阅读说明：本技术 一种适用于Littrow激光器的无跳模调谐区间拓展方法 (Mode-hopping-free tuning interval expansion method suitable for Littrow laser ) 是由 叶孜崇 江堤 张炜 靳琛垚 于 2021-07-20 设计创作，主要内容包括：本发明涉及一种适用于Littrow激光器的无跳模调谐区间拓展方法,该方法基于非线性的激光二极管电流输入,实现基于Littrow结构光栅外腔谐振原理的二极管窄线宽可调谐激光器的无跳模调谐区间大幅扩大,包括激光谐失振模的检测；失谐方式及失谐波长区间的判定；调整电流弧度获得失谐的压制三个技术过程。所述方法利用F-P干涉仪、波长计或原子频谱的激光失谐的波长区间及方式进行检测,进而通过与光栅角度调谐同步的二极管调制的非线性电流修正压制调谐波长区间的局部失谐问题,从而大幅拓展激光器稳定的可调谐空间。(The invention relates to a method for expanding a mode-hopping-free tuning interval of a Littrow laser, which is based on nonlinear laser diode current input and realizes that the mode-hopping-free tuning interval of a diode narrow-linewidth tunable laser based on a Littrow structure grating external cavity resonance principle is greatly expanded, and the method comprises the detection of a laser resonance loss mode; judging a detuning mode and a detuning wavelength interval; and adjusting the current radian to obtain the detuning suppression. The method utilizes an F-P interferometer, a wavelength meter or a wavelength interval and a mode of laser detuning of an atomic spectrum to detect, and further corrects and suppresses the local detuning problem of a tuned wavelength interval through a diode-modulated nonlinear current synchronous with grating angle tuning, so that the stable tunable space of the laser is greatly expanded.)

1. A method for expanding a mode-hopping-free tuning interval of a Littrow laser is disclosed, and the method is based on nonlinear laser diode current input modulation to realize that the mode-hopping-free tuning interval of the Littrow laser is greatly expanded, and is characterized by comprising the following steps:

the method comprises the following steps: detecting laser detuning;

step two: judging a detuning mode and a detuning wavelength interval according to the performance characteristics of the spectrum;

step three: aiming at different detuning modes, the current input of a laser diode is nonlinearly modulated to match the nonlinear evolution of a resonant mode in the tuning process of the Littrow laser, so that the problem of local detuning of a detuning wavelength interval is further solved, and the stable tunable space of the laser is greatly expanded.

Through the steps, stable frequency/wavelength tuning of more than 100GHz can be achieved in the Littrow laser.

2. The method for expanding the mode-hop-free tuning interval of the Littrow laser as claimed in claim 1, wherein:

in the first step, an F-P etalon is matched with an atomic or molecular spectrum reference pool to detect laser detuning, or a wavelength meter is used to detect laser detuning.

3. The method for expanding the mode-hop-free tuning interval of the Littrow laser as claimed in claim 2, wherein:

when the F-P etalon is adopted to be matched with an atomic or molecular spectrum reference pool to detect laser detuning, the detuning condition is shown in the peak distribution of the F-P etalon, and the detuning mode comprises simultaneous detuning of two edges and central detuning.

4. The method for expanding the mode-hop-free tuning interval of the Littrow laser as claimed in claim 3, wherein:

in step three, the nonlinear modulation laser diode current input comprises: adding a half-period positive spin wave function or a coefficient of a quadratic function on a straight-line time function of the FF, so that the detuning of the full period is suppressed;

for the detuning mode that the two edges are simultaneously detuned, the FF correction of the center is reduced through the coefficient, and the current of the whole tuning interval of the diode is reduced at the same time, so that the detuning of the two edges is suppressed;

for the detuning mode of center detuning, the center FF correction is increased by the coefficient so that the detuning gets a suppression.

5. The method for expanding the mode-hop-free tuning interval of the Littrow laser as claimed in claim 1, wherein:

in the method, the instant tuning detection is carried out while the laser carries out diagnosis or experiment in a light splitting mode.

6. The method for expanding the mode-hop-free tuning interval of the Littrow laser as claimed in claim 1, wherein:

the atomic or molecular spectrum reference cell is an iodine vapor emission or absorption spectrum.

Technical Field

The invention relates to the technical field of laser, in particular to a mode-hopping-free tuning interval expanding method suitable for a Littrow laser.

