A short-coherent light source and interferometer based on a noise-modulated external cavity feedback LD

Abstract

This invention discloses a short coherent light source and interferometer based on a noise-modulated external cavity feedback LD, comprising: a noise source, a DC drive power supply, an RF biaser, a temperature control module, a Fabry-Perot type semiconductor laser, an external cavity mirror, a half-wave plate, a polarization-maintaining fiber coupler, an orthogonal polarization system, and a Thyman-Green dynamic interferometer. Based on noise modulation, feedback is provided through the external cavity, completely suppressing the coherent sidelobes caused by the internal cavity. By changing the length of the external cavity, the position of the external cavity coherent sidelobes can be freely controlled, achieving a controllable sidelobe-free interference range. Compared to short coherent light sources that only use noise current modulation, the short coherent light source designed in this invention has a high coherent sidelobe suppression ratio, not only avoiding multi-surface fringe crosstalk caused by the superposition of interference fringes on the front and rear surfaces, but also solving the stray fringe crosstalk problem caused by coherent sidelobes, eliminating ripple errors in measurements on transparent parallel plate surfaces, and improving the accuracy of interferometric measurements.

Description

Technical Field

[0001] This invention pertains to interferometer technology, specifically relating to a short coherent light source and interferometer based on a noise-modulated external cavity feedback semiconductor laser (LD). Background Technology

[0002] Short coherent light sources can achieve optical interference at the zero optical path difference (OPD) position, and are widely used in surface error interferometry, microscopic interferometry, fiber optic gyroscopes, and other fields. In the interferometry measurement of surface errors of transparent parallel plates, short coherent light sources can avoid fringe crosstalk caused by multi-surface interference.

[0003] Over the past few decades, superluminescent diodes (SLDs), light-emitting diodes (LEDs), and halogen lamps have been used as short-coherence light sources. Due to their extremely short coherence lengths (typically measured in micrometers), these sources are difficult to optically path-match in interferometers without focal plane-assisted positioning. In classic Twyman-Green and Fizeau interferometers, micrometer-resolution zero-OPD positioning requires measurements within tens of millimeters or even meters.

[0004] The coherence length of laser diodes (LDs) can be controlled in the hundreds of micrometers by current modulation, making them suitable for fast and accurate zero-OPD matching in Twyman-Green and Fizeau interferometers. H. Lee et al. proposed a frequency modulation method to broaden the LD spectrum, achieving multi-longitudinal mode output through frequency modulation and shortening the coherence length to 200 μm (Precis. Eng. 2014, 38(4), 964–968). K. Wada et al. achieved short coherence lasers by superimposing lasers with different spectra under radio frequency (RF) modulation; although the suppression ratio of first-order coherent sidelobes was improved to 96%, the suppression ratio of second-order coherent sidelobes was 88%, and stray fringe crosstalk still occurred when the position of the sidelobes was matched with the optical path of the mirror under test (Jpn. J. Appl. Phys. 2005, 44(12), 8484–8490). 4D Technology has developed a GHz microwave-modulated short coherent laser, which has been successfully applied to a commercial 600 mm large-aperture Fizeau dynamic interferometer (Proc. SPIE 2017, 10329, 103291G). Han Zhigang et al. from Nanjing University of Science and Technology proposed an all-fiber orthogonally polarized noise-modulated short coherent laser for dynamic interferometry. This short coherent light source exhibits high vibration resistance and flexibility, and utilizes modulated short coherent light with a sidelobe suppression ratio (SLSR) of 80% to solve the multi-surface fringe crosstalk problem of parallel plates of specific thickness (Opt. Express 2023, 31(9), 14735–14749). However, this scheme does not completely eliminate the influence of coherent sidelobes, and stray fringe crosstalk still exists. In the patent "Interferometric Measurement Method Based on Short Coherent Light Source with All-Fiber Orthogonal Polarization Path Matching" (CN115112045A), the optical path matching of the short coherent light source is achieved by manually adjusting the delay line, which makes it difficult to select the optimal position for fringe contrast with high precision. In the patent "A Multi-Surface Morphology Testing Method Based on Coherent Controllable Laser" (CN116222426A), an adjustable radio frequency attenuator and a broadband noise source current modulated semiconductor laser are used to reduce the influence of interference sidelobes on the zero optical path difference position and interferometric measurement, but it does not completely eliminate the crosstalk problem caused by coherent sidelobes. Summary of the Invention

