Method and device for quantitatively sensing the power fraction of a radiation background of a pulsed laser

Abstract

The present disclosure provides a method and to a device for quantitatively sensing the power fraction of a radiation background of a pulsed laser. The disclosure further relates to the use of a saturable element. The method includes modulating a measurement beam, which is emitted by the laser, by means of a saturable element in accordance with the fluence of the measurement beam, detecting, by means of a modulation beam power detector, the power of the measurement beam modulated by the saturable element, and determining the power fraction of the radiation background of the pulsed laser on the basis of the detected power of the measurement beam modulated by means of the saturable element.

Claims

1. A method for quantitatively detecting the power proportion of a radiation background of a pulsed laser, the method comprising: modulating a measurement beam emitted by the laser using a saturable element, wherein the beam is modulated depending on a fluence of the measurement beam; detecting the power of the measurement beam modulated by the saturable element using a modulation beam power detector; and determining the power proportion of the radiation background of the pulsed laser based on the detected power, wherein the power proportion of the radiation background is the proportion of the radiation background to a mean pulse power of the pulsed laser over time.

2. The method according to claim 1, wherein the method comprises, prior to modulating the measurement beam, splitting a laser beam emitted by the laser into a reference beam and the measurement beam using a beam splitter, and wherein prior to the determining the power proportion, the method further comprises detecting the power of the reference beam using a reference beam power detector.

3. The method according to claim 2, wherein prior to detecting the power, the method further comprises: adjusting the fluence of the measurement beam occurring at the saturable element using a fluence adjusting element such that the saturable element is in an unsaturated state as a consequence of the radiation background of the measurement beam, and in a saturated state as a consequence of a pulse of the measurement beam.

4. The method according to claim 2, wherein modulating, detecting, and determining are repeated at least once with another saturable element, and wherein averaging over the determined power proportions of the radiation background of the pulsed laser is performed.

5. The method according to claim 2, further comprising, at the beginning of the method: calibrating the saturable element using a combined beam source, wherein the combined beam source includes a cw laser with adjustable cw power and a pulsed laser with adjustable pulse power.

6. The method according to claim 1, wherein prior to detecting the power, the method further comprises: adjusting the fluence of the measurement beam occurring at the saturable element using a fluence adjusting element such that the saturable element is in an unsaturated state as a consequence of the radiation background of the measurement beam, and in a saturated state as a consequence of a pulse of the measurement beam.

7. The method according to claim 6, wherein modulating, detecting, and determining are repeated at least once with another saturable element, and wherein averaging over the determined power proportions of the radiation background of the pulsed laser is performed.

8. The method according to claim 6, further comprising, at the beginning of the method: calibrating the saturable element using a combined beam source, wherein the combined beam source includes a cw laser with adjustable cw power and a pulsed laser with adjustable pulse power.

9. The method according to claim 1, wherein modulating, detecting, and determining are repeated at least once with another saturable element, and wherein averaging over the determined power proportions of the radiation background of the pulsed laser is performed.

10. The method according to claim 9, further comprising, at the beginning of the method: calibrating the saturable element using a combined beam source, wherein the combined beam source includes a cw laser with adjustable cw power and a pulsed laser with adjustable pulse power.

11. The method according to claim 1, further comprising, at the beginning of the method: calibrating the saturable element using a combined beam source, wherein the combined beam source includes a cw laser with adjustable cw power and a pulsed laser with adjustable pulse power.

12. A device for quantitatively detecting the power proportion of a radiation background of a pulsed laser, the device comprising: a saturable element for modulating a measurement beam of the laser incident on the saturable element, wherein the modulation of the measurement beam by the saturable element depends on the fluence of the measurement beam; a modulation beam power detector for detecting a power of the measurement beam modulated by the saturable element; and an evaluator for determining the power proportion of the radiation background of the pulsed laser based on the power of the measurement beam detected by the modulation beam power detector, wherein the power proportion of the radiation background is the proportion of the radiation background to a mean pulse power of the pulsed laser over time.

13. The device according to claim 12, wherein the saturable element is designed such that the saturable element is in an unsaturated state as a consequence of the radiation background of the measurement beam, and is in a saturated state as a consequence of a pulse of the measurement beam.

