Method for maintaining synchronization of a Fourier domain mode locked (FDML) laser

Abstract

The present invention relates to a method for maintaining the synchronization of a Fourier domain mode locked (FDML) laser, wherein the FDML laser has at least one dispersion compensation resonator having at least one variable wavelength selective optical filter, wherein the laser propagates in the resonator at a round-trip propagation frequency, and the filter repeatedly changes in terms of its wavelength selectivity at a tuning rate, and wherein the FDML laser is synchronized when the tuning rate is an integer multiple of the round-trip propagation frequency, characterized in that the method has the following steps: a. coupling out at least a part of the laser from the resonator; b. detecting at least a part of the coupled-out laser by means of at least one photodetector; c. counting the offset in the signal of the photodetector during successive counting time intervals; d. adjusting the round-trip propagation frequency or the tuning rate such that the ratio of the count value to the length of the counting time interval remains within a predetermined desired value range.

Description

Technical Field

[0001] The present invention relates to a method for maintaining the synchronization of a Fourier domain mode-locked (FDML) laser. Background Art

[0002] The FDML laser has at least one resonator having at least one variable wavelength-selective optical filter, wherein the laser propagates in the resonator at a round-trip propagation frequency (Umlauffrequenz), and the filter repeatedly changes in terms of its wavelength selectivity at a tuning frequency. Here, the average number of passes of the filter through its tuning bandwidth per unit time is referred to as the tuning rate. If, for example, the filter is tuned strictly periodically by means of electronic control, it can also be referred to as the tuning frequency.

[0003] For the continuous operation of the laser, it is important that the tuning rate of the filter is synchronized with the round-trip propagation frequency of the laser so that each of the rotating wavelengths in the rotating wavelength reaches the filter again at the time point when the filter is just penetrable for that wavelength. If the tuning rate is as precisely as possible an integer multiple of the round-trip propagation frequency—neglecting dispersion—, then the synchronization of the FDML laser can be referred to. Therefore, the FDML laser emits light with a repeatedly changing wavelength. The change in wavelength is usually also called "scanning" in German. A laser light source that emits light with a repeated wavelength change is usually called a "swept-source".

[0004] The FDML laser is characterized in that the FDML laser can form and emit laser modes for all wavelengths passing through the filter, and thus can generate light with a time-varying wavelength, a large coherence length, a small line width, and a high spectral energy density as a swept-source. The scanning rate of the FDML laser—corresponding to the tuning rate of the filter—is very high here; it is at least equal to its round-trip propagation frequency. Therefore, the FDML laser is a preferred light source for Fourier domain optical coherence tomography (FD-OCT), which has currently been widely used in material testing and medical imaging.

[0005] Optical filters that can be tuned over their tuning bandwidth in a very short time, for example, constructed in the manner of a Fabry-Perot interferometer and are known per se, for example, from the document EP 2557 441A1. For example, a tuning rate of up to 1 MHz can be achieved thereby.

[0006] The round-trip propagation frequency of the laser in the resonator of the FDML laser should correspond to an integer fraction of the tuning rate and is thereby limited upwards by the filter. This requires a resonator with an optical path length between several hundred meters and several kilometers. To achieve this, the laser is typically guided in a resonator that includes a sufficiently long, coiled glass fiber as a delay line. This fiber simultaneously introduces dispersion.

[0007] The actual structure of the FDML laser is known, for example, from the documents US 7,414,779 B2 and US 8,315,282 B2 and is generally entirely fiber-based. In these examples, in addition to the tunable filter, an optically or electrically pumped laser-active medium ("gain medium") is integrated into the fiber guiding the light.

[0008] The dispersion can be compensated for in terms of the device to maintain as constant an optical intensity as possible for all emitted wavelengths. This is achieved, for example, in an FDML laser with a fiber-based delay line by passive components that are integrated into the delay line and couple wavelength-selective optical components into an additional delay line, see, for example, Desmond C. Adler, Wolfgang Wieser, Francois Trepanier, Joseph M. Schmitt, and Robert A. Huber, "Extended coherence length Fourier domain mode locked lasers at 1310 nm", journal: Opt. Express 19, 20930 - 20939 (2011). In the context of this specification, a resonator with dispersion compensation is meant when the resonator has a design in terms of the device for compensating the dispersion.

