2 is pumped by a pump laser beam P generated by a pump laser 5 disposed outside the resonator 4.

(8) The resonator 4 may comprise several resonator mirrors 6a, 6b, 6c, 6d. In the embodiment illustrated, the resonator 4 is a linear resonator. In this case, two resonator mirrors, i.e., the mirrors 6a and 6b, form resonator end mirrors. Any number of further resonator mirrors 6c, 6d may be disposed on the optical axis 3 of the resonator 4 between the resonator end mirrors 6a, 6b. Alternatively, it is possible to provide the resonator 4 in the form of a ring resonator such that the resonator 4 does not have any resonator end mirrors.

(9) One of the resonator mirrors 6d is advantageously configured as a mirror appropriate for coupling in pump laser radiation P. Moreover, preferably a coupling-out mirror (in FIG. 1 the resonator mirror 6b) is provided for coupling laser light out of the resonator 4. For enhanced beam guiding, it may be appropriate to provide some of the resonator mirrors 6a, 6b, 6c, 6d as curved mirrors. It is also possible that one or more of the resonator mirrors are chirped mirrors. In this way, superior dispersion compensation may be achieved in the resonator 4. It may be advantageous to provide a mode coupling element 7 within the resonator 4, for instance, a Kerr lens or a saturable absorber.

(10) It is also possible that no laser active medium 2 is provided within the resonator 4, and instead, the laser radiation is coupled directly into the resonator 4 by an in-coupling mirror (similar to the pump laser radiation P). The radiation circulating in the resonator 4 may, in particular, represent a pulsed laser radiation, in particular, short or ultrashort laser pulses.

(11) FIG. 2 illustrates in the frequency domain representation the position of the modes of an optical frequency comb associated with the optical radiation circulating in the resonator 4. The optical frequency comb comprises a plurality of modes having intensities I(f) the frequencies of which may be described in the frequency domain f by the formula f.sub.m=mf.sub.rep+f.sub.0, wherein m is a natural number, f.sub.rep is a mode spacing and f.sub.0 is an offset frequency. The position of the modes of the frequency comb is, thus, determined by the parameters offset frequency f.sub.0 and mode spacing f.sub.rep.

(12) Within or at the resonator 4, one or more actuators 8a, 8b, 8c, 8d, 8e may be provided, with the aid of which the position of the modes of the frequency comb is adjustable. One actuator may represent, for instance, a device for adjusting the resonator length, in particular, for moving a resonator end mirror 6a along the optical axis 3 of the resonator 4, in particular, a mechanical step motor 8a, a piezo-electric motor 8b and/or an electro-optical modulator (EOM). In the context of the present invention, it has proven to be particularly advantageous to provide a plurality of actuators having different adjustment accuracies and different adjustment ranges for adjusting the resonator length. Depending on the circumstances, this allows adjustment either so as to have an increased range or to have superior accuracy. To this end, for moving the resonator end mirror 6a, a mechanical step motor 8a and a piezo-electric motor 8b are provided, wherein the adjustment step size of the piezo-electric motor 8b is finer compared to that of the mechanical step motor 8a. It is also possible to provide a piezo-electric motor 8b and an electro-optical modulator, wherein the adjustment step size of the electro-optical modulator is finer compared to that of the piezo-electric motor 8b.

(13) As further potential actuators that may be provided alternatively or additionally an apparatus for inserting an optical prism 9 into the optical path of the resonator 4 may be used. Again, this may be accomplished by a mechanical step motor 8a and/or a piezo-electric motor 8b. By inserting the optical prism 9 into the optical path of the resonator 4 or by moving the position of the prism 9 along a direction perpendicular to the optical axis 3 of the resonator 4, both the mode spacing f.sub.m of the frequency comb (through effects of the prism 9 onto the repetition rate) and the offset frequency f.sub.0 (through dispersive effects of the prism 9) may be varied.

(14) For variation of the offset frequency f.sub.0, it is also possible to provide, as an actuator, a tilting device 8e for an end mirror 6b of the resonator 4. To this end, for instance, the end mirror 6b may be tilted around an axis oriented perpendicularly to the optical axis 3 of the resonator 4.

(15) An example of an actuator that is not directly provided at or within the resonator 4 is a power controller 8d for the power of the pump laser 5. By varying the pump power, the position of the frequencies of the frequency comb may be adapted, in particular, by non-linear intensity dependencies of dispersive characteristics in the resonator 4, in particular, in the laser active medium 2.

(16) In FIG. 1, several actuators 8a, 8b, 8c, 8d, 8e are shown for illustrative purposes. A lower number of actuators may be provided as well, for instance, one, two or three. For instance, providing further actuators is also an option.