[0005] The purpose of this invention is to provide a short coherence light source and interferometer based on a noise-modulated external cavity feedback LD, so as to avoid interference crosstalk of interference patterns on the flat plate under test.

[0006] The technical solution of this invention is: a short coherent light source based on a noise-modulated external cavity feedback LD, comprising a noise source, a first high-frequency radio frequency line, a second high-frequency radio frequency line, a DC drive power supply, a first conductor, a radio frequency biaser, a TCM, a second conductor, a semiconductor laser, an external cavity mirror, a half-wave plate, an optical fiber coupler, and a polarization-maintaining fiber. The assembly steps are as follows:

[0007] Step 1: The noise source and DC drive power supply are connected to the RF input terminal and DC input terminal of the RF biaser respectively through the first high-frequency RF line and the first wire. The output terminal of the RF biaser is connected to the semiconductor laser through the second high-frequency RF line. The TCM is connected to the semiconductor laser through the second wire. The external cavity mirror, half-wave plate, and fiber coupler are placed in the output optical path of the semiconductor laser in sequence with the optical axis in the same direction. The fiber coupler and the polarization-maintaining fiber are connected. Proceed to Step 2.

[0008] Step 2: Read the temperature of the semiconductor laser in real time according to the TCM, and control the TEC of the semiconductor laser through the TCM to keep the semiconductor laser at 25±0.002 °C, then proceed to step 3.

[0009] Step 3: Turn on the DC drive power supply. The DC drive power supply provides DC power current to the semiconductor laser through the RF biaser. Proceed to Step 4.

[0010] Step 4: Adjust the pitch and tilt of the external cavity mirror to provide optical feedback to the semiconductor laser and form an external cavity laser. The feedback intensity changes with the reflection coupling coefficient of the external cavity mirror. Proceed to step 5.

[0011] Step 5: The noise source outputs high-intensity, wide-spectrum noise, which, together with the DC drive current, is injected into the semiconductor laser through the RF biaser, causing the laser spectrum output by the semiconductor laser to be modulated and broadened, achieving the effect of a short coherent light source. Proceed to step 6.

[0012] Step 6: Align the modulated external cavity laser with the fiber coupler by rotating the half-wave plate on the slow axis, and then couple it into the polarization-maintaining fiber for output.

[0013] An orthogonal polarization system for a short coherent light source using a noise-modulated external cavity feedback LD includes a polarization-maintaining fiber bundler, a fiber tunable attenuator, a fiber delay line, a polarization-maintaining fiber bundler, and a short coherent light source.

[0014] A short-coherent light source is connected to an orthogonal polarization system via a first polarization-maintaining flange. The output short-coherent light passes through a polarization-maintaining fiber and enters a polarization-maintaining fiber beam splitter. The reference light P-beam is split and enters port 1 of the polarization-maintaining fiber beam combiner via an adjustable fiber attenuator. The test light S-beam enters port 2 of the polarization-maintaining fiber beam combiner via a fiber delay line. The reference light and the test light are combined by the polarization-maintaining fiber beam combiner and output as a beam of orthogonal polarized light through port 3 via a polarization-maintaining fiber. All components in the system are connected via polarization-maintaining fibers.

[0015] Optical path matching is achieved by controlling the optical fiber delay line to compensate for the optical path difference between the reference light and the test light; optical intensity matching is achieved by using an optical fiber adjustable attenuator to adjust the intensity difference between two orthogonally polarized lights, so as to obtain clear and stable interference fringes.