[0009] The documents US 7,414,779 B2 and US 8,315,282 B2 already mentioned describe the components and variants of the FDML laser in great detail. In addition, it can be deduced from US 8,315,282 B2 that there is a hint for adjusting the tuning rate of the optical filter by the light coupled out of the FDML laser in the sense of feedback so as to set the tuning rate to be consistent with an integer multiple of the round-trip propagation frequency and to maintain it during the ongoing operation. This adjustment is based on an intensity measurement of the part of the radiation propagating in the resonator that is coupled out. The intensity fluctuations are detected and integrated as a function of time to determine a noise value, and then the minimization of the noise value is used as the adjustment target. The continuous adjustment requirement follows from the fact that the round-trip propagation frequency is related to the disturbing influences on the fiber-integrated delay line and changes during the ongoing operation.

[0010] Such adjustment generally involves a tuning rate, which can be controlled particularly simply by modifying the electronic manipulation of the filter. The main disturbing influences also include fluctuations in the ambient temperature of the optical fiber, which alternatively opens up the possibility of, for example, establishing the synchronization of the laser by controlling the temperature of the optical fiber.

[0011] However, as has been shown in the meantime, conventional adjustment is not precise enough.

[0012] When examining the radiation of the coupled output of a conventional synchronously operating FDML laser with a high-resolution oscilloscope (50 GHz detection bandwidth) according to the prior art, the inventors found that the laser is switched off many times within a very short time - in the sub-nanosecond range - during each scan. This switching off not only causes very rapid intensity fluctuations, but in particular also a reduction in the coherence of the laser radiation, since new laser modes have to be built up again and again. When manually changing the tuning rate of the filter, it was possible to indicate a hint of an extremely sensitive correlation between the switching off of the laser and the synchronization. Thus, at a round-trip frequency of a few hundred kilohertz, a change in the tuning rate of a few millihertz causes an increase in the switching off and thus a significant deterioration of the laser emission. Such a small change already occurs in the round-trip frequency when the temperature of the optical fiber fluctuates by about one thousandth of a degree. Summary of the Invention

[0013] Accordingly, it is an object of the present invention to provide a method for adjusting an FDML laser, which allows the laser to operate permanently and stably with less noise and higher coherence than in the prior art.

[0014] The object is achieved by a method for maintaining the synchronization of a Fourier domain mode locked (FDML) laser, wherein the FDML laser has at least one dispersion compensation resonator, the dispersion compensation resonator has at least one variable wavelength selective optical filter, wherein the laser propagates in the resonator at a round-trip frequency, and the filter repeatedly changes in terms of its wavelength selectivity at a tuning rate, wherein the FDML laser is synchronous when the tuning rate is an integer multiple of the round-trip frequency, characterized in that the method has the following steps:

[0015] a. Coupling out at least a part of the laser from the resonator;

[0016] b. Detecting at least a part of the coupled-out laser by means of at least one photodetector;

[0017] c. Counting the offsets in the measurement signal of the photodetector during successively following counting time intervals;

[0018] d. Adjust the round-trip propagation frequency or the tuning rate such that the ratio of the count value to the length of the counting time interval remains within a predetermined desired value range.

[0019] Embodiments of the present application provide advantageous design solutions for the present invention.

[0020] The method according to the present invention is based on the already performed operation of an FDML laser, meaning that the method is only used when the laser reaches a state that is evaluated as synchronous according to the prior art. Then it is ensured that this synchronism is maintained during a further operating duration, and instead of other adjustments, as long as the laser machine does not suddenly and significantly deviate from its controlled operating state, for example due to mechanical vibrations or fluctuations in the mains voltage. A person skilled in the art is particularly familiar from this document with the steps required to start or restart an FDML laser until synchronism.

[0021] For example, a simple measurement of the output power of an FDML laser according to different tuning rates can be used to roughly set the FDML laser to the tuning rate that is approximately present at the power maximum. This power maximization is described in US7,414,779 B2 and by Murari, and can allow the round-trip propagation frequency and the tuning rate to be adjusted to a deviation of up to 1 - 10 Hz. For common round-trip propagation frequencies in the order of magnitude of several hundred kilohertz, this corresponds to 10 -5 to 10 -6 accuracy. This method can be called "coarse tuning" and is used to start the FDML laser. In contrast, the present invention now provides an additional "fine tuning", which can be taken over by the coarse tuning and can further improve the laser quality.

