A method of controlling a thermally tuned semiconductor laser

Abstract

This invention discloses a method for controlling a thermally tunable semiconductor laser, which can be used to construct a low-cost frequency-sweeping light source and can be applied to weld penetration measurement, particularly for welding inspection of new energy batteries. Real-time monitoring of weld quality can influence the development of the new energy battery industry, improve industrial safety, and promote industrial upgrading.

Description

Technical Field

[0001] This invention relates to the field of semiconductor laser control and optical coherence measurement technology, specifically to a thermally tunable semiconductor laser control method and an optical coherence measurement system based on the thermally tunable semiconductor laser. It can be applied to various SS-OCT systems, and is particularly suitable for axial length measurement, weld penetration measurement and tooth morphology measurement. Background Technology

[0002] Optical coherence measurement (OCT) systems utilize a swept-frequency light source to output a swept-frequency light whose wavelength varies continuously over time. This swept-frequency light interferes with the reflected light from the sample, forming an interference signal. The depth distribution of the sample is then reconstructed through wavenumber domain resampling and Fourier transform. Compared to spectral domain OCT, swept-frequency OCT offers advantages such as a wider imaging depth range and lower sensitivity roll-off, and has been widely applied in ophthalmology, dermatology, dentistry, endoscopy, laser welding, and industrial inspection.

[0003] The wavenumber envelope of a thermally tunable semiconductor laser is determined by the residence time and output power of each wavenumber during the frequency sweep. Because the wavenumber-time curve is typically nonlinear due to the self-heating effect, the light source has a longer residence time at the beginning and end of the sweep, and a shorter residence time in the middle, resulting in a natural wavenumber envelope that often exhibits a convex edge and concave center shape. While this shape results in higher total output power, during OCT reconstruction, the window function applied to suppress the sidelobes of the point spread function filters out the strong components at the edges of the wavenumber envelope, leading to a significant decrease in the effective optical power actually used for imaging.

[0004] In existing technologies, the control of swept-frequency light sources typically focuses on a single objective, such as maximizing the swept bandwidth, maximum output power, or achieving the most linear wavenumber-time mapping, without considering the matching relationship between the wavenumber envelope shape and the reconstruction window function. Therefore, while the driving parameters determined by existing methods may perform well in a single objective, the actual reconstruction performance is often suboptimal, sometimes even exhibiting a phenomenon of "high total light source power but low effective imaging power."

[0005] Therefore, a comprehensive thermally tunable semiconductor laser control method is needed. By designing the periodic drive current waveform, actively shaping the sweep frequency wavenumber envelope, and comprehensively considering the sweep bandwidth, effective optical power, and the sidelobe level of the reconstruction point spread function, the optical coherent measurement system can obtain the optimal overall imaging performance. Summary of the Invention

[0006] The present invention aims to at least solve the technical problems existing in the prior art, and proposes a thermally tunable semiconductor laser control method and an optical coherence measurement system based on a thermally tunable semiconductor laser.

[0007] To achieve the above objectives, the present invention provides a method for controlling a thermally tunable semiconductor laser, comprising the following steps:

[0008] Step S1: Drive the thermally tunable semiconductor laser using multiple static drive current values, measure the output wavelength and output optical power corresponding to each static drive current value, and establish the output wavelength. Output optical power Respectively with drive current The correspondence between them;

[0009] Step S2: Based on the relationship between wavenumber and wavelength The relationship between output wavelength and drive current is converted into output wavenumber. With drive current Correspondence between ;

[0010] Step S3: Determine the target wavenumber envelope with Fourier transform sidelobe suppression capability. ;

[0011] Step S4: Based on the target wavenumber envelope The relationship between output wavenumber and drive current, and the relationship between output optical power and drive current, are used to determine the wavenumber-time trajectory. The dwell time and corresponding output optical power in different wavenumber ranges together form the target wavenumber envelope. ; based on the inverse function of the relationship between output wavenumber and drive current Wavenumber-time trajectory Converted to drive current waveform The driving current waveform was determined based on the thermal response time constant of the thermally tunable semiconductor laser. Perform dynamic compensation;

[0012] Step S5: Utilize the drive current waveform A thermally tunable semiconductor laser was driven, its wavelength-time curve and output optical power were measured, and the measured wavenumber envelope was obtained by conversion. ;

[0013] Step S6: Envelope the measured wavenumbers With target wavenumber envelope Align them according to their respective center wavenumbers and calculate their similarity. ;

[0014] Step S7: In similarity When the similarity is less than the preset similarity threshold, the wavenumber envelope is used as the basis for calculation. With target wavenumber envelope Adjusting the deviation between the drive current waveforms The parameters are then used, and steps S4 to S6 are repeated until the similarity is reached. If the similarity threshold is greater than or equal to the preset threshold, the corresponding drive current waveform parameters will be output.

[0015] Furthermore, the driving current waveform is a nonlinear continuous waveform or a segmented waveform, and each segment of the segmented waveform is a linear ramp, a nonlinear function curve, or a constant current step; the absolute value of the slope of the wavenumber-time trajectory in the central wavenumber interval of the target wavenumber envelope is less than the absolute value of the slope in the edge wavenumber interval of the target wavenumber envelope; when the segmented waveform includes a constant current step, the duration of each constant current step is greater than or equal to the thermal response time constant of the thermally tunable semiconductor laser.

[0016] Furthermore, similarity Calculate according to the following formula:

[0017] ;

[0018] in, and In calculating similarity Previously, they were normalized according to their respective peak values, and the center wavenumbers of the two were made to coincide by translating the wavenumber coordinates; the preset similarity threshold is greater than or equal to 0.5 and less than or equal to 0.99.

[0019] Furthermore, the target wavenumber envelope This is either a Gaussian wavenumber envelope or a Hamming wavenumber envelope. When the target wavenumber envelope... When the wavenumber envelope is Gaussian, the target wavenumber envelope is... satisfy:

[0020] ;

[0021] in, For the target center wavenumber, denoted as the standard deviation in the wavenumber domain.

[0022] Furthermore, the full width at half maximum (FWHM) of the target wavenumber envelope accounts for 20% to 80% of the total wavenumber scan range of the thermally tunable semiconductor laser.