14. The device according to claim 13, further comprising: a beam splitter for splitting the laser beam emitted by the laser into a reference beam and the measurement beam; a reference beam power detector for detecting the power of the reference beam; wherein the beam splitter is at least one of arranged and aligned such that the reference beam impinges on the reference beam power detector and the measurement beam impinges on the saturable element.

15. The device according to claim 13, wherein the saturable element is a saturable absorber.

16. The device according to claim 13, further comprising: a fluence adjusting element for adjusting the fluence of the measurement beam incident on the saturable element.

17. The device according to claim 12, further comprising: a beam splitter for splitting the laser beam emitted by the laser into a reference beam and the measurement beam; a reference beam power detector for detecting the power of the reference beam; wherein the beam splitter is at least one of arranged and aligned such that the reference beam impinges on the reference beam power detector and the measurement beam impinges on the saturable element.

18. The device according to claim 17, wherein the saturable element is a saturable absorber.

19. The device according to claim 12, wherein the saturable element is a saturable absorber.

20. The device according to claim 12, further comprising: a fluence adjusting element for adjusting the fluence of the measurement beam incident on the saturable element.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic drawing of a device according to an embodiment;

(2) FIG. 2 shows a schematic drawing of a device according to another embodiment;

(3) FIG. 3 shows an example profile of the reflectivity of a saturable absorber mirror as a function of the incident fluence;

(4) FIG. 4 shows a schematic drawing of a device according to another embodiment.

DETAILED DESCRIPTION

(5) FIG. 1 shows a schematic drawing of a device 100 for quantitatively detecting the power proportion of a radiation background of a pulsed laser according to one embodiment. The device includes a saturable member 13, a modulation beam power detector 15, and an evaluator (not shown in FIG. 1). In the embodiment of FIG. 1, the saturable element is transmissive, i.e. it transmits an impinging or incident laser beam 3, wherein the power of the transmitted beam 5 compared to the incident beam 3 is modulated depending on the fluence of the laser beam 3 impinging or incident on the saturable element. Depending on the fluence of the impinging or incident laser beam 3, the power of the transmitted beam 5 differs from the power of the incident beam 3, which is also referred to as a measurement beam.

(6) The laser beam or measurement beam 3 to be analyzed is propagated through the transmissive saturable element 13 and the power remaining in the modulated or transmitted beam 5 is measured by means of the modulation beam power detector 15, which in particular comprises a photodiode or a power measuring head.

(7) FIG. 2 shows a schematic drawing of a device 100 for quantitatively detecting the power proportion of a radiation background of a pulsed laser according to another embodiment. In the embodiment of FIG. 2, the saturable element is reflective, i.e. it reflects an impinging or incident laser beam 3, wherein the power of the reflected beam 5 compared to the incident beam 3 is modulated depending on the fluence of the laser beam 3 impinging or incident on the saturable element. Depending on the fluence of the impinging or incident laser beam 3, the power of the reflected beam 5 differs from the power of the incident beam or measurement beam 3.

(8) The laser beam or measurement beam 3 to be analyzed is reflected by the saturable element 13 and the power remaining in the modulated or reflected beam 5 is measured by means of the modulation beam power detector 15.

(9) The reflective saturable element may be a saturable absorber mirror. The modulation beam power detector 15 may include a photodiode or a power measuring head.

(10) FIG. 3 shows an example profile of the reflectivity of a saturable absorber mirror in dependence on the incident fluence. Here, the curve 40 shown in the diagram represents an ideal profile, whereas the curve 50 represents a real profile. In the fluence curve of the exemplary saturable element shown in FIG. 3, the degree of modulation or the reflectivity continuously increases as the fluence increases in the range of about 0.01 to 25 J/cm.sup.2. While the reflectivity with the ideal curve 40 increases even in case of a fluence increase in the range of >25 J/cm.sup.2, the reflectivity according to the real curve 50 decreases starting from about 25 J/cm.sup.2. This decrease may in particular result from a two-photon absorption. The fluence range in which the degree of modulation or the reflectivity decrease again with the real curve 50 is referred to as a roll-over range.