[0022] The combination of coarse tuning and fine tuning is also in principle more advantageous than the start-up process of an FDML laser. Because when the adjustment according to the method of the present invention cannot be achieved sometimes, then the coarse tuning can also be used as an "intercept and retrieve procedure". For example, this can occur, for example, in the case where too many counting events occur for any reason such that the measuring electronics can no longer detect a large part. When the predetermined maximum count value is exceeded within a counting time interval, then according to known methods, for example by means of the described power maximization, the fine tuning proposed here can be switched to coarse tuning. Once the coarse tuning has brought the laser back to an approximately synchronous state, then the fine tuning can take over the control again. An alternative method for applying the coarse tuning also lies in minimizing the integrated noise power or directly measuring the propagation time of the laser by means of a pulse signal from a photodiode.

[0023] The main difference between the method according to the present invention and the conventional integration of the measurement signal of a photodetector is that in the present invention, the actual duration of the shutdown or the length of the intensity drop is not taken into account at all.

[0024] Instead, only the occurrence of the steep drop is recorded, and if a steep drop occurs within the counting time interval, then the counter is incremented. Then, the counter reading at the end of the counting time interval is set proportionally to the length of the counting time interval, meaning that the offset frequency in the measurement signal of at least one photodetector is determined. This ratio is continuously re-determined for additional counting time intervals, meaning that the counter is set to zero at the start of each counting time interval.

[0025] The counting time interval can be predetermined such that before starting the counting process in each subsequent counting time interval, at least the start and end of the subsequent counting time interval are determined. Preferably, counting time intervals of the same length can be provided from the start, with uniform time intervals between them. However, it can also be advantageous to change the length of the counting time interval during the ongoing adjustment, for example, if one or more count values of an earlier counting time interval are higher (lower) than a predetermined threshold, then the predetermined percentage is shortened (lengthened) for a later counting time interval. Additionally, it can be advantageous to provide the duration of a predetermined time gap between two successive counting time intervals.

[0026] The predetermined counting time interval can be adjusted during operation. For example, at the start, a shorter time interval in an area where many steep drops occur makes sense in order not to overload the counter, i.e., to generate too many counting events. Then, if the adjustment has worked well after a certain time such that only very few counting events occur in each counting interval, then the counting window can be extended. It is also possible to move accordingly into optical frequency ranges with stronger and weaker noise. The adaptive method of adjustment can also involve the variable position and width of the counting time interval, the counting threshold, or the amplitude weighting factor. If the events become rarer, then the width of the interval can be increased, and the position can be moved more towards the center of the scan.

[0027] Alternatively, the length of the counting time interval can also be measured directly by using the counter as a stopwatch. The time measurement starts with the clearing of the counter and then ends when a predetermined count value is reached.

[0028] The adjustment according to the invention can work completely asynchronously, meaning that the counting time intervals do not have to be synchronized with the laser in any way.

[0029] In all of the above cases, it is possible to determine the ratio of the count value to the length of the counting time interval, and this ratio is suitable for the regulation according to the invention. This ratio should be repeatedly determined again and again in successive counting time intervals, and is maintained within a predetermined desired value range by regulating the round-trip propagation frequency or the tuning rate. Preferably, the lower limit of the desired value range can be set to zero, and the upper limit can be determined empirically by a person skilled in the art by observing the laser behavior according to the specific laser construction.

[0030] Generally, efforts are made to minimize the count value or the ratio, that is to say, for example, it is desired that the ratio value approaches the lower limit of the desired value range as closely as possible. However, due to the above-mentioned extreme sensitivity, the laser characteristics inevitably and very rapidly drift, and thus it is expedient to provide a reaction tolerance for the regulation in such a way that for the ratio value a value channel is allowed which does not cause a change in terms of the regulation technique of the round-trip propagation frequency or the tuning rate. Only when the ratio value breaks through from this range should the regulation become effective, and for example by changing the frequency of the frequency generator for tuning the optical filter, the ratio value returns to this channel again. In this way, it can be avoided that the regulation cannot keep up in time and / or starts to oscillate.

[0031] Furthermore, it would be advantageous to adaptively re-predetermine the desired value range during the regulation, that is to say, for example, if the ratio value of an earlier counting time interval is subject to very strong fluctuations overall and the reaction tolerance of the regulation should be increased until further notice, then the desired value range is changed with respect to at least one limit of the desired value range for a later counting time interval. Thus, the desired value range is pre-determined for deciding on the regulation measures in the sense of evaluating each determined ratio value; but for this purpose it is not necessary to fix it at fixed interval limits during the entire operating duration of the laser.