[0023] Furthermore, based on the above-mentioned thermally tunable semiconductor laser control method, the following comprehensive performance optimization steps are also included:

[0024] Step S11: Determine the target axial resolution based on the application scenario of optical coherence measurement. Target sensitivity and target sweep frequency And determine the constraints for peak drive current, average power and junction temperature;

[0025] Step S12: Based on the target axial resolution Determine the reference effective wavenumber bandwidth And based on this, determine the bandwidth of the target wavenumber envelope;

[0026] Step S13: Initialization including minimum drive current Maximum drive current The driving current waveform parameters, including duty cycle D, number of waveform segments, and slope or shape parameters of each waveform segment, are determined, and it is determined whether the initialized driving current waveform parameters meet the constraints.

[0027] Step S14: Drive the thermally tunable semiconductor laser using the driving current waveform parameters that satisfy the constraints, input the light into the Mach-Zehnder interferometer, obtain an interference signal of a fixed frequency, and obtain the measured wavenumber envelope corresponding to the j-th iteration. ;

[0028] Step S15: Analyze the measured wavenumber envelope under both windowed and unwindowed conditions. Perform a Fourier transform to obtain the point spread function, and then use the measured wavenumber envelope. The point spread function determines the effective wavenumber bandwidth. Effective optical power Side lobe level Sensitivity and comprehensive indicators ;

[0029] Step S16: When the comprehensive index When the preset performance conditions are not met, the effective wavenumber bandwidth is adjusted according to the drive current waveform parameters. Effective optical power Side lobe level and sensitivity To determine the direction of influence, adjust the drive current waveform parameters and repeat steps S14 to S16 until the overall performance is optimized. If the preset performance conditions are met or the preset iteration termination conditions are reached, the corresponding drive current waveform parameters will be output.

[0030] Furthermore, comprehensive indicators Calculate according to the following formula:

[0031] ;

[0032] in, For effective wavenumber bandwidth, For effective optical power, The sidelobe level of the point spread function. , and These are reference values ​​for effective wavenumber bandwidth, effective optical power, and sidelobe level, respectively. , and These are weighting coefficients for effective wavenumber bandwidth, effective optical power, and sidelobe level, respectively; for biological tissue tomography, the weighting coefficients are increased. For measurements on a single reflecting surface, the weighting coefficient is increased. .

[0033] Furthermore, adjusting the drive current waveform parameters includes: changing each drive current waveform parameter separately, and determining the effect of each drive current waveform parameter on the overall performance. The direction of influence, and along the comprehensive index The direction of increase involves joint adjustment of at least two drive current waveform parameters. Preset iteration termination conditions include comprehensive indicators. The rate of change after a preset number of iterations is less than a preset convergence threshold, or the number of iterations reaches a preset maximum number of iterations. The output drive current waveform parameters include the sweep frequency and minimum drive current. Maximum drive current Duty cycle D and parameters for each waveform segment.

[0034] Furthermore, the present invention provides an optical coherent measurement system employing the aforementioned thermally tuned semiconductor laser control method, comprising a swept frequency light source, a control module, an interference optical path, a first photodetector, an auxiliary interferometer, a data acquisition card, and a computer.

[0035] The frequency-sweeping light source includes a thermally tunable semiconductor laser, used to output frequency-sweeping light whose wavelength varies with the driving current. A portion of the light from the frequency-sweeping light source enters the auxiliary interferometer, and a portion enters the interference optical path. The control module is electrically connected to the frequency-sweeping light source and is used to generate a driving current waveform based on the correspondence between the target wavenumber envelope, the output wavenumber, and the driving current, as well as the correspondence between the output optical power and the driving current. It also adjusts the driving current waveform based on the deviation between the measured wavenumber envelope and the target wavenumber envelope. The interference optical path is connected to the frequency-sweeping light source optical path and is used to split the frequency-sweeping light into reference arm light and sample arm light, and to cause interference between the reference arm return light and the sample arm return light to output an interference optical signal. A first photodetector is connected to the interference optical path and is used to convert the interference optical signal into a first electrical signal S1. The auxiliary interferometer includes... Auxiliary interference channels with different optical path lengths are used for time-division or simultaneous generation. The circuit contains auxiliary interference electrical signals K1 to KN of different frequencies, among which... It is a positive integer greater than or equal to 2. The data acquisition card is electrically connected to the first photodetector and the auxiliary interferometer, respectively, and is used to acquire the first electrical signal S1 and The auxiliary interference electrical signals K1 to KN are used. The computer is electrically connected to the data acquisition card and is used to... The auxiliary interferometric electrical signals K1 to KN determine the wavenumber sampling relationship and depth calibration relationship, and the first electrical signal S1 is resampled in the wavenumber domain and corrected for depth based on the wavenumber sampling relationship and depth calibration relationship.

[0036] Furthermore, the auxiliary interferometer includes a second fiber beam splitter, a polarization controller, a third fiber beam splitter, an optical path switching component, a second photodetector, and a filter. The optical path switching component includes an input terminal and... One output terminal, Each output terminal is connected to an optical path of different... One auxiliary interference channel, the optical path switching component is used to sequentially select according to the timing control signal. There is one auxiliary interference channel. The second photodetector is used to convert the auxiliary interference optical signal formed after being combined by the third fiber beam splitter into an auxiliary interference electrical signal, and the filter is used to filter the auxiliary interference electrical signal.

[0037] Compared with the prior art, the present invention has at least the following beneficial effects:

[0038] (1) The present invention can actively adjust the wavenumber envelope shape of the thermally tuned semiconductor laser, reduce the mismatch between the natural wavenumber envelope and the OCT reconstruction requirements, and help to reduce the effective optical power loss and effective wavenumber bandwidth narrowing caused by additional windowing while suppressing the side lobes of the point spread function.

[0039] (2) Based on wavenumber envelope shaping, this invention further evaluates the effective wavenumber bandwidth, effective optical power, sidelobe level and sensitivity, and adjusts the driving current waveform parameters under the constraints of peak driving current, average power and junction temperature. This can avoid determining the driving parameters based on a single index such as sweep bandwidth, output power or wavenumber scanning linearity, and is beneficial to take into account axial resolution, sensitivity, sidelobe suppression capability and sweep speed.