(11) As shown in the diagram of FIG. 3, the saturable element or its fluence curve is characterized by the modulation depth R and by non-saturable losses R.sub.ns. To achieve a high measuring accuracy, it is advantageous that the modulation depth be as great as possible and the non-saturable losses be as small as possible. F.sub.Sat in FIG. 3 designates a saturation fluence of the saturable element. As shown in FIG. 3, for example, this saturation fluence can be determined or specified by the turning point of the fluence curve. However, it is also possible to define the saturation fluence in a different way, for example as a fluence value between the turning point and the maximum of the fluence curve shown in the FIG. 3, or as a fluence value corresponding to the maximum of the fluence curve. With a fluence greater than F.sub.Sat, the saturable element is substantially or fully saturated, i.e. is in a substantially or fully saturated state. Complete saturation of the saturable element is in particular the case at the maximum of the real curve 50, i.e. at about 50 J/cm.sup.2 in the example of FIG. 3.

(12) The transmissivity or reflectivity of the saturable element 13 of FIGS. 1 and 2 depends on the fluence, i.e. on the energy per unit area, of the laser pulse or laser beam 3. As the fluence dependence of the transmissivity or reflectivity of the saturable element 13 is known or can be determined experimentally, it is possible to calculate back to the power proportion of the laser pulses in relation to the cw background and/or to other noise by measuring the mean power P.sub.Sat of the transmitted or reflected laser beam 5. The incident measurement beam 3 has a power P.sub.Ref, which is composed of the power of the radiation or cw background P.sub.cw and the mean power of the laser pulses P.sub.puls:
P.sub.Ref=P.sub.cw+P.sub.puls (3).fwdarw.P.sub.cw=P.sub.RefP.sub.puls(3A).

(13) The beam is transmitted through the saturable element 13 or reflected thereon, wherein a proportion R.sub.ns of the beam is absorbed by non-saturable losses. In the fully saturated state, the saturable element has a reflectivity or transmissivity increased by the modulation depth R. If the saturable element is substantially or fully saturated (see FIG. 3), this results in the power in the modulated beam 5 according to:
P.sub.Sat=P.sub.cw(1R.sub.nsR)+P.sub.puls (1R.sub.ns)(4).

(14) Inserting equation (3A) into equation (4) yields P.sub.puls:

(15) $\begin{array}{cc}{P}_{\mathrm{Sat}}=\left(P_{\mathrm{Ref}}-P_{\mathrm{puls}}\right)\left(1-R_{\mathrm{ns}}-R\right)+P_{\mathrm{puls}}\left(1-R_{\mathrm{ns}}\right)=P_{\mathrm{Ref}}\left(1-R_{\mathrm{ns}}-R\right)+P_{\mathrm{puls}}R.fwdarw.P_{\mathrm{puls}}=\frac{P_{\mathrm{Sat}}-P_{\mathrm{Ref}}\left(1-R_{\mathrm{ns}}-R\right)}{R}.& \left(5\right)\end{array}$

(16) Inserting equation (5) into equation (3A) yields P.sub.cw:

(17) $\begin{array}{cc}.fwdarw.P_{\mathrm{cw}}=P_{\mathrm{Ref}}-\frac{P_{\mathrm{Sat}}-P_{\mathrm{Ref}}\left(1-R_{\mathrm{ns}}-R\right)}{R}=\frac{P_{\mathrm{Ref}}\left(1-R_{\mathrm{ns}}\right)-P_{\mathrm{Sat}}}{R}.& \left(6\right)\end{array}$

(18) Thus, the ratio of the power of the radiation background and the pulse power results from:

(19) $\begin{array}{cc}\frac{P_{\mathrm{cw}}}{P_{\mathrm{puls}}}=\frac{P_{\mathrm{Ref}}\left(1-R_{\mathrm{ns}}\right)-P_{\mathrm{Sat}}}{P_{\mathrm{Sat}}-P_{\mathrm{Ref}}\left(1-R_{\mathrm{ns}}-R\right)}=\frac{\left(1-R_{\mathrm{ns}}\right)-\frac{P_{\mathrm{Sat}}}{P_{\mathrm{Ref}}}}{\frac{P_{\mathrm{Sat}}}{P_{\mathrm{Ref}}}-\left(1-R_{\mathrm{ns}}-R\right)}.& \left(7\right)\end{array}$

(20) In case of a known overall power of the laser P.sub.Ref to be analyzed and with known values of R and R.sub.ns of the saturable element 13 used, it is possible to determine the ratio of the power of the radiation background and the pulse power by measuring the power P.sub.Sat of the modulated measurement beam 5 according to equation (7).

