Tunable laser with external cavity

Abstract

To provide: a tunable laser, in particular, an external cavity type laser designed for fast and repeatable wavelength tuning over a wide wavelength range; and a method of operating the tunable laser.SOLUTION: A tunable laser 10 comprises: a cavity 11; a gain medium 12 arranged in a region of the cavity the gain medium being configured to generate light by stimulated emission and to emit the light into the cavity; and a laser output configured to output a portion of the light in the cavity as a laser beam. The cavity includes a first resonator 13 and a second resonator 14 that are optically coupled to each other. Therein: a resonance frequency of the first resonator is tunable; a free spectral range (FSR) of the second resonator is an integer multiple of a FSR of the cavity.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to tunable lasers, particularly external cavity lasers. The present disclosure also relates to a method of operating a tunable laser. The tunable laser of the present disclosure is designed for fast and repeatable wavelength tuning over a wide wavelength range. [Background technology]

[0002] In integrated photonics, it is becoming increasingly important to integrate a suitable light source, e.g. a laser diode, into a photonic integrated circuit (PIC) to design a tunable laser. The tuning of the laser's spectral properties, such as wavelength, tunability, spectral linewidth, coherence length and relative intensity noise, can be achieved by properly designing and fabricating an additional external cavity in the PIC.

[0003] Examples of such external cavities include a set of tunable resonators, such as a set of two or more coupled ring resonators with slightly different circumferences. Such an external cavity with tunable ring resonators can be coupled, for example, to a linear resonator or cavity of a light source containing a gain medium, such as a laser diode.

[0004] The external cavity makes it possible to take advantage of the well-known Vernier effect, which effectively increases the tunability of the laser while reducing the linewidth and further increasing the spectral purity.

[0005] However, there remains a need to improve such integrated external cavity lasers, and in particular to find new designs for the external cavity. Summary of the Invention

[0006] The solution of the present disclosure is further based on the following considerations.

[0007] In external cavity lasers of the type mentioned above, wavelength tuning is typically achieved via a relatively slow mechanism, such as phasing the various resonators with phase shifters, e.g., heaters, etc. By phasing the resonators, a predetermined laser wavelength can be specified.

[0008] For example, the resonant mode of the second (untuned, i.e., stationary) ring resonator can also be addressed by tuning the phase of the first ring resonator of the external cavity, addressing a lower wavelength first and then moving to a higher wavelength. However, for each wavelength, the phase of the linear resonator must also be adjusted to fit the wavelength into the entire cavity of the laser and to avoid power loss. In other words, the speed of the phase shifter for the linear resonator becomes the bottleneck for the overall tuning speed of the laser. As a result, the above-mentioned switching scheme from the lowest to the highest wavelength of the first ring resonator is not practically scalable.

[0009] In view of this, the present disclosure aims to provide an improved tunable laser with an external cavity, in particular one that is fast tunable without experiencing power loss, in particular to avoid the bottleneck mentioned above of having to adjust the phase shifter of a linear resonator when implementing a scheme in which the first resonator is tuned while the second resonator is kept stationary.

[0010] These and other objects are achieved by the solution of the present disclosure which is set forth in the independent claims. Advantageous implementations are set forth in the dependent claims.

[0011] A first aspect of the present disclosure provides a tunable laser including a cavity; a gain medium disposed within a region of the cavity, the gain medium configured to generate light by stimulated emission and emit the light into the cavity; and a laser output configured to output a portion of the light in the cavity as a laser beam, the cavity including a first resonator and a second resonator optically coupled to each other, wherein a resonant frequency of the first resonator is tunable and a resonant frequency spacing (FSR: Free Spectral Range) of the second resonator is an integer multiple of the FSR of the cavity.

[0012] Since the FSR of the second resonator is matched to the FSR of the entire cavity of the tunable laser in the manner described above, the first resonator can be tuned while the second resonator is held stationary, without any power loss even if the other phase shifters are not in operation. Thus, in the laser of the first embodiment, only the first resonator needs to be tunable. Also, the tunable laser can be tuned at high speed without any power loss. The design of the second resonator and laser cavity allows to avoid the above-mentioned bottleneck of linear resonator phase shifters that need to be tuned while tuning the first resonator and keeping the second resonator stationary.