[0016] An interferometer employing a short coherent light source based on noise-modulated external cavity feedback LD includes a short coherent light source and an orthogonal polarization system, as well as a collimating mirror, a polarizing beam splitter, first to third quarter-wave plates, a standard mirror, a polarizing camera, and a mirror under test. Compared to an interferometer using only noise current modulation for a short coherent light source, the interferometer with external cavity feedback completely suppresses the coherent sidelobes caused by the internal cavity. The introduction of the external cavity also brings the influence of the external cavity coherent sidelobe peaks. When the thickness of the mirror under test matches the position of the external cavity sidelobe peaks, the coherent sidelobes caused by the external cavity can be freely controlled by controlling the cavity length, thus solving the stray fringe crosstalk problem caused by coherent sidelobes and improving the accuracy of interferometric measurements.

[0017] The short coherent light source based on noise-modulated external cavity feedback LD achieves a wide range of free optical path difference and a scanning resolution of up to 3μm through an orthogonal polarization system, in order to meet the measurement requirements of the interferometer under different cavity lengths.

[0018] Compared with existing light sources and interferometers, the significant advantages of this invention are:

[0019] (1) The present invention achieves high interference signal sidelobe suppression ratio and short coherence output through noise modulation and external cavity feedback, overcomes the crosstalk caused by coherent sidelobes, eliminates the ripple caused by crosstalk in flat surface interferometry, and improves measurement accuracy.

[0020] (2) This invention reduces single-pass gain and suppresses inner cavity side lobes by increasing external cavity feedback. Furthermore, the active region output end of the light source is coated with an anti-reflection and anti-reflection film, which has a low reflectivity. Compared with short coherent light sources that only use noise current modulation, it can achieve a high side lobe suppression ratio at a low noise power, effectively preventing damage to the laser from excessive noise power and improving the safety and stability of the system.

[0021] (3) The present invention has a controllable external cavity length, and the position of the external cavity side lobe can be freely controlled without affecting the output power, thus expanding the interference range without side lobe.

[0022] (4) The output light of the present invention is slow-axis aligned by a half-wave plate and then coupled out through a polarization-maintaining fiber, which has good polarization degree, spatial coherence and high output power, and can be applied to the surface error measurement of large-aperture flat optical components.

[0023] (5) The external cavity structure of the present invention is simple, the external cavity mirror has a wide reflection spectrum and low cost. Compared with traditional external cavity lasers, it does not require a complex diffraction grating to provide external cavity feedback. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of a short coherent light source and interferometer structure based on a noise-modulated external cavity feedback LD.

[0025] Figure 2 This is a schematic diagram illustrating the principle of an interferometer using a short coherent light source based on a noise-modulated external cavity feedback LD to scan the position of the external cavity peak as the external cavity mirror position changes, in order to match the thickness of the mirror under test with the position of the external cavity sidelobe peak.

[0026] Figure 3 This is a graph showing the output power of a short-coherent light source as a function of bias current.

[0027] Figure 4 The spectrum of a short coherent light source at different noise powers is shown.

[0028] Figure 5 This is a schematic diagram of the principle of optical path matching.

[0029] Figure 6 Normalized coherence curves for different noise levels and external cavity lengths.

[0030] Figure 7 Normalized coherence curves and interference fringe patterns for measuring fused silica parallel plates using a short coherent light source.

[0031] Figure 8 The front surface profile measured using a non-external cavity feedback semiconductor laser (noise power: 5.4 dBm).

[0032] Figure 9 The front surface profile measured using a short coherent light source (noise power: 30.5 dBm).

[0033] In the diagram, 1-noise source, 2-1 first high-frequency RF line, 2-2 second high-frequency RF line, 3-DC drive power supply, 4-first wire, 5-RF biaser, 6-temperature control module (TCM), 7-second wire, 8-semiconductor laser (LD), 9-external cavity mirror, 10-half-wave plate, 11-fiber coupler, 12-orthogonal polarization system, 13-Twyman-Green interferometer, 14-mirror under test, 1 5-1-First polarization-maintaining flange, 15-2-Second polarization-maintaining flange, 16-Polarization-maintaining fiber beam splitter (PBS), 17-Fiber adjustable attenuator (VOA), 18-Polarization-maintaining fiber bundler (PBC), 19-Fiber delay line (ODL), 20-Collimating lens, 21-Polarization beam splitter, First quarter wave plate-22-1, Second quarter wave plate-22-2, Third quarter wave plate-22-3, 23-Standard lens, 24-Polarization camera. Detailed Implementation

[0034] The present invention will now be described in further detail with reference to the accompanying drawings.