[0040] (3) The present invention can set the weight of each performance parameter in the comprehensive index according to different application scenarios, thereby improving the adaptability of the same control method to different optical coherent measurement scenarios.

[0041] (4) The optical coherent measurement system of the present invention is equipped with an auxiliary interferometer having multiple auxiliary interference channels with different optical paths. It uses multiple auxiliary interference electrical signals of different frequencies to determine the wavenumber sampling relationship and depth calibration relationship. It can perform wavenumber domain resampling and depth correction on the first electrical signal, reduce the sampling nonlinearity and depth measurement deviation caused by the wavelength drift of the thermally tuned semiconductor laser, and improve the stability of the optical coherent measurement results.

[0042] (5) The present invention can use a thermally tuned vertical cavity surface-emitting laser to form a low-cost sweep frequency light source, and can be applied to scenarios such as axial length measurement, biological tissue tomography, weld penetration measurement, tooth morphology measurement and industrial surface inspection, and has good applicability. Attached Figure Description

[0043] Figure 1 This is a schematic diagram of three wavenumber envelope shapes.

[0044] Figure 2 This is a schematic diagram of the envelopes of the three wavenumbers under the condition of Gaussian window.

[0045] Figure 3 This is a schematic diagram of the optical coherence measurement system according to the present invention.

[0046] Figure 4 This is a schematic diagram of the auxiliary interferometer. Detailed Implementation

[0047] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0048] The thermal dependence of the output wavelength of semiconductor lasers has been reported in various publications. For example, the RC22xxx1-F semiconductor laser (thermally tunable vertical-cavity surface-emitting laser, VCSEL) from Raycan Corporation of South Korea has peak wavelengths of 1312.8 nm, 1313.6 nm, 1314.6 nm, 1315.7 nm, 1317.1 nm, and 1318.4 nm, respectively, with relative peak powers of 0.18 mW, 0.38 mW, 0.65 mW, 0.91 mW, 0.95 mW, and 0.75 mW, respectively, when the injection currents are 2 mA, 4 mA, 6 mA, 8 mA, 10 mA, and 12 mA.

[0049] According to the instruction manual for this light source, the threshold current of this semiconductor laser is... Approximately 2 mA. When the drive current is less than... At this time, semiconductor lasers do not produce lasing, only spontaneous emission; when the driving current exceeds... Subsequently, the semiconductor laser enters the lasing state, and the output wavelength monotonically increases with increasing current, i.e., the wavelength redshifts as the current increases; simultaneously, the output optical power first increases and then decreases with increasing current, and there exists a peak power current that maximizes the output optical power. (Approximately 10mA), when the current exceeds Subsequently, the output optical power decreases due to thermal roll-off.

[0050] From a physical perspective, the aforementioned current-wavelength relationship originates from the self-heating effect. The lasing wavelength of a semiconductor laser is determined by the longitudinal mode resonance condition of the laser cavity.

[0051]

[0052] Where n(T) is the effective refractive index of the cavity as a function of temperature, L is the cavity length, and m is the longitudinal mode order. Taking the derivative with respect to temperature yields the wavelength-temperature coefficient:

[0053]

[0054] Increased injection current increases power dissipation in the active region. Joule heating, non-radiative recombination (including SRH and Auger recombination), and heat generated by photon reabsorption raise the lattice temperature in the active region. Increased lattice temperature increases the refractive index of the semiconductor material. Approximately 2 × 10⁻ 4 ~4×10⁻ 4 K⁻¹ increases the optical cavity length of the laser cavity, shifting the cavity resonant wavelength towards longer wavelengths. (Approximately 0.05–0.1 nm / K). In other words, the phenomenon of wavelength increasing with current is the result of the transmission of "current-junction temperature-refractive index-cavity resonant wavelength"; the redshift of the lasing wavelength can also be used as an indirect measurement of the internal lattice temperature rise caused by self-heating.

[0055] Under steady-state conditions, the increase in junction temperature in the active region Due to thermal resistance Determined by both power dissipation and other factors:

[0056]

[0057] Where I is the injection current and V is the device operating voltage. To determine the output optical power, increasing the junction temperature causes a redshift in the cavity resonant wavelength and also reduces material gain, increases the threshold current, and decreases slope efficiency. The threshold current increases approximately exponentially with junction temperature, and the output optical power satisfies:

[0058]

[0059]

[0060] in, Characteristic temperature, As a reference threshold current, This represents the slope efficiency. From the two equations above, it can be seen that an increase in junction temperature leads to... Enlarge The output optical power decreases. After reaching its peak value, thermal roll-off occurs. Thermal roll-off is a positive feedback process between temperature increase and power conversion efficiency decrease: increased temperature reduces power conversion efficiency, which in turn increases the proportion of power dissipation, leading to a further increase in temperature. In oxide-confined VCSELs, the power density is high in small-aperture devices, Joule heat generated by the DBR series resistor is concentrated, and the oxide layer acts as both an electrical and thermal barrier layer, exacerbating the aforementioned self-heating problems and making thermal roll-off more significant.

[0061] The wavelength corresponding to the maximum output optical power is the peak wavelength. It should be noted that the peak wavelength is different from the center wavelength: the center wavelength refers to the median wavelength of the wavelength scanning range, while the peak wavelength refers to the wavelength corresponding to the maximum output power. From the above characteristic data, it can be seen that this semiconductor laser has the highest output optical power when the injection current is approximately 10 mA, corresponding to a peak wavelength of approximately 1317.1 nm.

[0062] In some other embodiments, the center wavelength of the semiconductor laser is greater than or equal to 500 nm and less than or equal to 1600 nm; the wavelength scanning range is greater than or equal to 2 nm and less than or equal to 20 nm; and the thermal response time constant of the active region is greater than or equal to 0.1 μs and less than or equal to 100 μs.