[0013] The gain medium may be based on a semiconductor material, for example a semiconductor laser diode. The laser diode may include a cavity region in which the gain medium is located. The resonator may form an external cavity coupled to the laser diode. The laser diode and the external cavity may be integrated on the PIC.

[0014] In a laser embodiment, the cavity includes a linear resonator, the gain medium is disposed within a region of the linear resonator, and the linear resonator is optically coupled to the first and second resonators.

[0015] The gain medium and the linear resonator may form, for example, a laser diode. The laser diode may be coupled to an external cavity that includes the first and second resonators. The resulting tunable laser may be a hybrid external cavity laser.

[0016] In an embodiment of the laser, the linear resonator is optically coupled to the first and second resonators by an optical splitter configured to allow light to couple from the linear resonator to the first and second resonators and light to couple from the first and second resonators to the linear resonator.

[0017] In the laser embodiment, the optical splitter is further optically coupled to the laser output.

[0018] In one embodiment of the laser, the resonant frequency of the linear resonator is tunable.

[0019] That is, in the laser of the first embodiment, the linear resonator can be tunable, but this is not essential. The tunability of the linear resonator may improve the overall tunability of the laser, but is not essential for the above-mentioned wavelength tuning scheme. The tunability of the linear resonator may be useful, for example, to define the tunable wavelength range of the laser by setting the starting point.

[0020] In a laser embodiment, the resonant frequency of the second resonator is tunable.

[0021] That is, although the second cavity is tunable in the laser of the first embodiment, this is not required: tunability of the second cavity may increase the tunability of the laser as a whole, but is not required for the wavelength tuning scheme described above, particularly since the second cavity is kept stationary in this scheme anyway.

[0022] In an embodiment of the laser, the laser further comprises one or more phase shifters, a first of the one or more phase shifters configured to adjust a resonant frequency of the first resonator.

[0023] In an embodiment of the laser, the second phase shifter of the phase shifter is configured to adjust the resonant frequency of the second resonator and / or the third phase shifter of the phase shifter is configured to adjust the resonant frequency of the linear resonator.

[0024] In an embodiment of the laser, at least one of the resonant frequency of the second resonator and the resonant frequency of the linear resonator is not tunable.

[0025] In an embodiment of the laser, the first resonator is a first ring resonator and / or the second resonator is a second ring resonator.

[0026] The first and second resonators may alternatively be Mach-Zehnder interferometers, or Fabry-Perot interferometers, or similar types of resonators.

[0027] In an embodiment of the laser, the first ring resonator has a different circumference than the second ring resonator.

[0028] This results in different resonant frequencies of the ring resonators, thereby improving the tunability of the laser.

[0029] In a laser embodiment, the cavity includes one or more additional resonators optically coupled to each other and to the first and second resonators.

[0030] In a laser embodiment, the resonant frequency of at least one of the additional resonators is tunable.

[0031] The additional resonator or resonators contribute to the overall cavity and therefore to the FSR of the overall cavity.

[0032] A second aspect of the present disclosure provides a method for operating a tunable laser system of the first aspect or any implementation thereof, the method including tuning a resonant frequency of a first resonator and tuning the resonant frequency of the first resonator while keeping a resonant frequency of a second resonator fixed.

[0033] In an embodiment of this method, adjusting the resonant frequency of the first resonator produces multiple peaks in the power of the laser beam output output by the laser output, each peak having equal power.

[0034] The method of the second aspect achieves the same advantages as the tunable laser of the first aspect and can be extended by each of the embodiments described above for the tunable laser of the first aspect, in that the laser can be tuned without power loss and at a faster speed compared to a similar laser where the FSR of the second resonator is not matched to the FSR of the cavity.

[0035] In summary of the above features and embodiments, the present disclosure provides a fast tunable external cavity laser with a set of resonators, e.g., ring resonators, in the external cavity. A preferred tuning scheme operates by tuning a first resonator of the resonator set from the lowest wavelength to the highest wavelength while keeping a second resonator of the resonator set (and optionally one or more additional resonators) stationary. The tunable laser is based on a specific cavity design in which the FSR of the second resonator and the entire laser cavity are matched as described above to enable fast tuning. [Brief description of the drawings]

[0036] The above aspects and embodiments are illustrated in the following description of examples which refer to the accompanying drawings.