Background

The diode narrow linewidth tunable laser is a laser with high precision, low cost and wide application, and has popular application in atomic physics, quantum information, spectral analysis, plasma, such as flame speed and density distribution measurement. The core is that the advantages of laser broadening smaller than 1MHz, no mode-hopping tuning function with the scanning range larger than 10GHz, stable tuning function larger than hour magnitude, large-amplitude miniaturization of the whole set of laser system, relatively simple operation and the like can be realized simultaneously on the premise of low cost. Compared with the traditional dye laser which needs to push the dye body through high-power YAG laser to obtain hundreds of milliwatts, the diode laser can obtain the light beam with the same power of hundreds of milliwatts from the seed laser of tens of milliwatts through the arrangement of a Tapered Amplifier (TA), and the operation stability is greatly improved. In plasma diagnostics, the mode-hopping-free tunable amplitude of the laser determines the diagnostic range available for particle velocity distribution, and thus the mode-hopping-free tunable range of the laser is one of the fundamental performance indicators that can be used for plasma diagnostics.

The diode external cavity grating structure laser is generally divided into a Littrow design and a Littman-Metcalf design, wherein the Littrow design directly adopts a zero-order light spot of a diode to perform resonance, so that the power is high, a TA amplifier is properly used for further power enhancement in more wavelengths, but a light path is only diffracted once through a grating every time, and the selectivity of a resonance mode is low, so that the problems of mode hopping and mode mixing are easy to occur besides large line width. The Littman-Metcalf obtains performance improvement on the line width and the mode-hopping-free tuning range by a method that the light path passes through the grating diffraction twice at each time, but the Littman-Metcalf design abandons zero-order light spots, so that the light output power is relatively low, and the Littman-Metcalf is difficult to cooperate with a TA amplifier to obtain high-power light beams. Also, the top Littman-Metcalf laser supplier on the international market is New Focus in the united states, however, the commercially available laser is often software-locked within the tunable range of 60 GHz. On the contrary, manufacturers for producing Littrow lasers based on German laser diodes are available in China. Therefore, on the premise that the technology of the narrow-linewidth laser diode is limited, obtaining a higher tunable interval from the Littrow laser through the precise regulation and control of control parameters is an important technical development direction for realizing the autonomous controllable process of related devices in China.

The tuning process of a diode external cavity grating structure laser is affected by three important factors: (1) the first most significant and most common factor is that fine tuning of the grating angle will precisely change the resonant wavelength. Generally, the grating external cavity laser can finely adjust the grating angle through the characteristic so as to obtain accurate laser wavelength change, and the most important tuning mode is to perform accurate driving adjustment through adding a piezoelectric ceramic driver on a grating fixing piece; (2) secondly, the temperature of the laser often changes the diffraction coefficient of the grating, and further changes the resonant wavelength, and some old brands of lasers often use the method to control the tuning interval, but because the temperature tuning has considerable uncontrollable property, the more novel laser technical scheme generally pursues the stability of the temperature, thereby controlling the stability of the tuning wavelength; (3) finally, the drive current of the laser diode also changes the resonance of the laser, and compared with temperature tuning, power tuning is a simpler and more effective tuning method, and is even used as a main tuning mode in some internal cavity lasers. Meanwhile, with the technical development of lasers, manufacturers also find that the driving current which is most matched with the resonant mode of a laser diode can be changed simultaneously when the grating angle is tuned, so that the function of changing the driving current of the diode synchronously with the grating angle is added on the basis, and the function is generally called Feed Forward (FF). The existing FF function generally changes linearly with the linear drive of the grating piezoceramic. The addition of this function greatly increases the stability and usable tuning range of the laser.

However, in many experimental experiences, the specific tuning of the external cavity diode laser does not exhibit a linear characteristic, and with the voltage drive of the piezoelectric ceramic, we see that the laser wavelength variation is often characterized by a quadratic function curve. It follows that we have no reason to believe that the diode resonant current, which varies with the grating angle, is linearly characterized with the input of the piezoelectric ceramic. In practice, we can observe a nonlinear relationship between the grating angle and the resonant current in experiments. The invention aims at the nonlinear relation of the tuning to carry out nonlinear correction on FF, thereby realizing the effective increase of the mode-hopping-free tuning interval of the laser under the premise that the laser diode, the grating and other laser control technologies have higher dependence to foreign countries or have limited technologies at present in China. The technology disclosed by the patent can realize a stable tuning interval with a scanning range exceeding 100GHz on the basis of the existing and relatively mature Littrow external cavity structure diode narrow linewidth tunable laser technology in China, so that the technical blockade of China to our country is broken through.