[0035] Combination Figure 1 Taking the Thyman Green interferometer as an example, this invention proposes for the first time an interferometer based on a short coherent light source using a noise-modulated external cavity feedback LD. The interferometer includes a short coherent light source, an orthogonal polarization system 12, a collimating mirror 20, a polarizing beam splitter 21, a first quarter-wave plate 22-1, a second quarter-wave plate 22-2, a third quarter-wave plate 22-3, a standard mirror 23, a test mirror 14, and a polarization camera 24. The innovation of this invention lies in introducing external cavity feedback based on the spectral broadening of noise current modulation. This can suppress coherent sidelobes caused by the internal cavity of the semiconductor chip and other singular feedbacks. Adjusting the length of the external cavity allows for free control of the sidelobe-free coherent range of the interferometer, avoiding interference crosstalk from the flat test plate. This new light source and interferometer can avoid stray fringe crosstalk caused by coherent sidelobes in the measurement of surface errors of parallel plates, eliminate interference ripples caused by stray fringe crosstalk, and improve measurement accuracy.

[0036] Combination Figure 2 Compared to short coherent light source interferometers that only use noise current modulation, short coherent light source interferometers with external cavity feedback completely suppress coherent sidelobes caused by the internal cavity. However, the introduction of the external cavity also brings the influence of external cavity coherent sidelobe peaks. When the thickness of the measured mirror matches the position of the external cavity sidelobe peak, fringe crosstalk will occur. This invention can freely control the coherent sidelobes caused by the external cavity by controlling the cavity length, thus solving the stray fringe crosstalk problem caused by coherent sidelobes and improving the accuracy of interferometric measurements.

[0037] Combination Figure 1 The short coherent light source based on noise-modulated external cavity feedback LD includes the following construction steps:

[0038] Step 1: Connect the short coherent light source based on the noise-modulated external cavity feedback LD as follows: connect the noise source 1 and the DC drive power supply 3 to the radio frequency (RF) input and DC input terminals of the RF bias 5 through the first high-frequency RF line 2-1 and the first wire 4, respectively. Connect the output terminal (OUT) of the RF bias 5 to the semiconductor laser 8 through the second high-frequency RF line 2-2. Connect the TCM6 to the semiconductor laser 8 through the second wire 7. Place the external cavity mirror 9, half-wave plate 10, and fiber coupler 11 on the output optical axis of the semiconductor laser 8 in sequence. Connect the fiber coupler 11 to the polarization-maintaining fiber 12. Proceed to Step 2.

[0039] Step 2: Based on the real-time temperature reading of the semiconductor laser 8 by the TCM6, control the TEC of the semiconductor laser 8 through the TCM6 to maintain the semiconductor laser 8 at 25±0.002 °C; control the temperature first and then turn on the semiconductor laser 8 to obtain a more stable output laser; proceed to step 3.

[0040] Step 3: The DC drive power supply 3 provides DC power to the semiconductor laser 8 through the RF biaser 5, and the output power of the semiconductor laser 8 as a function of the bias current is measured, as shown in the figure. Figure 3 As shown in the curve with solid points, proceed to step 4.