[0037] [Figure 1] 1 shows a tunable laser according to the present disclosure. [Diagram 2]1 shows a tunable laser according to the present disclosure with two ring resonators and a linear resonator. [Diagram 3] 1 shows a tunable laser according to the present disclosure with two ring resonators, a linear resonator, and three phase shifters. [Figure 4] The filter response is shown when tuning the first or second ring resonator alone, or when tuning the coupled ring resonator. [Diagram 5] 13 shows the filter response for various settings when tuning two or more ring resonators of a tunable laser according to the present disclosure. [Figure 6] 1 shows the laser output power when the first cavity is tuned and the second cavity is kept stationary for (a) an exemplary laser and (b) a laser according to the present disclosure. [Figure 7] 1 illustrates a method of operating a tunable laser according to the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] 1 shows a tunable laser 10 according to the present disclosure. The tunable laser 10 includes a cavity 11, which includes at least a first resonator 13 and a second resonator 14. The tunable laser 10 may be referred to as an external cavity tunable laser, in which case the cavity 11 includes one or more external components (in this case the first and second resonators 13, 14, which may form an external cavity) to allow for greater control over the spectral characteristics of the laser beam generated by the tunable laser 10.

[0039] The laser 10 includes a gain medium 12 disposed within a region of the cavity 11. The gain medium 12 is configured to generate light by stimulated emission and emit the generated light into the cavity 11. The gain medium 12 need not be distributed throughout the cavity 11. For example, the gain medium 12 may be disposed within an external cavity that provides an optical path for the emitted light that extends beyond the immediate region containing the gain medium 12. The region containing the gain medium 12 may be a linear resonator, for example, the internal cavity of a laser diode, which may be coupled to first and second resonators 13, 14 that are part of an external cavity.

[0040] Laser 10 also includes a laser output 15, which is configured to output a portion of the light in cavity 11 as a laser beam. The light in cavity 11 may be amplified by passing through gain medium 12 one or more times and causing constructive interference within cavity 11, particularly in resonators 13, 14.

[0041] The first and second resonators 13, 14 are optically coupled to each other and at least the resonant frequency of the first resonator 13 is tunable. The resonant frequency of the second resonator 14 may or may not also be tunable. Because at least the first resonator 13 is tunable, the characteristics of the cavity 11 can be altered and the wavelength of the laser beam provided at the laser output 15 can be adjusted or "tuned."

[0042] The solution of the present disclosure is specifically in the design of the second resonator 14 and the cavity 11, in particular in matching the second resonator 14 to the cavity 11 as follows: According to the solution of the present disclosure, the FSR of the second resonator 14 is an integer multiple of the FSR of the cavity 11 (i.e., the entire cavity 11 including the second resonator 14). This is expressed mathematically as the following equation (1): TIFF2025016426000002.tif10153

[0043] In this formula, Δν ris the FSR of the second resonator 14 (in Hz), Δν tot is the FSR (in Hz) of the entire laser cavity 11, and n is a positive integer greater than zero. It is worth noting that the FSR may indicate the distance between adjacent resonance peaks, as explained below.

[0044] The advantages of this laser 10 design, and in particular the matching FSR of the second resonator 14 and the cavity 11, are explained in more detail below. Hereby, an example of a tunable laser 10 is used, which in particular comprises two ring resonators as the first and second resonators 13, 14. However, the solution of the present disclosure is not limited to the use of such ring resonators. The first and second resonators 13, 14, respectively, may be Mach-Zehnder interferometers, or Fabry-Perot interferometers, or others.

[0045] Figure 2 shows a tunable laser 10 according to the present disclosure that includes at least two ring resonators, based on the tunable laser 10 shown in Figure 1. The tunable laser 10 shown in Figure 2 has a first ring resonator 13 and a second ring resonator 14 that are part of an external cavity.

[0046] In FIG. 2, the layout of the laser 10 is shown including a gain region (left side) in which the gain medium 12 is contained, and showing a waveguide (right side) that defines the remaining part of the cavity 11. The cavity 11 includes a linear resonator 21, for example a laser diode, and the gain medium 12 may be located within the region of this linear resonator 21. The linear resonator 21 may be, for example, an internal cavity of a laser diode. The linear resonator 21 is optically coupled to an external cavity that includes at least a first and a second ring resonator 13, 14. For example, the linear resonator 21 may be coupled to the two ring resonators 13, 14 by an optical splitter 22. The optical splitter 22 is configured to couple light from the linear resonator 21 into the first and second ring resonators 13, 14, respectively, and also configured to couple light from the first and second ring resonators 13, 14, respectively, into the linear resonator 21.