[0032] However, it should also be noted in this case that experiments with an FDML laser regulated according to the invention, specifically for imaging of the type according to optical coherence tomography (OCT), have shown that a readjustment of a few millihertz has no significant influence on the image quality. That is to say, the slight jitter caused by the continuous readjustment is completely acceptable for OCT. This then again allows a pure minimum regulation method, in which the regulation slightly changes the frequency after each counting time interval. Whenever the count value increases in one interval with respect to the previous interval, the regulation direction in the subsequent interval is reversed, so that it is regulated towards a count value of zero. Here, the duration of the counting time interval is preferably adjusted adaptively, and preferably the fewer the counting events occur, the longer the duration.

[0033] The offset in the measurement signal of the photodetector can be partly directly related to the timing of the laser propagating in the resonator or the timing of the filter. However, the frequency range of the round-trip propagation frequency and its higher harmonics is much lower than the frequencies that may be meaningful for the sub-nanosecond drops of the laser that are of interest here. Therefore, preferably, the frequency components in the measurement signal should be suppressed to the first N multiples of the round-trip propagation frequency, where N is a predetermined natural number. Particularly preferably, the measurement signal is high-pass filtered with a cut-off frequency up to 100 MHz. Thus, for an exemplary round-trip propagation frequency of 411 kHz, 1 ≤ N ≤ 243 can be selected, where at least N ≥ 100 is preferred.

[0034] Also regarded as a particularly advantageous design of the present invention is that the counter is increased only during the counting time interval when an unusually large offset appears in the measurement signal. Starting from the initially synchronously operating laser, in which the offset in the measurement signal is mainly caused by noise, for example, the amplitude A(k) of the offset can first be considered as a Gaussian-distributed random variable, and the conventional standard deviation can be calculated over a very short time period - a finite sample - for example, It is calculated as the average value of the measurement signal.

[0035]

[0036] Accordingly, a threshold value "Bias", B can be set, for example, B = 3*σ. Then, once - due to deteriorated synchronization or the like - under the condition |A(k)– A shift occurs below B, which can be considered unusually large and is then evaluated in relation to the regulation, while the remaining part is ignored as being caused by random noise. Preferably, only those shifts in the measurement signal are counted whose difference from the average value of the measurement signal exceeds a pre-determined threshold. This method also additionally increases the difference from the conventional regulation of the laser based on the integration of the measurement signal, since in this case a non-linear weighting is performed on the measurement signal components. In other words, in this case too, relatively little attention is paid to the amplitude of the shift in the measurement signal and the degree of the measured intensity drop. It is only classified as relevant and irrelevant.

[0037] In an alternative design, the threshold for counting is determined by a simple comparison with the maximum amplitude occurring in the measurement signal. For example, the threshold can be set to 5%, 10%, 30%, 50% or 75% of the maximum shift.

[0038] By introducing the threshold, the method is highly robust against external interference, dark current and electronic noise.

[0039] However, the above regulation is carried out with particularly high precision, which is approximately 100 to 1000 times the precision of the previously proposed regulations that only regulate the power, for example. In a good case, the precision is below millihertz, which corresponds to a relative operating precision of approximately 10 -9 at a tuning rate of 400 kHz. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The invention will now be explained in more detail with the aid of the drawings. Shown here are:

[0041] Figure 1 shows a graph (voltage signal) of the laser emission of an approximately synchronous FDML laser in the case of adjustment according to the prior art measured with a high-resolution oscilloscope (50 GHz);

[0042] Figure 2 shows a graph of the laser emission of an FDML laser adjusted according to the present invention as in Figure 1 ;

[0043] Figure 3 shows a graph of the counted drops in the measurement signal when the tuning rate changes by ±5 Hz near the round-trip propagation frequency;

[0044] Figure 4 shows a graph of the number of intensity drops directly counted with a high-resolution oscilloscope (50 GHz) when the tuning rate changes by ±100 mHz near the round-trip propagation frequency;

[0045] Figure 5 shows a graph of the tuning rate of the laser when actively adjusting a resonator with a temperature-stable cavity;

[0046] Figure 6 shows a schematic diagram of the wavenumber variation curve of the filter selection rate for illustrating the measurement of residual dispersion;