[0063] Furthermore, there are interdependent relationships between the driving current parameters, wavelength scanning range, wavenumber envelope shape, and optical power: increasing the maximum current can expand the wavelength scanning range, but it will exceed the peak power current. This leads to a decrease in output optical power and an increase in junction temperature; increasing the minimum current or duty cycle will increase the average junction temperature, causing baseline wavelength drift and exacerbating heat accumulation; increasing the sweep frequency will increase the time constant of the junction temperature change. The limitations reduce the wavelength scanning range; the shape of the driving current waveform determines the residence time of each wavelength during the frequency sweep, and thus the shape of the scan wavenumber envelope. Therefore, determining the driving current parameters requires a comprehensive trade-off between wavelength scanning range, spectral shape, and optical power, which forms the physical basis for subsequent driving waveform design and iterative optimization. Figure 1The simulation graphs show the relationship between three typical wavenumber envelopes—Gaussian, rectangular, and triangular—and time. The intensity values ​​have been normalized, with a maximum of 1 and a minimum of 0 for all three. The horizontal axis represents time, ranging from 0 to 10 µs. The areas of the three curves represent their respective optical powers: 3, 10, and 5, respectively. It can be observed that the Gaussian envelope has the lowest power, only about 30% of the rectangular envelope and about 60% of the triangular envelope. This is because OCT data reconstruction requires windowing of the wavenumbers to reduce spectral leakage and sidelobe artifacts caused by FFT. Commonly used window functions include the Gaussian window and the Hamming window. Here, the Gaussian window is used as an example to demonstrate its effect. Figure 1 The first Gaussian envelope in the diagram does not need to be multiplied by a Gaussian window again, but the second rectangular envelope does, and the third triangular envelope also needs to be multiplied by a Gaussian window. For example... Figure 2 As shown, without windowing, the power of the Gaussian wavenumber envelope is 3 and the half-width at half-maximum (WHM) is 2.83 µs; with windowing, the power of the rectangular wavenumber envelope is 3 and the WHM is 2.83 µs; with windowing, the power of the triangular wavenumber envelope is 1.6 and the WHM is 2.75 µs. From the power and WHM metrics, it can be seen that the rectangular and Gaussian envelopes have the same effect, both better than the triangular envelope. However, considering that the effective power utilization of the rectangular envelope is only about 30%, achieving the same... At that time, the incident light power of the sample with a rectangular envelope is 3.33 times that with a Gaussian envelope. Therefore, a Gaussian original wavenumber envelope is a more ideal situation, as it will not cause power attenuation due to windowing, thus preventing [further issues]. The reduction will not cause the effective wavenumber range to narrow due to windowing, thus avoiding a decrease in axial resolution.

[0064] Example 1

[0065] This embodiment provides a thermally tunable semiconductor laser control method. By controlling the driving current waveform to actively adjust the dwell time and corresponding output optical power in different wavenumber intervals, the thermally tunable semiconductor laser forms a target wavenumber envelope with Fourier transform sidelobe suppression. The target wavenumber envelope can be Gaussian, Hamming, or other shapes with sidelobe suppression function. For example, the swept frequency light source forms a Gaussian or approximately Gaussian wavenumber envelope in the wavenumber domain rather than the wavelength domain, so that in OCT reconstruction, no additional window function needs to be applied, or only a weak window function needs to be applied, to obtain a low sidelobe point spread function and reduce the axial resolution reduction and effective optical power loss caused by windowing. (1) The definition of the target wavenumber envelope is given by Figure 1 and Figure 2 The results show that the Gaussian wavenumber envelope retains both power and full width at half maximum (FWHM) before and after windowing, representing the most ideal wavenumber envelope shape. Therefore, the target envelope of this invention is defined in the wavenumber domain as Gaussian:

[0066]

[0067] in, The center wavenumber of the sweep frequency, Let be the standard deviation of the Gaussian envelope in the wavenumber domain, and the corresponding full width at half maximum (FWHM) is . Central wavenumber With center wavelength satisfy The bandwidth in the wavenumber domain and the bandwidth in the wavelength domain approximately satisfy the following conditions: .

[0068] It should be noted that, due to The Gaussian envelope in the wavelength domain is not equal to the Gaussian envelope in the wavenumber domain. The goal of this invention is not that the wavelength-time trajectory is Gaussian, but that the wavenumber-time trajectory and its envelope are Gaussian or approximately Gaussian.

[0069] (2) Structure of the driving current waveform

[0070] To achieve a Gaussian wavenumber envelope, the driving current employs a nonlinear waveform or a piecewise waveform, where each segment is a linear ramp or a nonlinear function (such as a quadratic function, exponential function, or sine function). The waveform has a small slope in the time interval corresponding to the center wavenumber and a large slope in the time interval corresponding to the edge wavenumbers. The stepped waveform is a special case of the piecewise waveform, where the residence time of each segment is greater than the thermal response time constant. The slope distribution of the driving current waveform over time determines the residence time of each wavenumber, and thus the shape of the wavenumber envelope.

[0071] (3) Waveform design steps

[0072] Step S1: Under static current drive, discretely measure the input current and measure the output wavelength of the light source under different drive currents. and output optical power The correspondence between the output wavelength and the driving current is established by interpolation fitting. And the relationship between output optical power and drive current. The peak wavelength and relative peak power data of the aforementioned RC22xxx1-F semiconductor laser under different injection currents can be used as a set of specific measurement data to establish the aforementioned correspondence.

[0073] Step S2: Based on the wavenumber-wavelength relationship ,Will Convert to the correspondence between output wavenumber and drive current .

[0074] Step S3: Determine the initial expression for the target Gaussian wavenumber envelope:

[0075]

[0076] in, and The shape of the Gaussian envelope can then be completely determined: Determine the location of the envelope center. The envelope width is determined by the corresponding wavenumber half-width. . The initial value can be initially selected based on the wavenumber scanning range of the light source being debugged, so that the full width at half maximum (FWHM) of the Gaussian wavenumber envelope occupies about 20% to 80% (e.g., 40%) of the total wavenumber scanning range, and then adjusted based on the actual measurement results. The initial value can be equal to the wavenumber corresponding to the peak power of the light source. Simultaneously, a lower limit constraint is set for the wavelength scanning range, such as the wavelength scanning range... Greater than 2 nm, based on the center wavelength The lower limit of the wavenumber scan range is obtained through conversion. The actual wavenumber scanning range of the light source should be greater than This ensures the Gaussian envelope is fully realized. If the wavelength scanning range provided by the light source is less than... If the target bandwidth is not met, it means that a quasi-Gaussian envelope cannot be achieved under that target bandwidth, and the target should be lowered and then reset.