[0047] The first ring resonator 13 and the second ring resonator 14 may have different circumferences, as shown in the schematic diagram of the tunable laser 10 in Figure 2. Also shown is a laser output 15 configured to output a laser beam and coupled to an optical splitter 22.

[0048] It should be noted that even when using ring resonators 13, 14, the solution of the present disclosure is not limited to this scheme. For example, cavity 11 may include one or more additional (e.g., ring) resonators that are optically coupled to each other and to the first and second resonators 13, 14. In one example, the resonant frequency of at least one of these additional resonators may be tunable.

[0049] In the tunable laser 10 shown in FIG. 2, wavelength tuning can be achieved by tuning at least the first ring resonator 13 with a phase shifter. For example, as shown in FIG. 3, the tunable laser 10 may include one or more phase shifters 31, 32, 33. The first phase shifter 32 may be arranged and configured to tune the resonance frequency of the first resonator 13. The first phase shifter 32 may be the only phase shifter of the tunable laser 10, i.e., the laser 10 may include only a single phase shifter 32 for tuning the first resonator 13. In this case, the resonance frequency of the second resonator 14 and the resonance frequency of the potential linear resonator 21 are not tunable.

[0050] However, to facilitate understanding of all possible implementations of the tunable laser 10, as shown in Fig. 3, the laser 10 may also include a second phase shifter 33 arranged and configured to adjust the resonant frequency of the second resonator 14. The laser 10 may also include a third phase shifter 31 that may be arranged and configured to adjust the resonant frequency of the linear resonator 21. In other words, at least one of the resonant frequency of the second resonator 14 and the resonant frequency of the linear resonator 21 is likewise adjustable. The (typical) positions of the phase shifters 31, 32, 33 are shown in Fig. 3 by thick black lines.

[0051] Each phase shifter 31, 32, 33 may be or include a heater, i.e., the tuning of the various resonators of laser 10 may be performed using heaters. Other phase shifters 31, 32, 33 and phase shifting mechanisms may also be used, for example, using stress to change the refractive index of the resonator or using liquid crystal techniques. By phase tuning at least the first ring resonator 13, a predetermined wavelength of laser 10 may be specified.

[0052] Figure 4(a) shows the filter response (power vs. wavelength) of an exemplary first ring resonator 13, and Figure 4(b) shows the filter response of an exemplary second ring resonator 14. Combining these filter responses results in the frequency response shown in Figure 3(d), while superimposing all three filter responses results in Figure 3(c).

[0053] An important parameter of the spectral response of a resonator is the so-called FSR. The FSR is defined as the distance between adjacent resonant frequencies of the resonator. For example, Fig. 4(a) shows that the FSR of the first ring resonator 13 is peak-to-peak. It should be noted in Fig. 4 that the FSRs of the ring resonators 13, 14 are slightly different. If the two ring resonators 13 and 14 are coupled in series (as in Figs. 2 and 3), the combined filter response is as shown by the bold lines in Figs. 3(c) and 3(d). In Fig. 3(c), the bold dotted line represents the filter response of the second ring resonator 14 and the thin dotted line represents the filter response of the first ring resonator 13.

[0054] The tunable laser 10 of the present disclosure may be operated according to a tuning regime in which the first ring resonator 13 is tuned over 2π and the second ring resonator 14 is kept stationary (i.e., not tuned, whether tunable or not). During such tuning, the combined filter response moves over the entire spectrum, thereby accommodating all peaks of the second ring resonator 14. Portions of the resulting spectra corresponding to different settings during tuning are shown in FIG. 5. Looking at λ1 in FIG. 5(a), λ2 in FIG. 5(b), λ3 in FIG. 5(c), and finally λx in FIG. 5(d), respectively, the small dotted line moves towards the right side of the graph. The arrows between FIG. 5(c) and FIG. 5(d) indicate that the sequence λ1, λ2, λ3 is repeated equally until λx is reached. When the frequency response again looks like λ1, a phase shift of 2π will have occurred.