[0041] Step 4: Adjust the pitch and tilt of the external cavity reflector 9 to provide optical feedback to the semiconductor laser 8, forming an external cavity laser. The feedback intensity changes with the reflection coupling coefficient of the reflector. The output power of the external cavity laser as a function of the bias current is measured, as shown in the figure. Figure 3 As shown in the curve, combined with the data measured in step 3, it can be seen that the threshold current of the light source decreased after the external cavity feedback was added, from 85 mA to 66 mA; proceed to step 5; the external cavity modulation step should be before the noise current injection step. After the noise current is injected into the semiconductor laser 8, the laser outputs at a higher power. At this time, when the external cavity mirror 9 is used for modulation, it is impossible to observe obvious external cavity oscillation phenomenon, which has a significant impact on the experiment; proceed to step 5.

[0042] Step 5: Noise source 1 outputs high-intensity, wide-spectrum noise, which, together with the DC drive current, is injected into the semiconductor laser through RF biaser 5. The output power of the modulated external cavity laser as a function of the bias current is measured, as shown in the figure. Figure 3 The six-point curve is shown; the output laser spectrum is modulated and broadened, and the output spectrum at different noise powers is shown in the figure. Figure 4 As shown, the output spectral linewidth is broadened from 0.11 nm to 0.51 nm, achieving the effect of a short coherent light source. Proceed to step 6.

[0043] Step 6: Align the modulated external cavity laser with the fiber coupler 11 on the slow axis by rotating the half-wave plate 10, and then couple it into the polarization-maintaining fiber output.

[0044] Semiconductor laser 8 is a butterfly-shaped Fabry-Perot laser with an antireflection coating on the output end of the gain chip. Its reflectivity is less than 0.5%, the output center wavelength is 630 nm, and the output spectral linewidth is 0.1 nm.

[0045] The external cavity reflector 9 is a wedge or plate made of fused silica. The reflector does not require coating treatment and has a wide reflection band ranging from 200 nm to 2.2 µm and a single-sided reflectivity of 4%.

[0046] Half-wave plate 10 uses a zero-order wave plate with a center wavelength of 630 nm. The wave plate is coated with an anti-reflection and anti-dispersion film to reduce dispersion and backlighting.

[0047] A noise signal and a driving DC signal are superimposed in the RF biaser 5. The noise signal has a power of 30.5 dBm and a frequency bandwidth of 10 GHz. The bias current of the driving DC signal is 85 mA. The semiconductor laser (8) is directly modulated using the noise current. According to the multi-beam interference theory, the cavity gain spectrum of the semiconductor laser 8 is... Represented as:

[0048] ,

[0049] In the formula, R1 is the reflectivity of the high-reflectivity end of the semiconductor laser 8, R2 is the reflectivity of the low-reflectivity end of the semiconductor laser 8, v is the optical frequency, and Δv is the longitudinal mode spacing of the cavity. q1 = c / (2nL), where c is the speed of light, n is the refractive index of the resonant cavity, and L is the cavity length; v c (t) represents the center frequency of the laser, which varies linearly with the noise current, v c (t) = v0 [1 + MN(t)], where v0 is the center frequency at the bias DC, M is the frequency response coefficient of the noise power, N(t) is the normalized noise, and S(v,t) is the single-pass gain spectrum of the LD.

[0050] S(v,t) is represented as:

[0051] ,

[0052] In the formula, the center frequency v of the gain spectrum m (t) = v0[1 + m MN(t)], which varies linearly with the noise current, where m is the gain response coefficient of the noise power, Δv is the full width at half maximum (FWHM) of the gain spectrum, and S is the maximum value of the single-pass gain, S = S0 exp [-(S - 1) P out / (SPsat )], where S0 is the small-signal gain, P out For output power, P sat The saturation power is reached; when the saturation power is reached, the single-pass gain decreases as the output power increases.

[0053] External cavity gain spectrum of short coherent light source Represented as:

[0054] ,

[0055] In the formula, R3 is the reflectivity of the external cavity mirror 9, and Δv q2 The longitudinal mode spacing of the external cavity laser. The external cavity's reflection coupling coefficient is denoted as .