[0077] Step S4: Inversely deduce the driving current waveform based on the target Gaussian wavenumber envelope. Since the measured wavenumber envelope is related not only to the dwell time in different wavenumber intervals but also to the output optical power under the corresponding driving current, it can be deduced from the target Gaussian wavenumber envelope. , and Determine the required residence time distribution for each wavenumber interval. Ignoring the variation of output optical power with drive current or normalizing the output optical power, the residence time distribution can be approximately expressed as |dt / dk|∝ Considering variations in output optical power, the dwell time distribution can be corrected based on the output optical power at the corresponding wavenumber, so that the dwell time and output optical power together form the target wavenumber envelope. Integrating the dwell time distribution yields the wavenumber-time trajectory. This results in a longer residence time in the center wavenumber band and a shorter residence time in the edge wavenumber band; then by inverse function Obtain the driving current waveform Dynamic compensation is performed based on the thermal response time constant. The initial value and range of the drive current are selected according to the device characteristics, and a fixed starting wavelength or starting wavenumber is not required.

[0078] Step S5: Generate a driving current waveform and drive the light source. Use a spectrometer and / or photodetector to measure the wavelength-time curve and output optical power. Convert the wavelength-time curve to a wavenumber-time curve, and obtain the measured wavenumber envelope based on the residence time and output optical power in each wavenumber interval. .

[0079] Step S6: Envelope the measured wavenumbers With ideal Gaussian envelope Comparisons were made after aligning the wavenumbers by center wavenumber, allowing for overall wavenumber shift, and similarity was used. As a convergence criterion:

[0080]

[0081] in, and Before calculation, both are normalized to their respective peak values ​​and their center wavenumbers are aligned. Since changes in the driving current will cause an overall shift in the center wavenumbers, and this overall shift does not affect the shape and depth relationship of the point spread function, the center wavenumbers of both are aligned before similarity calculation. The value range is from 0 to 1, when If the measured envelope is greater than or equal to a preset threshold (e.g., 0.8), it is considered to meet the quasi-Gaussian requirement, and the iteration stops.

[0082] Step S7: If the measured wavenumber envelope does not meet the quasi-Gaussian requirement, adjust the current waveform parameters according to the deviation direction, repeat steps S3 to S6 until the convergence criterion is met, and output the optimal drive current waveform parameters.

[0083] Example 2

[0084] This embodiment adds a comprehensive performance optimization step to the thermally tuned semiconductor laser control method described in Embodiment 1. By balancing the relationship between each drive current waveform parameter and imaging performance index, the drive current waveform parameters that meet the hardware constraints and enable the OCT system's comprehensive imaging performance to reach the preset performance conditions are determined.

[0085] (1) Relationship between each variable and sensitivity and axial resolution

[0086] In a swept-frequency light source optical coherent measurement system, variables directly related to imaging performance include: the power of the sample arm returning to the detector. Sampling time interval Number of spectral sampling points Quantum efficiency of detectors Photon energy Sweep frequency Effective wavenumber bandwidth and the sidelobe level of the reconstructed point spread function .

[0087] Sensitivity ( The relationship between sensitivity and the variables is as follows: Sensitivity and sample arm returned optical power. The sensitivity is directly proportional to the sample light intensity; the stronger the sample light, the stronger the interference signal, and the higher the sensitivity under the shot noise limit. Sensitivity is also related to the sampling time interval. and the number of spectral sampling points product Proportional: Among them This is the time interval between two adjacent sampling points, which is directly related to the sampling clock period of the A / D converter; This represents the total number of interference signal data points acquired in a single complete wavelength scan (one A-line). Sensitivity is also related to the detector's quantum efficiency. Proportional to photon energy It is inversely proportional to the center wavelength of the sweep frequency, that is, it is related to the center wavelength of the sweep frequency.

[0088] The relationship between axial resolution and various variables is as follows. Axial resolution is determined by the effective wavenumber bandwidth during reconstruction. Decide, The larger the frequency, the higher the axial resolution. The effective wavenumber bandwidth is determined by the width of the swept wavenumber envelope and the windowing condition; in Example 1, the wavenumber envelope is Gaussian, eliminating the need for additional windowing and thus avoiding narrowing of the effective wavenumber bandwidth and loss of effective optical power. Sweep frequency The sweep frequency determines the A-line rate. A higher sweep frequency results in a shorter acquisition time per A-line, requiring a higher sampling rate for the same number of sampling points, but reducing sensitivity. Conversely, a lower sweep frequency increases sensitivity but reduces imaging speed and exacerbates motion artifacts. Furthermore, increasing the maximum current expands the sweep range, but exceeding the peak power current... The output optical power decreases, the sensitivity decreases, and the junction temperature increases. Therefore, there are interdependent relationships between the sweep range, sweep frequency, number of sampling points, sampling time interval, and sample optical power, which need to be balanced through comprehensive indicators.

[0089] The above relationship can be expressed by the following formula. Under the shot noise limit, the sensitivity of the SS-OCT system is:

[0090]

[0091] in, Photon energy, For the quantum efficiency of the detector To return optical power to the sample arm reaching the detector, The time interval between two adjacent sampling points. This represents the number of spectral sampling points within a single A-line. Because the output power of thermally tunable semiconductor laser sweep light sources is generally only 1-2 mW in practice, which is below the power level where optical damage needs to be considered, it can be assumed that the higher the output power of this type of light source, the higher the sample light power, and ultimately, the higher the OCT system's output power. The higher the resolution, the better. Furthermore, the relationship between axial resolution and effective wavenumber bandwidth is:

[0092]

[0093] in, This represents the effective wavenumber bandwidth after windowing; if expressed in the wavelength domain, .visible The larger, The smaller the value, the higher the axial resolution.

[0094] To comprehensively balance sensitivity, axial resolution, and sidelobe level, a comprehensive index is defined. :

[0095]

[0096] in, It is the effective wavenumber bandwidth. It is the effective optical power. It is the sidelobe level of the reconstructed point spread function; Deduced from the required axial resolution, Determined by sensitivity requirements, Determined by the maximum permissible sidelobe. (Comprehensive index) The expression highlights that bandwidth and power are "the larger the better," so they are treated as positive terms; sidelobes are "the smaller the better," so they are treated as negative terms. Theoretically, the range of values ​​for is not fixed: each ratio can range from 0 to infinity. It can range from negative values ​​to positive infinity. For example, using weights. =0.40、 =0.35、 =0.25 Calculate a baseline: if all three items meet the standard (all three ratios equal to 1), then = 0.40 + 0.35 - 0.25 = 0.50. Therefore, 0.50 is the baseline value for "just meeting all targets". When all targets are met... > 0.50; when some parts do not meet the standard or the sidelobes exceed the tolerance. < 0.50; when the sidelobe deviation is very severe It may become a negative number.