Disclosure of Invention

The core purpose of the invention is to obtain a wider tunable interval without mode hopping by controlling a method for changing the input current of a diode on the premise of limited laser diode technology, and the method greatly expands the stable tunable space of a laser to obtain the suppression of the detuning by the problem of local detuning of a nonlinear tuning wavelength interval of diode current modulation synchronous with grating angle tuning.

The invention provides the following technical scheme:

a method for expanding a mode-hopping-free tuning interval of a Littrow laser is disclosed, and the method is based on nonlinear laser diode current input modulation to realize the mode-hopping-free tuning interval of the Littrow laser to be greatly expanded, and comprises the following steps:

the method comprises the following steps: detecting laser detuning;

step two: judging a detuning mode and a detuning wavelength interval according to the performance characteristics of the spectrum;

step three: aiming at different detuning modes, the current input of a laser diode is nonlinearly modulated to match the nonlinear evolution of a resonant mode in the tuning process of the Littrow laser, so that the problem of local detuning of a detuning wavelength interval is further solved, and the stable tunable space of the laser is greatly expanded.

Through the steps, stable frequency/wavelength tuning of more than 100GHz can be achieved in the Littrow laser.

In the first step, an F-P etalon is matched with an atomic or molecular spectrum reference pool to detect laser detuning, or a wavelength meter is used to detect laser detuning.

When the F-P etalon is matched with an atomic or molecular spectrum reference pool to detect laser detuning, the detuning condition is shown in the peak distribution of the F-P etalon, and the detuning mode comprises simultaneous detuning of two edges and central detuning.

In the third step, the nonlinear modulation laser diode current input is: adding a half-period positive spin wave function or a coefficient of a quadratic function on a straight-line time function of the FF, so that the detuning of the full period is suppressed;

for the detuning mode that the two edges are simultaneously detuned, the FF correction of the center is reduced through the coefficient, and the current of the whole tuning interval of the diode is reduced at the same time, so that the detuning of the two edges is suppressed;

for the detuning mode of center detuning, the center FF correction is increased by the coefficient so that the detuning gets a suppression.

In the method, the instant tuning detection is carried out while the laser is used for diagnosis or experiment in a light splitting mode.

Wherein, the atomic or molecular spectrum reference cell is an iodine vapor emission or absorption spectrum, namely an iodine cell.

The technical scheme provided by the invention is a tuning-mode-free tuning interval expansion method aiming at nonlinear change of a resonance condition in the tuning process of a diode tunable laser, and the tuning-mode-free tuning interval expansion method comprises a detection and judgment method of a detuning mode generated due to the nonlinear change of the resonance condition and a detuning suppression method aiming at a detuning mode forming mechanism.

When the technical scheme is used for detuning detection, the detuning detection can be realized by matching an F-P etalon with an atomic or molecular spectrum reference pool or a wavelength meter. The atomic/molecular spectral reference cell is an optional accessory element and a tool for calibrating absolute wavelengths in the process of using a laser, and is not absolutely necessary for the solution of the present invention. The technical scheme of the invention judges whether the resonance condition of the laser has nonlinear characteristics by observing the conditions of detuning near the central wavelength, detuning at two ends simultaneously or other local detuning of the laser tuning wavelength interval, and judges whether the diode driving current is higher or lower in the tuning of the local wave band of the laser according to the detuning characteristics, thereby carrying out targeted correction.

Furthermore, the detuning suppression method adopted in the invention can increase a quadratic curve coefficient and a half-cycle positive spin wave in a function of diode current synchronous regulation (Feed Forward) in a tuning process, or increase a higher-order curve function coefficient or more higher-frequency positive spin wave current inputs under a more complex detuning condition, thereby satisfying the nonlinear resonance condition of the laser so as to achieve the purpose of suppressing local detuning. In a non-numerical control laser power supply consisting of an analog circuit, the pressing method can be realized by adding a positive spin wave output power supply; in a numerical control laser power supply, the suppressing method can be directly realized by cutting of a digital control signal.