[0056] Combination Figure 5 When measuring a transparent parallel plate, a long coherent light source, due to its long coherence, causes the reference light to interfere with the light reflected from the front and rear surfaces simultaneously, and the reflected light from the front and rear surfaces also interferes with each other, resulting in crosstalk. A short coherent light source can avoid eliminating other unwanted interference fringes, allowing only the reference light to interfere with the light reflected from the front or rear surfaces. By using software to control the fiber delay line 19 to compensate for the optical path difference between the reference light and the test light in the interferometer, optical path matching is achieved, and interference fringe patterns of the front and rear surfaces are obtained respectively. The fiber adjustable attenuator 17 is used to adjust the intensity difference between two orthogonally polarized lights to achieve intensity matching, thereby obtaining clear and stable interference fringes.

[0057] Combined gain spectrum of short coherent light source output The integral of the product of the semiconductor laser (8) gain spectrum and the external cavity gain spectrum over the spectral detection time T is as follows:

[0058] ,

[0059] Complex coherence of short coherent light sources The inverse Fourier transform of its spectral distribution is as follows:

[0060] ,

[0061] In the formula, the time delay τ between the two interfering beams is ∆. OPD / c, optical path difference ∆ OPD = 2h, where h is the length difference between the two interference arms; j represents the imaginary part;

[0062] Assuming both beams of light have the same intensity I0, the interference fringe intensity I(h) is expressed as:

[0063] ,

[0064] Where Δφ is the additional phase and |γ| is the mode of complex coherence. In interferometry, |γ| is measured by the contrast as a function of optical path difference.

[0065] As can be seen from the above formula, the high-frequency components of the spectrum will cause coherent sidelobes; under the high-speed modulation of noise source 1, the more continuous the optical spectrum output by semiconductor laser 8 is modulated, the higher its coherent sidelobe suppression ratio is.

[0066] Combination Figure 6 Using a fused silica parallel plate as the test piece 14, normalized coherence curves as a function of optical path length were measured under different noise levels and external cavity lengths. The light intensity between the reference light source and the test light in the interference path was matched to obtain the maximum contrast fringes. Normalized coherence was calculated using a four-step phase-shift algorithm. As the noise power increased from 5.4 dBm to 28.6 dBm, the sidelobe suppression ratio of the external cavity-less semiconductor laser 8 improved from 6% to 53%. Due to high-speed modulation, the multi-longitudinal mode frequencies varying with the noise current overlapped during the spectral detection time, increasing spectral continuity and suppressing coherent sidelobes. At a low noise power of 5.4 dBm, periodic high-order coherent peaks were generated, and the optical path between adjacent peaks should be twice the optical path of the resonant cavity. The blue curve in the figure shows that a 93% sidelobe suppression ratio was achieved when a 38.4 mm long external cavity was introduced, with a significant reduction in internal cavity peaks. This is because the single-pass gain decreases with increasing output power, leading to suppression of internal cavity sidelobes. Due to the introduction of the external cavity, a sidelobe with the same suppression ratio appears at a distance of 76.8 mm optical path from the inner cavity sidelobe. Increasing the external cavity length by 26.1 mm doubles the increase in external cavity length, shifting the external cavity peak by a distance of 52.2 mm, as shown by the red curve. The coherent sidelobe caused by the external cavity accounts for approximately 3% of the main peak, almost identical to the external cavity peak value measured at an external cavity length of 38.4 mm. The results indicate that although coherent sidelobes from the external cavity may be generated during inner cavity sidelobe suppression, the position of the external cavity-induced coherent sidelobes can be controlled by shifting the external cavity length. Therefore, a controllable coherence range can be achieved without sidelobes.

[0067] Example 1

[0068] Combination Figure 1 The test was conducted using the short coherent light source and interferometer based on the noise-modulated external cavity feedback LD of the present invention. The test piece 14 was a fused silica parallel plate with a thickness of 10 mm.

[0069] The short coherent light source is connected to the orthogonal polarization system 12 and the interferometer 13 in sequence through the first polarization-maintaining flange 15-1 and the second polarization-maintaining flange 15-2, respectively.