[0097] In particular, for applications involving complex biological tissues, such as ophthalmic OCT, it can improve... The proportion of [unclear] highlights its high axial resolution performance; for single-reflective-surface applications such as laser welding inspection OCT or dental inspection OCT, it can improve [unclear] The proportion of [specific component] highlights high sensitivity performance.

[0098] (2) Specific steps

[0099] Step S11: Determine the target axial resolution based on the application scenario. Sensitivity and sweep frequency Requirements and constraints;

[0100] Step S12: Based on the target axial resolution The required reference effective wavenumber bandwidth for back-calculation In conjunction with Example 1 The initial selection method determines the target wavenumber envelope;

[0101] Step S13: Initialize the drive current waveform parameters, including the minimum current. Maximum current Check the duty cycle D, the number of waveform segments and the slope or shape parameters of each segment, and check hardware constraints such as peak current, average power and junction temperature;

[0102] Step S14: Generate a driving current waveform according to the method described in Example 1 and drive the thermally tunable semiconductor laser. Input the light into a Mach-Zehnder interferometer to obtain an interference signal of a fixed frequency, and obtain the measured wavenumber envelope corresponding to the j-th iteration. ;

[0103] Step S15: Analyze the measured wavenumber envelope under both windowed and unwindowed conditions. Perform a Fourier transform to reconstruct the point spread function and calculate the effective wavenumber bandwidth. Effective optical power Side lobe level Sensitivity and comprehensive indicators ;

[0104] Step S16: Determine the comprehensive index Does it meet the requirements or reach the optimal state? If yes, lock the current driving parameters and output them; otherwise, proceed to step S17.

[0105] Step S17: Adjust according to each parameter The influence of axial resolution on the trend of adjusting drive current waveform parameters, such as adjusting , Check the duty cycle and slope of each waveform segment, check the hardware constraints, and repeat steps S14 to S16 until... If the change is less than a preset threshold (e.g., 1%) for three consecutive rounds or the maximum number of iterations (e.g., 100 times) is reached, the optimal performance driving parameters are output, including the sweep frequency. , Duty cycle, waveform parameters, and corresponding , , , and value.

[0106] Example 3

[0107] like Figure 3 As shown, this embodiment provides an optical coherent measurement system using the thermally tuned semiconductor laser control method. The system includes a swept frequency light source 1, a first fiber beam splitter 2, an interference optical path 3, a sample arm optical path 4, a first photodetector 5, an auxiliary interferometer 6, a data acquisition card 7, a computer 8, and a control board 9. The computer 8 and the control board 9 together constitute a control module for controlling the swept frequency light source 1.

[0108] The swept frequency light source 1 can be a thermally tunable semiconductor laser swept frequency light source with a quasi-Gaussian wavenumber envelope, implemented based on the present invention. The emitted beam from the swept frequency light source 1 is connected to the first port of the first fiber beam splitter 2 via an optical fiber. The second port of the first fiber beam splitter 2 is connected to the interference optical path 3, and the third port of the first fiber beam splitter 2 is connected to the auxiliary interferometer 6.

[0109] The first fiber beam splitter 2 is used to split the beam output by the frequency sweep light source 1 and guide the split beam to the interference optical path 3 and the auxiliary interferometer 6 respectively.

[0110] The interference optical path 3 is connected to the optical paths of the first fiber beam splitter 2, the sample arm optical path 4, and the first photodetector 5, respectively. The interference optical path 3 is used to split the light incident from the first fiber beam splitter 2 into reference arm light and sample arm light, and to cause interference between the reference arm return light and the sample arm return light, forming an interference optical signal. The first photodetector 5 is used to receive the interference optical signal output from the interference optical path 3 through its third and fourth ports, and convert the interference optical signal into a first electrical signal S1.

[0111] The sample arm optical path 4 is connected to the interference optical path 3. The sample arm optical path 4 is used to guide the sample arm light output from the interference optical path 3 to the sample to be tested, and to transmit the sample return light back to the interference optical path 3. The sample arm optical path 4 may include optical devices for performing two-dimensional or three-dimensional scanning of the sample to be tested, so that the sample arm light illuminates different areas of the sample to be tested, thereby realizing multi-dimensional measurement or imaging.

[0112] The auxiliary interferometer 6 is optically connected to the first fiber optic beam splitter 2 and electrically connected to the data acquisition card 7. The auxiliary interferometer 6 is used to perform auxiliary interferometric measurements on a portion of the light output from the first fiber optic beam splitter 2 and output... The auxiliary interference electrical signals are K1~KN.

[0113] The first photodetector 5 and the auxiliary interferometer 6 are electrically connected to the data acquisition card 7. The data acquisition card 7 is used to simultaneously or time-divisionally acquire the first electrical signal S1 output by the first photodetector 5 and the auxiliary interference electrical signals K1~KN output by the auxiliary interferometer 6 under the triggering of the third timing signal T3, and send the acquired data to the computer 8.

[0114] The computer 8 is electrically connected to the data acquisition card 7 and the control board 9, respectively. The computer 8 receives the acquired data output from the data acquisition card 7 and processes the acquired data to obtain measurement results or imaging data of the object under test. The measurement results may include interface position, tomographic images, axial length, surface height, weld penetration parameters, or other OCT detection data. The computer 8 is also used to determine the wavenumber sampling relationship and depth calibration relationship based on the multi-channel auxiliary interference electrical signals K1~KN output by the auxiliary interferometer 6, and to perform wavenumber domain resampling and depth correction on the first electrical signal S1. The computer 8 is also used to determine the driving current waveform parameters based on the correspondence between the target wavenumber envelope, the output wavenumber and the driving current, and the correspondence between the output optical power and the driving current, and to adjust the driving current waveform parameters based on the deviation between the measured wavenumber envelope and the target wavenumber envelope. The computer 8 sends the driving current waveform parameters and system control commands to the control board 9.