The invention achieves the following beneficial effects:

under the current conditions that the quality and the technology of the laser diode are limited by the current conditions and the dependence on foreign components, the mode-hopping-free wavelength tuning interval of the diode narrow-linewidth Littrow external cavity laser is greatly increased by utilizing a mode of laser diode driving current waveform cutting which is relatively easy to realize. The method can improve the mode-hopping-free wavelength tuning interval of the Littrow laser from more than 20GHz to more than 90GHz, has the potential of expanding the tuning amplitude to more than 120GHz under specific conditions, and realizes small-magnitude stable tuning which is far beyond the tuning amplitude limited by software. The method has the potential that the original diode Littrow external cavity laser which is relatively low in cost, high in power and easy to add a cone-shaped amplifier is comparable to a partial Littman-Metcalf laser in the key performance of a mode-hopping-free tuning wavelength range.

Drawings

FIG. 1 is a laser detuning detection optical path;

FIG. 2 is a diagram of the signals of an F-P etalon and an iodine cell with a laser normally implementing mode-hopping-free tuning;

FIGS. 3A and 3B show the signals of the F-P etalon and iodine cell with edge detuning of the laser in the case of mode-mixing and mode-hopping, respectively;

FIG. 4 is a diagram of the F-P etalon and iodine cell signals with bilateral detuning of the laser;

FIG. 5 is a signal of an F-P etalon and an iodine cell with center detuning of the laser;

FIG. 6 is a diode current correction used for the laser double-sided detuning case;

fig. 7 is a diode current correction used for the laser center detuning case.

Detailed Description

The technical solutions of the embodiments of the present invention will be described in detail below with reference to the accompanying drawings in the examples of the present invention, and it is obvious that the described examples are only a part of examples of the present invention, and not all examples, and all other examples obtained by a person of ordinary skill in the art without creative efforts based on the examples of the present invention belong to the protection scope of the present invention.

The invention relates to a method for expanding a mode-hopping-free tuning interval of a Littrow laser, which is based on nonlinear laser diode current input modulation to realize the mode-hopping-free tuning interval of the Littrow laser to be greatly expanded, and comprises the following steps:

detecting laser detuning;

judging a detuning mode and a detuning wavelength interval according to the performance characteristics of the spectrum;

aiming at different detuning modes, the current input of a laser diode is nonlinearly modulated to match the nonlinear evolution of a resonant mode in the tuning process of the Littrow laser, so that the problem of local detuning of a detuning wavelength interval is further solved, and the stable tunable space of the laser is greatly expanded.

Through the steps, stable frequency/wavelength tuning of more than 100GHz can be achieved in the Littrow laser.

Firstly, we will use F-P etalon, atomic spectrum reference cell or wavelength meter to detect the tuning condition of the laser, and at this time, laser needs to be emitted into the detection system, in this embodiment, we use the method of light splitting to implement the instant tuning detection while the laser is diagnosed or tested, and the light path diagram is shown in fig. 1. The detection light path, whether on-line or off-line, has various implementation modes, and is used as an implementation case here to facilitate the description of the technical scheme of the invention. Wherein, the atomic or molecular spectrum reference cell is an iodine cell.

Furthermore, we will use the measured data to obtain the condition of the laser stray mode and the mode jump. The data of the laser mode-mixing and mode-hopping states can be obtained by matching an F-P etalon and an atomic reference cell, some precise wavelength meters have the function of simple spectrometers, and the tuning condition of the laser can be known in real time by obtaining the real-time wavelength change of the precise wavelength meters, but the difficulty of the method for integrating the data analysis is slightly higher than that of the method for applying the F-P etalon, so the technical scheme of the invention is described by using an example of detecting the F-P etalon in the description. In the case of an F-P etalon, the signal without mode hopping, without mode-spurt tuning, forms equidistant peaks in the free spectral range of the F-P etalon. Ideally, these peaks should remain equidistant in time. However, since the relationship between the laser frequency tuning and the piezoelectric ceramic voltage is nonlinear (in many cases, a quadratic curve can be estimated more accurately), the time interval of each peak of the F-P etalon is also approximately characterized by a quadratic curve in the case of a large tuning range. Meanwhile, in order to avoid the situation that the wavelength is decoupled from the rest scanning intervals in normal scanning after mode hopping, an atomic or molecular spectrum reference pool is added, and the absolute wavelength interval of the scanning process is confirmed through the spectral distribution of the atomic or molecular spectrum reference pool. Of course, this particular mode-hopping situation is extremely rare and almost impossible to occur, and the main role of the reference cell is still to identify the absolute wavelength of the laser. The results of the normal tuning are shown in fig. 2.