[0047] Figure 7A graph showing comparative data between a proxy signal generated interferometrically (below) and an associated measurement signal of a laser emission without control;

[0048] Figure 8 It is shown that when the regulation is activated based on the agent signal Figure 5 Graph of the curve. DETAILED DESCRIPTION

[0049] The exemplary experimental configuration of an FDML laser has a resonator length of approximately 500 m, a round-trip propagation frequency of 411 kHz, a tuning bandwidth of approximately 130 nm and a tuning rate of 411 kHz. The passband wavelength λ(t) follows the temporal profile of a sine function around the central wavelength λ0=1295 nm, but only the most linearly extended, rising section of the sine function is used for the laser, which corresponds to one eighth of the function profile. Accordingly, the laser-active medium is optically pumped only during one eighth of each propagation period. The duration of the scan is therefore 1 / 3.288 MHz=304 nanoseconds.

[0050] exist Figure 1 The intensity distribution of the coupled-out laser component measured with a high-resolution oscilloscope (detection bandwidth 50 GHz) during the scanning duration is shown in FIG. Therefore, the time axis corresponds to the wavelength axis at the same time, and the average intensity varies with the wavelength. Figure 1 The measurement curve in shows the intensity of a synchronously operated FDML laser according to the prior art. At this resolution, large, very brief deviations in the measurement signal can be detected very well, but cannot be detected with commercially available measuring electronics (detection bandwidth <2 GHz) that are provided for conventional regulation. If these deviations in the positive or negative direction are utilized according to the method according to the invention for regulation, a significantly improved intensity distribution can be achieved, as in Figure 2 As can be seen in .

[0051] The accuracy of the regulation according to the invention becomes particularly clear if the offset count values ​​are observed in more detail in the case of a controlled variation of the tuning rate, in which case the measurements are made in regularly spaced count time intervals of equal size. Figure 3 The graph in shows the count values ​​around the optimal tuning rate ±5 Hz, which leads to synchronization of the laser. During synchronization, a global minimum of the deviation can be found, wherein the edge of the measurement curve close to the minimum already shows a strong rise in variations of the order of 100 mHz. This enables the application of a control which is already able to detect and compensate for frequency differences of a few millihertz. Figure 4In order to clarify, the number of average intensity dips directly measured with a high-resolution oscilloscope (50 GHz) during the laser scan when the tuning rate is changed by only ±100 mHz is visible. Here, the measurement data are differentiated according to whether the wavelength is shortened (red to blue) or lengthened (blue to red) during the scan.

[0052] By means of the adjustment according to the invention, a persistent and stable operation of an FDML laser is achieved for the first time, which has, while having optimal coherence properties, a hitherto unattainable low noise. The adjustment works very precisely and shows insensitivity with respect to external influences, such as radiation, and with respect to changes in the laser spectrum propagating in the resonator.

[0053] Figure 5 The curve of the tuning rate of a regulated and temperature-stable FDML laser over a time segment of 60 minutes is shown. The long-term drift of approximately 80 mHz that can be seen is attributed to the inevitable change in the round-trip propagation frequency of the laser even in the case of temperature stability. The fine tuning can follow this drift without problems and is also not disturbed by the temporary switching on of the vacuum pump (approx. 0 and around 50 minutes), meaning that the fine tuning is robust.

[0054] It is to be emphasized here that the above-mentioned oscilloscope for rapid detection is used here not only for adjustment, but also equally for displaying the unregulated and regulated laser emission in Figure 1 and 2 However, this measuring device is not required only for the adjustment. There are also very simple and low-cost counting mechanisms implemented in hardware (usually microprocessors) that are designed for very short trigger times. For example, such counting mechanisms are used in photon-counting detectors. For the implementation of the invention, only the count value and the length of the counting time interval are required. The latter can be fixed - and can itself be arbitrarily pre-determined according to the user's choice - for example, around 0.1 s. In general, using a counting mechanism implemented in hardware is a preferred design option of the invention.

[0055] Furthermore, it can be advantageous if the counting time interval is shorter than the reciprocal of the round-trip propagation frequency. In particular, it is thus feasible and advantageous to start and end the counting time interval at a pre-determined time after the start of the wavelength scan.