[0115] The first timing signal T1 is output from the control board 9 to the swept frequency light source 1 and is a periodic driving current. The control board 9 generates a second timing signal T2, a third timing signal T3, and a fourth timing signal T4 based on the starting position of the first timing signal T1 and the system control commands output by the computer 8. The second timing signal T2 is used to control the working state of the auxiliary interferometer 6; the third timing signal T3 is used to trigger the data acquisition card 7 to acquire data; and the fourth timing signal T4 is used to control the sample arm optical path 4 to achieve two-dimensional or three-dimensional scanning.

[0116] like Figure 4 As shown, the auxiliary interferometer 6 includes a second fiber beam splitter 601, a third polarization controller 602, a third fiber beam splitter 603, an optical path switching component 604, a second photodetector 605, and a filter 606.

[0117] The optical path switching component 604 is connected to the optical paths of the second fiber beam splitter 601 and the third fiber beam splitter 603, respectively, and is used to switch the optical path in the auxiliary interferometer 6 according to the second timing signal T2. The second fiber beam splitter 601 is connected to the optical path of the third polarization controller 602, and the third polarization controller 602 is connected to the optical path of the third fiber beam splitter 603; the third fiber beam splitter 603 is also connected to the optical path of the second photodetector 605, and the second photodetector 605 is electrically or signal-connected to the filter 606.

[0118] The second fiber beam splitter 601 is used to split the light input into the auxiliary interferometer 6; the third polarization controller 602 is used to adjust the polarization state of one of the light paths so that the auxiliary interferometer 6 meets the polarization conditions required for interference detection; the other light path needs to pass through the optical path switching component 604 before entering the third fiber beam splitter 603; the two light paths interfere at the third fiber beam splitter 603, and are then converted into auxiliary interference electrical signals K1~KN by the second photodetector 605; the filter 606 is used to filter the auxiliary interference electrical signals K1~KN to suppress common-mode noise, background signals or unwanted frequency components.

[0119] The number of channels of the third fiber optic beam splitter 603 is The input is The output is 1 path.

[0120] The number of channels of the optical path switching component 604 is of, It is a positive integer greater than or equal to 2; the optical path switching component 604 has one channel as input and output as... This channel The difference between the channels is their respective optical path lengths; the optical path switching component 604 Each output channel can enable the auxiliary interferometer to generate outputs at 6 time intervals or simultaneously. The auxiliary interference signals K1~KN for different paths are characterized by different electronic frequencies.

[0121] The optical path switching component 604 can switch the optical path in the optical path switching component 604 according to the second timing signal T2.

[0122] The optical path switching component 604 can be a flip mirror driven by a motor or servo motor, a mechanical optical switch, a MEMS optical switch, a galvanometer, or other devices capable of dynamically switching optical paths. Preferably, the optical path switching component 604 is a MEMS optical switch.

[0123] Because the wavelength scanning parameters of thermally tunable semiconductor laser sweep light sources are easily affected by changes in ambient temperature and humidity, some literature reports the use of narrow-linewidth FBGs in auxiliary interferometers for wavelength alignment, thereby addressing the problem of OCT system parameter fluctuations caused by changes in light source performance at different times and under different environments. However, FBG-based solutions are costly in practice, and FBG feature peak extraction increases computational complexity. This invention proposes a scheme for OCT correction based on multiple auxiliary interferometric electrical signals K1~KN, which can perform both interpolation correction and depth correction, offering significant cost advantages, a simple optical path, and eliminating the need for complex spectral alignment calculations.

[0124] This embodiment employs an auxiliary interferometer 6 with multiple optical path channels to reduce the impact of wavelength drift of the thermally tuned semiconductor laser sweep frequency source on the parameters of the OCT system, and realizes a low-cost, high-sensitivity, and high-imaging-depth sweep frequency OCT system.

[0125] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0126] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for controlling a thermally tunable semiconductor laser, characterized in that, Includes the following steps: Step S1: Drive the thermally tunable semiconductor laser using multiple static drive current values, measure the output wavelength and output optical power corresponding to each static drive current value, and establish the output wavelength. Output optical power Respectively with drive current The correspondence between them; Step S2: Based on the relationship between wavenumber and wavelength The correspondence between the output wavelength and the driving current is converted into the output wavenumber. With drive current Correspondence between ; Step S3: Determine the target wavenumber envelope with Fourier transform sidelobe suppression effect. ; Step S4: Based on the target wavenumber envelope The correspondence between the output wavenumber and the drive current, and the correspondence between the output optical power and the drive current, are used to determine the wavenumber-time trajectory. The dwell time and corresponding output optical power in different wavenumber ranges together form the target wavenumber envelope. ; based on the inverse function of the correspondence between the output wavenumber and the drive current The wavenumber-time trajectory Converted to drive current waveform The driving current waveform is determined based on the thermal response time constant of the thermally tuned semiconductor laser. Perform dynamic compensation; Step S5: Utilize the aforementioned drive current waveform The thermally tunable semiconductor laser is driven, its wavelength-time curve and output optical power are measured, and the measured wavenumber envelope is obtained by conversion. ; Step S6: Envelope the measured wavenumber With the target wavenumber envelope Align them according to their respective center wavenumbers and calculate their similarity. ; Step S7: In the similarity When the similarity is less than a preset similarity threshold, the measured wavenumber envelope is used as the reference. With the target wavenumber envelope Adjusting the deviation between the drive current waveform The parameters are then used, and steps S4 to S6 are repeated until the similarity is achieved. If the similarity is greater than or equal to the preset similarity threshold, the corresponding drive current waveform parameters are output.

2. The thermally tunable semiconductor laser control method according to claim 1, characterized in that, The driving current waveform is a nonlinear continuous waveform or a segmented waveform, and each segment of the segmented waveform is a linear ramp, a nonlinear function curve, or a constant current step; the absolute value of the slope of the wavenumber-time trajectory in the central wavenumber interval of the target wavenumber envelope is less than the absolute value of the slope in the edge wavenumber interval of the target wavenumber envelope; when the segmented waveform includes a constant current step, the duration of each constant current step is greater than or equal to the thermal response time constant of the thermally tuned semiconductor laser.