The detuning condition is typically apparent in the peak distribution of the F-P etalon. The non-serious detuning condition of the Littrow laser is generally a mode hopping condition and a mode scrambling condition, wherein the mode hopping can enable the wavelength of light emitted by the laser to suddenly change to another wavelength, the continuous frequency/wavelength tuning is lost, and the generation of the mode scrambling can enable the light with several wavelengths to exist simultaneously; the two detuning modes are respectively the disappearance of the F-P etalon peak and the dense appearance of the stray peak in the F-P etalon, the general detuning condition appears at the edge of the tuning range, as shown in fig. 3A and 3B, fig. 3A and 3B respectively show the stray mode and the mode hopping, and the detuning interval is marked by the range of the dashed line frame in the figure. In general, the appearance of the stray mode can be suppressed by reducing the diode current, and the abrupt mode hopping often represents that the diode current is too low to form resonance at the original wavelength. Edge detuning can basically restore mode-hop-free tuning through linear modulation of diode current with piezoelectric ceramic voltage, which is the root cause of Feed Forward invention. However, with the gradually increasing tuning range, especially beyond 60GHz, the Littrow laser suffers from simultaneous detuning of two edges, as shown in fig. 4; or center detuning, as shown in fig. 5. This example is for the convenience of the reader to understand, the two detuning cases are selected from the case of the spurious mode, because in the case of the current deficiency mode-hopping detuning, the continuous scanning to the original edge does not necessarily restore the tuning by itself, and the (DC) current in the whole tuning scanning process is increased to suppress the detuning of the frequency band, and the obtained tuning state is similar to the cases shown in fig. 4 and 5.

Simultaneous detuning of the two edges or central detuning is a characteristic of nonlinear diode resonant currents: since linear current tuning cannot fit resonance condition changes gradually exhibiting non-linear characteristics, linear FF current tuning cannot form perfect tuning within the entire tuning range, resulting in multiple or non-unidirectional detuning intervals of the laser in the tuning range. In this case, the diode current of the present invention is curved in accordance with the voltage of the piezoelectric ceramic, thereby suppressing the detuning. The realization principle is that the synchronous adjustment (FF) mode of changing the diode current: the original current regulation function FF is time-proportional, i.e. the rate at which it reduces the current is a constant, and is realized by adding a triangular wave-type current to the DC current, which is synchronized with the piezo-ceramic scanning voltage. In most cases, the voltage of the piezo ceramic controlling the grating is also a triangular wave, and we can also understand this time linearity FF as the current drop is proportional to the piezo ceramic voltage by some constant. The advanced correction mode is that the coefficient of a half-period positive spin wave function or a quadratic function is added on a linear time function of the current correction time function, and the detuning of the full period is suppressed by matching the nonlinear current correction with the nonlinear resonance condition. For the case of both side detuning of fig. 4, we can reduce the FF correction of the center by this factor while reducing the (DC) current of the whole tuning region of the diode, so that both side detuning is suppressed. The time variation of the total diode current corrected for this case is shown in fig. 6. Similarly, for the center detuning case of fig. 5, we can obtain a suppression by increasing FF correction of the center, and the corrected current variation is shown in fig. 7.

In analog circuits, a forward rotating wave or similar power supply circuit is a more mature product. Therefore, it is estimated that the method of the present invention can be more easily implemented in a laser controller formed by a conventional analog circuit by adding a half-wave positive spin wave power supply with adjustable amplitude to the FF power supply system. On the contrary, in the controller implemented by numerical control, it is theoretically possible to output a current of any waveform by a numerical control command, and thus the two methods do not implement a difference in method in the numerical control laser power supply.

With the further increase of the mode-hopping-free scanning range of the Littrow laser, a detuning condition that a half-wave FF curve can be smoothed over a quadratic factor is given to different characteristics of gratings, and at the moment, the detuning condition can be indicated by numerical control FF of a higher-power function, or an adjustable positive spin wave auxiliary power supply formed by more wave periods or even multiple frequencies can further increase the range of the flattenable detuning condition, so that the tuning range is further increased. This is also one of the methods protected by this patent.

Although illustrative embodiments of the present invention have been described above to facilitate the understanding of the present invention by those skilled in the art, it should be understood that the present invention is not limited to the scope of the embodiments, and various changes may be made apparent to those skilled in the art as long as they are within the spirit and scope of the present invention as defined and defined by the appended claims, and all matters of the invention which utilize the inventive concepts are protected.