(17) According to the present invention, at least one signal S1, S2, S3, S4 may be determined, which is correlated with an actual value of a degree of freedom F of the frequency comb. In the context of the present invention, the degree of freedom may be an arbitrary linear combination of the offset frequency f.sub.0 and the mode spacing f.sub.rep of the optical frequency comb. In particular, it may be advantageous when the degree of freedom corresponds to the mode spacing f.sub.rep or the offset frequency f.sub.0 and, therefore, the signal correlates with an actual value of the mode spacing f.sub.rep or the offset frequency f.sub.0.

(18) In case that the degree of freedom F corresponds to the mode spacing f.sub.rep of the frequency comb, the at least one signal S1, S2, S3, S4 may be determined by evaluating a beat of adjacent modes of the frequency comb. To this end, for instance, the number of oscillations of the beat signal may be determined within a specified time interval by means of a photodetector M1, M2.

(19) In case that the offset frequency f.sub.0 is used as the degree of freedom F, determining the at least one signal may be accomplished by means of an f:2f-interferrometer. To this end, one component of the optical radiation associated with the frequency comb is doubled in frequency and is superimposed with a non-frequency doubled component of the optical radiation. The generated beat includes a frequency that corresponds to the offset frequency f.sub.0 of the frequency comb and may be measured by known means.

(20) In FIG. 1 there are illustrated two measurement devices M1, M2 that perform one or more measurements for determining the at least one signal S1, S2, S3, S4. In this case, a laser pulse corresponding to the frequency comb may be measured. As a further option, only one or more than two measurement devices M1, M2 may be provided. A laser pulse exiting the resonator 4 at the out-coupling mirror 6b is divided into two subpulses at a first beam splitter D1 in FIG. 1. One of the subpulses is passed on for an application-specific use. The other subpulse is further split at a second beam splitter D2. The two subpulses of second generation generated in this manner are each supplied to the measurement devices M1, M2. A different positioning of the measurement devices M1, M2 is also an option, for instance, within the resonator 4. The measurement devices M1, M2 may represent an interferometer or photo diode or other measurement devices.

(21) As discussed later on, the actual value of the degree of freedom F is controlled to a target value or intended value F.sub.intended. Such a control is accomplished when the actual value of the degree of freedom F is within a stabilizing range F.sub.stabilizing that includes the target value F.sub.intended, as will be described later on in more detail. FIG. 3 illustrates the mutual relationship between the target value F.sub.intended and the stabilizing range F.sub.stabilizing in relation to further ranges of values of the actual value of the degree of freedom F, as will be described later on, i.e., the capture range F.sub.capture of the second control unit 40, described later on, and the validity range F.sub.validity of the at least one signal S1, S2, S3, S4.

(22) FIG. 4 illustrates a signal processing scheme by which the inventive method may be realized according to an embodiment.

(23) Measurement values W1, W2 as obtained from the measurement devices M1, M2 may directly represent a signal correlating with the actual value of the degree of freedom F of the frequency comb. It is also an option that the at least one signal S1, S2, S3, S4 is obtained from a measurement value W1, W2 by further processing. For example, a measurement value W2 may be converted into the signal by a post-processing unit 20, 22, 24, in particular, by amplification, normalization, frequency mixing and/or level measurement. As shown in FIG. 4, several signals S2, S3, S4 may be obtained from a measurement value W2 of the measurement device M2 by respective different post-processings by means of the post-processing units 20, 22, 24, wherein the signals correlate with the actual value of the degree of freedom F. The measurement value W1 of the measurement device M1, on the other hand, may be directly used as signal S1.

(24) According to the present invention, it is possible that merely a single signal S1, S2, S3, S4 that correlates with the actual value of the degree of freedom F be determined. Advantageously, however, two, three, four, five or more signals S1, S2, S3, S4, correlating with the actual value with the degree of freedom F may be determined.

(25) A signal S1 obtained by a first measurement device M1 and correlating with the actual value of the degree of freedom F is transmitted to a first control unit 10 as a first input signal. A first control loop 12 of the first control unit 10 receives this first input signal. It is an option that further first control loops 12 are provided that may also receive this first input signal or alternatively may receive a different signal correlating with the actual value of the degree of freedom F. In the embodiment shown in FIG. 4, a further signal S2 derived from a measurement value W2 of the second measurement device M2 by means of the post-processing unit 20 is transmitted to a further first control loop 12 of the first control unit 10 as a further first input signal.