3. The thermally tunable semiconductor laser control method according to claim 1, characterized in that, The similarity Calculate according to the following formula: ； in, and In calculating the similarity Previously, they were normalized according to their respective peak values, and the center wavenumbers of the two were made to coincide by translating the wavenumber coordinates; the preset similarity threshold is greater than or equal to 0.5 and less than or equal to 0.

99.

4. The thermally tunable semiconductor laser control method according to claim 1, characterized in that, The target wavenumber envelope The target wavenumber envelope is either a Gaussian wavenumber envelope or a Hamming wavenumber envelope; When the target wavenumber envelope is a Gaussian wavenumber envelope, the target wavenumber envelope is... satisfy: ； in, For the target center wavenumber, denoted as the standard deviation in the wavenumber domain.

5. The thermally tunable semiconductor laser control method according to claim 1, characterized in that, The full width at half maximum (FWHM) of the target wavenumber envelope accounts for 20% to 80% of the total wavenumber scan range of the thermally tunable semiconductor laser.

6. The method for controlling a thermally tunable semiconductor laser according to any one of claims 1 to 5, characterized in that, It also includes the following comprehensive performance optimization steps: Step S11: Determine the target axial resolution based on the application scenario of optical coherence measurement. Target sensitivity and target sweep frequency And determine the constraints for peak drive current, average power and junction temperature; Step S12: Based on the target axial resolution Determine the reference effective wavenumber bandwidth And thereby determine the bandwidth of the target wavenumber envelope; Step S13: Initialization including minimum drive current Maximum drive current The driving current waveform parameters, including duty cycle D, number of waveform segments, and slope or shape parameters of each waveform segment, are determined, and it is determined whether the initialized driving current waveform parameters meet the constraints. Step S14: Drive the thermally tunable semiconductor laser using the driving current waveform parameters that satisfy the constraints, input the light into a Mach-Zehnder interferometer, obtain an interference signal of a fixed frequency, and obtain the measured wavenumber envelope corresponding to the j-th iteration. ; Step S15: Analyze the measured wavenumber envelope under both windowed and unwindowed conditions. Perform a Fourier transform to obtain the point spread function, and then apply the measured wavenumber envelope. The effective wavenumber bandwidth is determined by the point spread function. Effective optical power Side lobe level Sensitivity and comprehensive indicators ; Step S16: When the comprehensive index When the preset performance conditions are not met, the effective wavenumber bandwidth is adjusted according to the drive current waveform parameters. The effective optical power The side lobe level and the sensitivity To determine the direction of influence, adjust the driving current waveform parameters and repeat steps S14 to S16 until the comprehensive index is obtained. If the preset performance conditions are met or the preset iteration termination conditions are reached, the corresponding drive current waveform parameters are output.

7. The thermally tunable semiconductor laser control method according to claim 6, characterized in that, The comprehensive indicators Calculate according to the following formula: ； in, For effective wavenumber bandwidth, For effective optical power, The sidelobe level of the point spread function. , and These are reference values ​​for effective wavenumber bandwidth, effective optical power, and sidelobe level, respectively. , and These are weighting coefficients for effective wavenumber bandwidth, effective optical power, and sidelobe level, respectively; for biological tissue tomography, the weighting coefficients are increased. For measurements on a single reflecting surface, the weighting coefficient is increased. .

8. The thermally tunable semiconductor laser control method according to claim 6, characterized in that, Adjusting the drive current waveform parameters includes: changing each drive current waveform parameter separately, and determining the effect of each drive current waveform parameter on the overall index. The direction of influence, and along the comprehensive index The direction of increase involves joint adjustment of at least two drive current waveform parameters; the preset iteration termination condition includes the comprehensive index. The rate of change after a preset number of iterations is less than a preset convergence threshold, or the number of iterations reaches a preset maximum number of iterations; the output drive current waveform parameters include the sweep frequency and minimum drive current. Maximum drive current Duty cycle D and parameters for each waveform segment.

9. An optical coherence measurement system employing the thermally tunable semiconductor laser control method according to any one of claims 1 to 5, characterized in that, include: Frequency-sweeping light sources, including thermally tunable semiconductor lasers, are used to output frequency-sweeping light whose wavelength varies with the driving current. Part of the light from the frequency-sweeping light source enters the auxiliary interferometer, and part enters the interference optical path; The control module is electrically connected to the swept frequency light source and is used to generate a drive current waveform based on the correspondence between the target wavenumber envelope, the output wavenumber and the drive current, and the correspondence between the output optical power and the drive current, and to adjust the drive current waveform based on the deviation between the measured wavenumber envelope and the target wavenumber envelope. An interference optical path, connected to the sweep frequency light source optical path, is used to split the sweep frequency light into reference arm light and sample arm light, and to cause the reference arm return light and the sample arm return light to interfere with each other in order to output an interference optical signal; A first photodetector is connected to the interference optical path and is used to convert the interference optical signal into a first electrical signal S1; Auxiliary interferometer, including Auxiliary interference channels with different optical path lengths are used for time-division or simultaneous generation. The circuit contains auxiliary interference electrical signals K1 to KN of different frequencies, among which... It is a positive integer greater than or equal to 2; The data acquisition card is electrically connected to the first photodetector and the auxiliary interferometer, respectively, and is used to acquire the first electrical signal S1 and the... Path-assisted interference electrical signals K1 to KN; A computer, electrically connected to the data acquisition card, is used to... The wavenumber sampling relationship and depth calibration relationship are determined by the auxiliary interferometric electrical signals K1 to KN, and the first electrical signal S1 is resampled in the wavenumber domain and corrected in depth according to the wavenumber sampling relationship and the depth calibration relationship.

10. The optical coherence measurement system according to claim 9, characterized in that, The auxiliary interferometer includes a second fiber beam splitter, a polarization controller, a third fiber beam splitter, an optical path switching component, a second photodetector, and a filter; the optical path switching component includes an input terminal and... One output terminal, the Each output terminal is connected to an optical path of different... One auxiliary interference channel; the optical path switching component is used to sequentially select the [channel name missing] according to the timing control signal. A second photodetector is used to convert the auxiliary interference optical signal formed after being combined by the third fiber beam splitter into the auxiliary interference electrical signal, and the filter is used to filter the auxiliary interference electrical signal.

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