[0055] When the exemplary spectra shown in Figures 4 and 5 are fed back into the gain region, i.e., the gain medium 12 of the laser 10 shown in Figure 2, a laser beam is emitted when there is sufficient coherent accumulation of light in the cavity 11. This occurs when an integer wave number fits within the cavity 11, causing constructive interference. If the wavelength is slightly larger, constructive interference still occurs in the cavity 11, but it is less pronounced and the laser output drops. If the wavelength is large enough, destructive interference becomes dominant and in this case the laser 10 emits (almost) no coherent light. This can be measured by a significant reduction in the laser output, but also seen in the output spectrum of the laser 10 (loss of laser lines).

[0056] In the tunable laser 10, if the FSR of the second ring resonator 14 is not an integer multiple of the FSR of the cavity 11, the above wavelength tuning scheme must be performed as follows. When tuning the first resonator 13 from the lowest wavelength to the highest wavelength, the wavelengths do not necessarily all fit into the cavity 11 to correspond to the respective wavelengths of the second resonator 14. Therefore, the phase shifter of the linear resonator 21 must be operated further to tune the linear resonator 21 to ensure that the wavelengths match. That is, the wavelength tuning scheme is performed from setting to setting. If a typical type of phase shifter such as a heater is used, this is a rather slow tuning mechanism. Since the tuning is not instantaneous, a certain time is required each time to reach a steady state. In other words, the wavelength tuning is very slow and almost impractical.

[0057] However, if the linear resonator 21 is not tuned, power losses will occur. FIG. 6 compares the laser 10 of the present disclosure with an exemplary laser in this regard. Laser 10 is shown in FIG. 6(b) where the FSR of the second ring resonator 14 is designed to be an integer multiple of the FSR of the cavity 11. An exemplary laser is shown in FIG. 6(a) and is similar to laser 10, but the FSR of the second ring resonator 14 is not matched to the FSR of the cavity 11. In both cases, the switch from the lowest wavelength λ1 to the higher wavelength λx is measured by monitoring the output power of the laser versus time (while the tuning mechanism is performed). In both cases, only the first resonator 13 is tuned and the other phase shifters are not operational.

[0058] In Fig. 6(a), λ1 fits into the laser cavity 11 at an integer multiple, resulting in constructive interference and coherent laser output. However, λ2 and λ3 already show low output power, and at λ4, a low-power incoherent output is seen. If the wavelength of the external cavity 11 is further tuned, at a certain moment, the next integer multiple wavelength fits into the cavity 11 again, and the output power and coherence increase sharply (shown as λx).

[0059] In FIG. 6(b), such degradation is not observed due to the design of the laser 10 of the present disclosure. It is noteworthy that in the exemplary laser of FIG. 6(a), the degradation of the output power and the decrease of the coherence are resolved by tuning the linear resonator 21. In this case, the output power over time may be similar to that of FIG. 6(b). However, since the third phase shifter 31 is a physical phase shifter, a phase shift larger than 2π is required to accommodate the different wavelengths imposed by the 2π phase shift of the ring resonator 13, and the speed of wavelength tuning is significantly limited by the tuning speed of the third phase shifter 31. Fast tuning across the entire spectrum is not possible.

[0060] The laser 10 of the present disclosure does not require such tuning of the linear resonator 21. In fact, a tunable linear resonator 21 is not required at all to tune the laser 10 at high speed. In a preferred measurement shown in FIG. 6(b), all wavelengths are addressed with equal power over the entire tuning range. Thus, the laser 10 of the present disclosure avoids the bottleneck discussed above of having to continuously tune the linear resonator 21 to equalize the laser output.

[0061] The relevance of the FSR can be explained as follows: To ensure that the necessary constructive interference occurs, an equal number of waves must fit inside the cavity 11. The wavelength difference between these waves is the FSR of the cavity 11. All wavelengths that fit inside the cavity 11 (that match the wavelengths of the FSR of the cavity 11) are so-called cold cavity modes. In the case of ring resonators such as the second ring resonator 14, similar cavity modes exist, which have their own FSR. The FSR of this second ring resonator 14 is related to the circumference of the second ring resonator 14. The cold cavity modes of a laser with at least two ring resonators as external waveguide circuits have a smaller FSR than the ring resonator itself. By matching the spacing of the multiple cold cavity modes to the FSR of the stationary second ring resonator 14, all modes of the stationary ring resonator 14 of interest resonate in the entire laser. As already mentioned above, this FSR agreement can be mathematically described by equation (1).