[0056] In Figure 6 the wavelength change curve of the FDML laser output is schematically shown - here represented by the wave number k = 2π / λ - where only certain time gaps between the scans have been removed for the sake of clarity. Thus, the period length T corresponds to the duration of the scan. As shown, the wavelength change curve is actually never strictly linear. Furthermore, Figure 6Three counting time intervals dt1, dt2, and dt3 are drawn, and the counting time intervals are periodically repeated at a period duration T, respectively. The counting time intervals are associated with wavenumber intervals dk1, dk2, and dk3, respectively.

[0057] The adjustment according to the invention can be arranged specifically according to one of a series of counting time intervals, for example, according to dt1, in such a way that the counter is set to zero at the start of dt1 and read at the end of dt1 in each scan. The counting time interval dt1 here refers to the start of the wavelength scan, because only in this way can a fixed reference to the wavenumber interval dk1 remain unchanged over all scans. For its part, the counting time interval starts and ends at a predetermined time after the start of the scan.

[0058] Then, the adjustment automatically optimizes the synchronization of the laser only for the wavenumber interval dk1. Thus, the average round-trip propagation frequency of the wavelength in the interval dk1 can be measured very precisely. The same is feasible for the remaining intervals dt2 or dk2 and dt3 or dk3. Thus, finally, the average propagation times of different wavelength ranges are known and the residual dispersion in the laser can be measured and, if necessary, optimized. In addition, either a counting time interval particularly suitable for frequency adjustment can be selected, or alternatively, a less suitable counting time interval can be excluded from the application for adjustment.

[0059] Advantageously, instead of the light regularly propagating in the resonator, the light reflected at the filter is coupled out and analyzed, where the sudden drop in intensity in the transmitted light is reflected as a peak in intensity in the reflected light, and the measurement signal has less background due to the average reflection intensity.

[0060] As already mentioned, the sudden drop or shift in the intensity of the laser emission in the measurement signal of at least one photodetector has not been detectable to date at a commercially reasonable cost, because the detection bandwidth required for this is too high and measurement electronics with the corresponding capabilities are usually too expensive to be used only for the operation adjustment of the laser. However, the actual "invisibility" of the sudden drop and / or temporary shutdown can also enable high-quality OCT to be achieved exactly with a conventional FDML laser.

[0061] Therefore, there are two suggestions: how to make the shift of the measurement signal more easily detectable at least for the adjustment electronics, so that the application of low-cost technologies can also be realized.

[0062] One possibility for making the intensity drop visible even with detection hardware having a relatively small detection bandwidth is to make the intensity drop itself longer and / or slower, so that the direct measurement signal has lower frequency components. As shown in the theoretical consideration, the duration of the drop and its edge steepness are directly related to the finesse of the optical filter; a higher finesse results in a steeper edge and an extended duration of the drop. Thus, a higher finesse reduces the required detection bandwidth for the regulation.

[0063] In an FDML laser, it is common to use a tunable Fabry - Perot filter as a variable wavelength - selective optical filter. The optical filter is an optical resonator having two mirrors with a high reflectivity, e.g., 99.5%, and the distance between the mirrors can be adjusted. A change in the mirror distance results in a change in the transmitted wavelength here. All wavelengths are transmitted that are an integer factor of the mirror distance. The finesse of the Fabry - Perot filter represents the ratio of the wavelength spacing of the transmitted wavelengths to the wavelength bandwidth at a fixed mirror distance. The greater the reflectivity of the mirrors, the narrower the range of transmitted wavelengths and the greater the finesse of the filter.

[0064] The duration of the drop is again approximately within the characteristic time range of the inverse wavelength bandwidth - and thus proportional to the finesse. As an example for estimating the characteristic time involved here, a filter with a wavelength bandwidth of 0.1 nm, which corresponds to an optical frequency bandwidth of 17 GHz, will be used. Based on the usual correlation: the rise time and decay time of a band - pass filter correspond to half of the inverse filter bandwidth, a drop can be expected here on a time scale of 1 / (2 * 17 GHz)=30 ps. Now, if an optical band - pass filter with less than 0.01 nm is preferably selected, the drop duration will increase to more than 300 ps, and detection can be performed with a smaller bandwidth.