(26) Preferably, the first control loops 12 are used sequentially. The sequential use of the first control loops 12 is preferably controlled by a state machine 30 of the first control unit 10. To this end, the state machine 30 may also receive the at least one first input signal and may decide, depending on at least one first input signal, to further continue in the sequence of the first control loops 12. To this end, the state machine 30 may activate or deactivate, preferably individually, the first control loops 12. It is advantageous when the state machine 30 is supplied with all the signals from the set of the at least one determined signal S1, S2, S3, S4.

(27) According to the present invention, by means of the first control unit 10 the actual value of the degree of freedom F is adjusted to a predetermined capture range F.sub.capture on the basis of the at least one first input signal. To this end, the first control loops 12 have access to one or more actuators 8a, 8b,8d. In this respect, it is possible that each of the first control loops 12 may drive the same actuator 8a, 8b, 8d. Alternatively, the first control loops 12 may drive different actuators 8a, 8b, 8d to gain access to different adjustment ranges. In the embodiment shown in FIG. 4 the two first control loops 12 control the step motor 8a for moving the resonator end mirror 6a.

(28) In particular, immediately after turning on the frequency comb, there may exist the possibility that the actual value of the degree of freedom F is outside of a given validity range F.sub.validity of a signal from the set of the at least one determined signals S1, S2, S3, S4. The validity range F.sub.validity may be different for each of the signals S1, S2, S3, S4 and may be the range of values of the actual value in which the signal S1, S2, S3, S4 correlates with the actual value of the degree of freedom F such that a one-to-one relation exists between the signal S1, S2, S3, S4 and the actual value. In other words, the validity range F.sub.validity is the range in which the actual value is correctly determined by determining the signal S1, S2, S3, S4.

(29) It may be verified whether the actual value is within the validity range F.sub.validity. This may be accomplished, for instance, by evaluating one of the signals S1, S2, S3, S4 correlating with the actual value of the degree of freedom F by means of the state machine 30. If, for instance, a beat of two subsequent frequency comb modes is provided by the second measurement device M2 and is converted into an electrical signal, the post-processing unit 22 may be configured as a beat analyzer. It may be used to first supply the electrical beat signal to a frequency filter configured to pass frequencies within a determined range only, and then the power level downstream of the frequency filter is supplied to the state machine 30 as signal S3. The state machine 30 may then determine on the basis of the power level whether the actual value of the degree of freedom F, for example, the mode spacing f.sub.rep, is within the validity range F.sub.validity of a signal S1, S2, S3, S4.

(30) In case that the actual value is outside of the set validity range F.sub.validity of a signal used as first input signal for driving an actuator 8a, 8b, 8c, 8d, 8e, the adjustment process is temporarily interrupted by those first control loops 12 that use the corresponding signal S1, S2.

(31) If the actual value for any signals S1, S2 used as first input signals is outside of the corresponding validity range F.sub.validity, all of the first control loops 12 are deactivated and the control value of at least one actuator (for example, of the step motor 8a in FIG. 4) is varied in a stochastic or uniform manner until the validity range F.sub.validity is reached again in order to reenter into the range of reliable measurements and, thus, a meaningful adjustment by the first control loops 12. To this end, the state machine 30 communicates with the function generator G that controls the stochastic or uniform variation of the actuator control value depending on an input of the state machine 30. After having received information on preceding signals S1, S2, S3, S4 and actuator adjustments, the state machine 30 may control the function generator G depending on preceding states so as to reenter as fast as possible the validity range F.sub.validity of at least one signal S1, S2 used as first input signal.

(32) On the basis of at least one signal S1, S2, S3, S4 provided to the state machine 30 and correlating with the actual value of the degree of freedom F, the state machine 30 determines whether the actual value is within the capture range F.sub.capture of the second control unit 40. As illustrated in FIG. 3, the capture range F.sub.capture of the second control unit 40 may be within the validity range F.sub.validity, wherein the target value F.sub.intended is within the validity range F.sub.validity and within the capture range F.sub.capture, preferably substantially centered. The capture range F.sub.capture may be determined as the range in which the second control unit 40 is capable of controlling the actual value to the target value F.sub.intended. To this end, the target value F.sub.intended is within the capture range F.sub.capture, preferably centered. A control of the actual value to the target value F.sub.intended is accomplished, according to the present invention, if the actual value is within a stabilizing range F.sub.stabilizing that includes the target value and, in particular, is centered around the target value F.sub.intended, i.e., a portion of the capture range F.sub.capture. The stabilizing range F.sub.stabilizing in this respect may allow a deviation of the actual value from the target value F.sub.intended by less than 1%, less than 3%, less than 5% or less than 10% of the target value F.sub.intended. Alternatively, the stabilizing range F.sub.stabilizing may allow a deviation of the actual value from the target value F.sub.intended by only several Hz or even mHz, for instance by 1 mHz, 10 mHz, 2 Hz, 5 Hz, 10 Hz, 20 Hz or 50 Hz at most.