[0062] Figure 7 illustrates a method 70 of operating the tunable laser 10 of the present disclosure shown in Figure 1 or 2. The method 70 involves tuning 72 the resonant frequency 71 of the first resonator 13 while keeping the resonant frequency of the second resonator 14 fixed. Due to the design of the laser 10, and in particular the matching of the FSR of the second ring resonator 14 with the FSR of the cavity 11, tuning the resonant frequency of the first resonator 13 results in multiple peaks in the power of the laser beam output at laser output 15, with each peak having equal power, as shown in Figure 6(b).

[0063] In summary, by matching the second ring resonator 14 to the cavity 11 in terms of its FSR, all peaks of the second resonator 14 can be tuned without power degradation by tuning the first resonator 13. Thus, tuning of other parts of the cavity 11, such as the linear resonator 21, can be omitted, and as a result, the speed of tuning can be significantly improved.

[0064] In the claims and description of the present disclosure, the word "comprising" does not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude a plurality. A single element may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage in practice.

Claims

1. A tunable laser (10), comprising: Cavity (11); a gain medium (12) arranged within the cavity (11) and configured to generate light by stimulated emission and emit the light into the cavity (11); and a laser output (15) configured to output a portion of the light in the cavity (11) as a laser beam; The cavity (11) includes a first resonator (13) and a second resonator (14) optically coupled to each other, the resonant frequency of the first resonator (13) being adjustable; A laser (10) in which the free spectral range (FSR) of the second resonator (14) is an integer multiple of the FSR of the cavity (11).

2. 2. The laser (10) of claim 1, wherein the cavity (11) includes a linear resonator (21), the gain medium (12) is disposed within the region of the linear resonator (21), and the linear resonator (21) is optically coupled to the first and second resonators (13, 14).

3. The linear cavity (11) is optically coupled to the first and second resonators (13, 14) by an optical splitter (22); 3. The laser (10) of claim 1 or 2, wherein the optical splitter (22) is configured to allow light to couple from the linear resonator (21) to the first and second resonators (13, 14) and to allow light to couple from the first and second resonators (13, 14) to the linear resonator (21).

4. 4. The laser (10) of claim 3, wherein the optical splitter (22) is further optically coupled to the laser output (15).

5. 5. The laser (10) of claim 3 or 4, wherein the resonant frequency of the linear resonator (21) is adjustable.

6. The laser (10) of any one of claims 1 to 5, wherein the resonant frequency of the second resonator (14) is tunable.

7. further comprising one or more phase shifters (31, 32, 33); The laser (10) of any one of claims 1 to 6, wherein a first phase shifter (32) of the one or more phase shifters (31, 32, 33) is configured to adjust a resonant frequency of the first resonator (13).

8. a second phase shifter (33) of the phase shifters (31, 32, 33) configured to adjust the resonant frequency of the second resonator (14); and / or 7. The laser (10) according to claim 5 or 6, wherein a third phase shifter (31) of the phase shifters (31, 32, 33) is configured to adjust the resonant frequency of the linear resonator (21).

9. The laser (10) of any one of claims 1 to 8, wherein at least one of the resonant frequency of the second resonator (14) and the resonant frequency of the linear resonator (21) is not tunable.

10. The laser (10) of any one of claims 1 to 9, wherein the first resonator (13) is a first ring resonator and / or the second resonator (14) is a second ring resonator.

11. 11. The laser (10) of claim 10, wherein the first ring resonator (13) has a different circumference than the second ring resonator (14).

12. The laser (10) of any one of claims 1 to 11, wherein the cavity (11) comprises one or more additional resonators optically coupled to each other and to the first and second resonators (13, 14).

13. 13. The laser (10) of claim 12, wherein the resonant frequency of at least one of the additional resonators is tunable.

14. A method (70) for operating a tunable laser (10) according to any one of claims 1 to 13, comprising: Tuning (71) the resonant frequency of the first resonator (13); and A method (70) comprising: tuning (71) the resonant frequency of the first resonator (13) while keeping fixed (72) the resonant frequency of the second resonator (14).

15. 15. The method (70) of claim 14, wherein the resonant frequency of the first resonator (13) is adjusted to produce a plurality of peaks in the power of the laser beam output by the laser output (15), each peak having an equal power.

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