[0065] Therefore, a targeted selection of a sufficiently small wavelength bandwidth of the optical filter is a measure for increasing the duration of the drop so that detection can be achieved with an economically favorable detection bandwidth. At a wavelength bandwidth of, for example, only 0.005 nm, a counting unit with an input analog bandwidth of only 1.7 GHz can be used. However, since drops shorter than the inverse analog bandwidth can also be detected - but with a reduced amplitude - the measurement can also be performed, for example, by lowering the counting threshold. Thus, an optical filter width of 0.2 nm, 0.1 nm, 0.05 nm, or 0.02 nm is suitable for an analog detection bandwidth of 6 GHz, 3 GHz, 2 GHz, or 1 GHz.

[0066] However, this prolongation in time of the steep drop significantly affects the quality of the FDML laser emission, as there is now a non-negligible "flickering" of the laser. This is undesirable at least for some applications.

[0067] Another possibility of performing the regulation according to the invention with the aid of low-cost measuring technology equipment consists in, instead of the laser intensity detection optical surrogate, which has the same origin as the rapid intensity drop, but reacts to this origin on a different time scale. A suitable surrogate for this is obtained by superimposing a part of the propagated laser coupled out of the resonator with itself at a later time point.

[0068] It is currently assumed that, especially when reaching the optical filter, recurring errors in the phase matching of the laser propagated in the resonator cause very short interruptions. If this is the case, then said phase errors must also be present in the interfering light and may in particular also concern those parts of the light field which, after superimposing two laser components delayed with respect to each other - and thus also wavelength-shifted - have the difference frequency of the superimposed light field. Therefore, in the beats of this interfering light, an indication of the phase error can be expected, which occurs on a longer time scale than the intensity drop itself. As has been found experimentally, this is also the case.

[0069] It has proven to be particularly advantageous to modify the regulation method described hitherto in such a way that the laser coupled out of the resonator is fed to an interferometer having at least two different arm lengths, wherein at least one optoelectronic detector detects the interfering light at at least one output of the interferometer.

[0070] To produce such an interference, in the simplest case, the coupled-out laser can be radiated through a plane-parallel plate. Here, the reflections from the front and back of the plate interfere with each other, and the partial beams thus have a path length difference. The plate can be formed from glass, a semiconductor such as silicon, zinc selenide, germanium, or another common material having as high a refractive index as possible for strong interference. Similarly, the light guidance by means of the interferometer can be effected in a fiber-optically coupled manner with the aid of two 50 / 50 fiber couplers and two fibers of different lengths between the couplers. The first fiber coupler serves here as a beam splitter which feeds the split light via two fibers of different lengths to the input of the second optical coupler, in which the partial beams interfere. In addition or alternatively, two partial beams can also be removed at two different locations of the resonator to produce an interference. Here, one or two partial beams can also be removed from the light reflected by the filter.

[0071] At the output of the interferometer, the interference light is preferably detected as an electrical differential signal heterodyne by means of a pair of balanced photodetectors.

[0072] Preferably, the arm length difference is between 3 mm and 150 mm here. Of particular importance here is that the corresponding beat frequency is greater than the detection bandwidth of the downstream electronics. Then, in addition to the remaining variations in the coupling ratio of the beam splitting, the measurement signal detected by the balanced photodetector with a detection bandwidth of less than 6 GHz, preferably less than 4 GHz, particularly preferably less than 2 GHz, and more particularly preferably less than 1 GHz should show a smooth zero line especially according to the wavelength and according to what a person skilled in the art expects for an FDML laser. However, in fact, a temporary offset is detected, which can be interpreted as the longer phase of the interfered interference signal at the detector. The offset occurs sporadically with positive and negative signs and amplitudes of different sizes.

[0073] Noteworthy about the measurement signal of the interference light is that its offset is excellently correlated with the much shorter intensity drop of the laser emission. Figure 7 and Figure 8 Shows a direct comparison in time of the measurement curve of the interference light signal (respectively below) with the intensity change curve of the laser (respectively above) during the scan. If the laser is optimally synchronized, then the offset in the interference light is significantly reduced. Therefore, it is possible to easily perform the laser adjustment according to the present invention based on the count value of only the surrogate offset in the measurement signal of the interference light. This is achieved in the same way in the inventor's laboratory with a commercially available and low-cost measurement card with a bandwidth of 400 MHz.

[0074] When adjusting according to the surrogate, the FDML laser emits without any degradation at all, and in particular, "flickering" does not occur. The coherence of the radiation can be further improved for all wavelengths through the adjustment.

[0075] The purpose of the two above-mentioned variants of the adjustment method according to the present invention is to detect the measurement signal with a detection bandwidth of several gigahertz, and more particularly preferably with a detection bandwidth of less than 1 GHz. This can also be achieved with low-cost measurement techniques.