(21) FIG. 4 shows a schematic drawing of a device 100 for quantitatively detecting the power proportion of a radiation background of a pulsed laser in accordance with another embodiment. In comparison to the embodiments of FIGS. 1 and 2, the device according to the embodiment of FIG. 4 additionally includes a beam splitter 20 and a fluence adjusting element or lens 30. The beam splitter 20 splits the laser beam 1 emitted by a laser into a measurement beam 3 and a reference beam 7. The measurement beam impinges on the saturable element or the saturable absorber mirror 13 and is modulated or reflected thereby. The modulated or reflected beam 5 will eventually be directed to a modulation beam power detector by means of the beam splitter 20, with which the power of the modulated beam is 5 measured. By means of a reference beam power detector 17, the power P.sub.Ref of the reference beam 7 is measured. As shown in FIG. 4, the beam splitter 20 may be a conventional beam splitter, which performs a splitting of the laser beam into power proportions.

(22) According to FIG. 4, the laser beam 1 to be analyzed is split into the measurement beam 3 and the reference beam 7 by means of the beam splitter 20 with a known split ratio, for example with 50% of the power in each of the partial beams. Alternatively or in addition, the beam splitter 20 may be calibrated prior to the measurement e.g. by means of a highly reflective mirror disposed at the location of the saturable element 13.

(23) The reflectivity of the saturable absorber mirror depends on the fluence of the measurement beam 3. Since this fluence dependence of the reflectivity of the saturable absorber mirror is known, it is possible to calculate back to the power proportion of the laser pulses in relation to the cw background and/or to other noise by measuring the mean power P.sub.Sat of the modulated or reflected laser beam.

(24) If the reflectivity of the beam splitter 20 is designated with S, the power of the reference beam 7 is:
P.sub.Ref=S(P.sub.cw+P.sub.puls)(8).

(25) It follows for the power P.sub.cw of the background radiation:

(26) $\begin{array}{cc}{P}_{\mathrm{cw}}=\frac{P_{\mathrm{Ref}}}{S}-P_{\mathrm{puls}}.& \left(8A\right)\end{array}$

(27) The power P.sub.Sat the modulated measurement beam 5 is:
P.sub.Sat=S(1S)[P.sub.cw(1R.sub.nsR)+P.sub.puls (1R.sub.ns)](9).

(28) Inserting equation (8A) into equation (9) yields for the mean power P.sub.puls of the pulse radiation:

(29) $\begin{array}{cc}{P}_{\mathrm{Sat}}=S\left(1-S\right)\left[\left(\frac{P_{\mathrm{Ref}}}{S}-P_{\mathrm{puls}}\right)\left(1-R_{\mathrm{ns}}-R\right)+P_{\mathrm{puls}}\left(1-R_{\mathrm{ns}}\right)\right]=S\left(1-S\right)\left[\frac{P_{\mathrm{Ref}}}{S}\left(1-R_{\mathrm{ns}}-R\right)+P_{\mathrm{puls}}R\right].fwdarw.P_{\mathrm{puls}}=\frac{\frac{P_{\mathrm{Sat}}}{S\left(1-S\right)}-\frac{P_{\mathrm{Ref}}}{S}\left(1-R_{\mathrm{ns}}-R\right)}{R}.& \left(10\right)\end{array}$

(30) Inserting equation (10) into equation (8A) yields for P.sub.cw:

(31) $\begin{array}{cc}{P}_{\mathrm{cw}}=\frac{P_{\mathrm{Ref}}}{S}-\frac{\frac{P_{\mathrm{Sat}}}{S\left(1-S\right)}-\frac{P_{\mathrm{Ref}}}{S}\left(1-R_{\mathrm{ns}}-R\right)}{R}=\frac{\frac{P_{\mathrm{Ref}}}{S}\left(1-R_{\mathrm{ns}}\right)-\frac{P_{\mathrm{Sat}}}{S\left(1-S\right)}}{R}.& \left(11\right)\end{array}$

(32) Thus, the ratio of the power of the radiation background and pulse power results from:

(33) $\begin{array}{cc}\frac{P_{\mathrm{cw}}}{P_{\mathrm{puls}}}=\frac{\frac{P_{\mathrm{Ref}}}{S}\left(1-R_{\mathrm{ns}}\right)-\frac{P_{\mathrm{Sat}}}{S\left(1-S\right)}}{\frac{P_{\mathrm{Sat}}}{S\left(1-S\right)}-\frac{P_{\mathrm{Ref}}}{S}\left(1-R_{\mathrm{ns}}-R\right)}=\frac{\left(1-R_{\mathrm{ns}}\right)-\frac{P_{\mathrm{Sat}}}{\left(1-S\right)P_{\mathrm{Ref}}}}{\frac{P_{\mathrm{Sat}}}{\left(1-S\right)P_{\mathrm{Ref}}}-\left(1-R_{\mathrm{ns}}-R\right)}=\frac{\left(1-S\right)\left(1-R_{\mathrm{ns}}\right)-\frac{P_{\mathrm{Sat}}}{P_{\mathrm{Ref}}}}{\frac{P_{\mathrm{Sat}}}{P_{\mathrm{Ref}}}-\left(1-S\right)\left(1-R_{\mathrm{ns}}-R\right)}.& \left(12\right)\end{array}$

(34) With a known reflectivity S of the beam splitter 20 used and with known values of R and R.sub.ns of the saturable element 13 used, the ratio of the power of the radiation background and the pulse power can be determined according to equation (12) by measuring the power P.sub.Sat of the modulated measurement beam 5 and by measuring the power of P.sub.Fef of the reference beam 7.

(35) The saturable element 13 has a profile of the transmission/reflection that is dependent on the fluence (see FIG. 3). In order to avoid a systematic measurement error due to insufficient saturation of the saturable element 13, the fluence of the pulses to be measured and of the cw background can be adapted to the fluence curve of the saturable element 13. To this end, as shown in FIG. 4, an axially displaceable lens or lens 30 displaceable along the optical path may be used. This lens 30 is arranged between the beam splitter 20 and the saturable element 13. If the saturable element 13 is a saturable absorber mirror, for example, it can be guaranteed that the pulses saturate the saturable absorber mirror maximally. In this case, a saturation is achieved at a minimum of the ratio P.sub.cw/P.sub.puls.

(36) For the above-described measurements it must be provided that the power of the radiation background, i.e. in particular the mean cw power of the laser to be analyzed is not sufficient to saturate the saturable element. In contrast, the saturable element must be saturated, and may be fully saturated for a pulse peak power. This condition is usually easily met for common pulsed lasers. This is because the peak power of an individual laser pulse is generally much higher than the mean power of the background radiation or the cw background of the laser. Thus, for example, a laser with a pulse duration of 5 ps, a repetition rate of 1 MHz, and a mean pulsed power of 50 W (mean total power of 100 W) has a pulse peak power of 10 MW. Thus, the cw background proportion is 50% of the mean total power of the laser. Hence, the peak power of a pulse is greater than the power of the cw background by a factor of 2.Math.10.sup.5. Based on this high power difference, the above-mentioned requirement can be met easily. For many pulsed laser systems to be measured an even greater difference between pulse peak power and mean power of the cw background can be expected, as the proportion of the cw background to the total power is typically much lower.

(37) In the case of saturable elements having a roll-over range in the fluence curve (see FIG. 3), it is crucial to the measuring accuracy that the saturable element does not get into the roll-over range upon impingement of a laser pulse. To avoid this, the fluence impinging on the saturable element 13 can be adapted or adjusted by means of the fluence adjusting element or the lens 30 shown in FIG. 4. By appropriately adjusting the fluence, i.e. by a suitably displacing the lens along the optical path between the beam splitter 20 and the saturable element 13, one can achieve that the saturable element 13 assumes an unsaturated state upon impingement of the radiation background, on the one hand. On the other hand, on can achieve that the saturable element 13 assumes a saturated state upon impingement of a laser pulse, without getting into the roll-over range.

(38) The fluence required for a complete saturation can be determined from the ratio P.sub.cw/P.sub.puls with the measurement setup or device 100 according to the invention. Due to the characteristic saturation behavior of the saturable element or saturable absorber mirror 13, for example, it is expected according to the exemplary curve of FIG. 3 that the ratio P.sub.cw/P.sub.puls in the fluence region of the complete saturation (for the laser pluses) has a minimum, since here the maximum reflectivity of the saturable element or saturable absorber mirror 13 is achieved.