(33) When the state machine 30 determines that the actual value is within the capture range F.sub.capture of the second control unit 40, then the second control unit 40 is activated, preferably by the state machine 30. Thereafter, the actual value is controlled to the target value F.sub.intended by means of the second control unit 40. To this end, the second control unit 40 comprises one or more second control loops 42. These control loops control one or more actuators 8a, 8b, 8c, 8d, 8e so as to affect the actual value. To this end, the second control unit 40 may access actuators 8a, 8b, 8c, 8d, 8e that are also used by the first control unit 10. This configuration is implemented in the embodiment shown in FIG. 4 for the step motor 8a. Additionally or alternatively, the second control loops 42 may control one or more actuators 8a, 8b, 8c, 8d, 8e that are not used by the first control unit 10. This is the case in FIG. 4 for the piezo-electric motor 8b and the power controller 8d for the pump laser radiation P. It goes without saying that also different or further actuators may be controlled, such as an electro-optical modulator 8c or a tilting device 8e for a resonator mirror 6a-6e or a rotatable waveplate, as is known from post-published document DE 10 2014 204 941.5.

(34) The second control loops 42 of the second control unit 40 control the corresponding actuators 8a, 8b, 8d on the basis of a signal S4 transmitted thereto as a second input signal from the set of the at least one signal S1, S2, S3, S4 that correlates with an actual value of degree of freedom F. It is also an option that the second control unit 40 may be supplied with several second input signals, based on which the second control loops 42 may control the actuators 8a, 8b, 8c, 8d, 8e. In the embodiment of FIG. 4, the single second input signal is derived from the measurement value W2 of the second measurement device M2 by means of the post-processing unit 24.

(35) As shown in FIG. 4, the second control loops 42 are preferably provided as a cascade. This is particularly advantageous when each of the second control loops 42 accesses a different one of the actuators 8a, 8b, 8c, 8d, 8e. In view of optimizing usage of the signal, it may be advantageous for the first control unit 10 to obtain at least a second input signal. It may be output to one or more first control loops 12. In particular, the state machine 30 should receive the second input signals. This is particularly advantageous when the state machine 30 controls the cascading of the second control loops 42. To this end, the first control unit 10, in particular, the state machine 30, may activate or deactivate, preferably individually, one or more of the second control loops 42.

(36) Based on available data, in particular, based on the first or second input signals, the state machine 30 may assess whether a state of successful stabilizing of the frequency comb has been reached. This is so when the actual value of the degree of freedom F coincides with the target or intended value F.sub.intended or is within a stabilizing range F.sub.stabilizing. Thereafter, the state machine 30 may display or forward a message indicating the state of successful stabilizing. For example, such a message may be output on the screen 80 or may be input into a computer.

(37) The state machine 30, as well as none, one or several of the first control loops 12 and/or the function generator G may be implemented as a computer executable program, preferably stored on a computer-readable medium.

(38) In particular, fiber lasers may be provided as the inventive frequency comb generators, in particular, the fiber lasers including a non-linear optical mirror (NOLM, non-linear optical loop mirror) or saturable absorbers. A preferred embodiment includes the polarization-maintaining fibers. This results in a particularly efficient stability of the generated frequency comb.

(39) As an example, the automatic stabilizing of the mode spacing of a frequency comb may be considered and may include the following steps:

(40) a) detecting the mode spacing, for instance, by counting the pulse repetition rate of the fs-laser by means of an electronic counter, for instance 252 MHz,

(41) b) entering the counter values in a computer, forwarding the counter values to a software-based state machine for controlling the further steps,

(42) c) comparing to the given target value of, for instance, 250 MHz,

(43) d) setting the step size and direction of change (repetition rate is too high by 2 MHz in the example selected),

(44) e) coarsely varying the mode spacing by means of a step motor that changes the resonator length of the fs-laser towards the capture range of the second control,

(45) f) iterating the last three steps c) to e) (first control),

(46) g) if the actual value is within the capture range of the second control (for example, with a deviation of 1 KHz at the most), activating the second control,

(47) h) from this point on: performing detection by means of a phase detector 24 that outputs a signal proportional to the phase difference between the target value and the actual value.

(48) Thereafter, the control to the target value is accomplished with a deviation of, for instance, only 1 mHz by means of a piezo-electric actuator. In case the stabilizing process exits the lock range, for instance, due to external influences, the method is started again at the beginning, that is, starting with step a).