[0076] Instead of adjusting the tuning rate, alternatively, the round-trip propagation frequency can also be changed. In particular, it is feasible that based on the fine tuning, the resonator length can be changed by a short free beam path, by a fiber heating device, or by a common piezoelectric-based fiber stretcher.

[0077] Measures for stabilizing the temperature of a laser resonator, especially with regard to the temperature stability of an optical fiber coil, can be advantageous for regulation. For this purpose, the glass fiber coil can be mounted on a carrier with good heat conductivity, such as aluminum, instead of on plastic which is commonly used for glass fibers, and then the temperature of the carrier can be stabilized via a Peltier element. It can be advantageous if most of the resonator is thermally insulated, for example by means of foam plastic.

Claims

1. A method for maintaining synchronization of a Fourier domain mode locked (FDML) laser, wherein the FDML laser has at least one dispersion compensation resonator having at least one variable wavelength selective optical filter, wherein laser light propagates in the resonator at a round-trip propagation frequency, and the filter repetitively changes in its wavelength selectivity at a tuning rate, wherein the FDML laser is synchronized when the tuning rate is an integer multiple of the round-trip propagation frequency, characterized in that, The method has the following steps: a. Coupling out at least a part of the laser from the resonator; b. Detecting at least a part of the laser coupled out by means of at least one photodetector; c. Counting the offsets in the measurement signal of the photodetector during successive counting time intervals, wherein in the case of an intensity drop, if the occurrence takes place within a pre-determined counting time interval, the counter is incremented; d. Adjusting the round-trip propagation frequency or the tuning rate such that the ratio of the count value to the length of the counting time interval remains within a pre-determined expected value range.

2. The method according to claim 1, characterized in that, Suppressing frequency components in the measurement signal to the first N multiples of the round-trip propagation frequency, where N is a pre-determined natural number.

3. The method according to claim 2, wherein High-pass filtering the measurement signal with a cut-off frequency up to 100 MHz.

4. The method according to any one of claims 1 to 3, characterized in that, Only counting the offsets in the measurement signal whose difference from the average value of the measurement signal exceeds a pre-determined threshold.

5. The method according to any one of claims 1 to 3, characterized in that The counting time interval is shorter than the reciprocal of the round-trip propagation frequency.

6. The method according to claim 5, characterized in that, Starting and ending the counting time interval at a pre-determined time after the start of the wavelength scan.

7. The method according to any one of claims 1 to 3, characterized in that, Using a variable wavelength selective optical filter having a wavelength bandwidth less than 0.2 nanometers.

8. The method according to any one of claims 1 to 3, characterized in that, Feeding the laser coupled out to an interferometer having at least two different arm lengths, wherein at least one of the photodetectors detects interference light at at least one output of the interferometer.

9. The method according to claim 8, wherein The interferometer has two arm lengths with an arm length difference between 3 millimeters and 150 millimeters.

10. The method according to claim 7, wherein Detecting the measurement signal with a detection bandwidth less than 6 GHz.

11. The method according to any one of claims 1 to 3, characterized in that, When exceeding a pre-determined maximum count value within the counting time interval, switching to coarse tuning until it is again below the maximum count value.

12. The method according to any one of claims 1 to 3, characterized in that Adapting at least one of the following adjustment parameters: the duration and position of the counting time interval, the adjustment step size, the counting threshold, the counting hold time.

13. The method according to any one of claims 1 to 3, characterized in that, Providing active and / or passive measures for stabilizing the temperature of the resonator.

14. The method according to any one of claims 1 to 3, characterized in that, Using the light reflected on the filter for adjustment.

15. The method according to any one of claims 1 to 3, characterized in that, Removing light at different parts of the resonator and causing interference.

16. The method according to claim 7, characterized in that, The wavelength bandwidth is less than 0.1 nanometers.

17. The method according to claim 7, wherein The wavelength bandwidth is less than 0.05 nanometers.

18. The method according to claim 7, wherein The wavelength bandwidth is less than 0.02 nanometers.

19. The method according to claim 10, wherein The detection bandwidth is less than 4 GHz.

20. The method according to claim 10, characterized in that, The detection bandwidth is less than 2 GHz.

21. The method according to claim 10, wherein The detection bandwidth is less than 1 GHz.

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