[0070] The fused silica test mirror 14 is placed in the test optical path of the interferometer 13. The length of the interferometer test path cavity is coarsely adjusted so that the zero optical path position of the optical path matching between the reference optical path and the test optical path is within the 200 mm scanning optical path range of the fiber delay line 19.

[0071] Turn on the short coherent light source and output short coherent light into the interferometer 13 through the orthogonal polarization system 12.

[0072] By controlling the fiber delay line 19 in the orthogonal polarization system 12 with interferometric measurement software, the coherence of the interference signal is scanned in steps within a 200 mm optical path range.

[0073] By changing the delay of fiber delay line 19 to locate the zero optical path difference position, optical path matching is achieved; by adjusting VOA17 to match the light intensity of the reference optical path and the test optical path, interference fringes with the highest contrast are obtained.

[0074] The current interference fringe pattern is acquired by the polarization camera 24 in the interferometer 13, and the surface shape data of the fused silica test mirror 14 is obtained by solving the four-step phase shifting algorithm.

[0075] Combination Figure 7 At a noise power of 5.4 dBm, due to the relatively low modulation power, the coherence curve exhibits strong sidelobes, as shown by the gray curve in the figure. The coherent sidelobes on the rear surface coincide with the main peak on the front surface, forming typical stray fringe crosstalk, as shown in the typical stray fringe crosstalk diagram. When using the light source designed in this invention at a noise power of 30.5 dBm, the normalized sidelobe suppression ratio is at most 98.4%, as shown by the blue curve in the figure. Since there are no obvious coherent sidelobes, the interferogram can only be obtained at the position where the scanning optical path of the fiber delay line 19 matches the front and rear surfaces. Based on the fringe peak spacing of 30.0 mm and the refractive index of fused silica (1.457), the thickness of the measured part can be calculated to be 10.3 mm, which is consistent with the actual thickness.

[0076] Combination Figure 8 , Figure 9 The ripples on the measured surface are caused by stray fringe crosstalk. When the noise power of the injected short coherent light source increases to 30.5 dBm, the ripples caused by the coherent sidelobes disappear, and the peak-to-valley (PV) and root mean square (RMS) values ​​of the front surface are 31.9 nm and 4.5 nm, respectively, which are close to the accuracy limit of the dynamic interferometer (PV = λ / 20, λ = 633 nm).

[0077] In summary, the short coherence light source and interferometer designed in this invention have a high coherence sidelobe suppression ratio, which not only avoids multi-surface fringe crosstalk caused by the superposition of interference fringes on the front and back surfaces, but also solves the problem of stray fringe crosstalk caused by coherence sidelobes, eliminates ripple error in the measurement of transparent parallel plate surfaces, and improves the accuracy of interferometric measurements.

Claims

1. A short-coherent light source based on a noise-modulated external cavity feedback LD, characterized in that, The components include a noise source (1), a first high-frequency radio frequency line (2-1), a second high-frequency radio frequency line (2-2), a DC drive power supply (3), a first wire (4), a radio frequency biaser (5), a TCM (6), a second wire (7), a semiconductor laser (8), an external cavity mirror (9), a half-wave plate (10), and an optical fiber coupler (11). The assembly steps are as follows: Step 1: The noise source (1) and the DC drive power supply (3) are connected to the RF input terminal and DC input terminal of the RF bias unit (5) respectively through the first high-frequency RF line (2-1) and the first wire (4). The output terminal of the RF bias unit (5) is connected to the semiconductor laser (8) through the second high-frequency RF line (2-2). The TCM (6) is connected to the semiconductor laser (8) through the second wire (7). The external cavity mirror (9), the half-wave plate (10), and the fiber coupler (11) are placed in the output optical path of the semiconductor laser (8) in sequence with the same optical axis. The fiber coupler (11) is connected to the polarization-maintaining fiber. Proceed to Step 2. Step 2: Read the temperature of the semiconductor laser (8) in real time according to the TCM (6), control the TEC of the semiconductor laser (8) through the TCM (6) to keep the semiconductor laser (8) at 25±0.002 °C, and then proceed to step 3; Step 3: Turn on the DC drive power supply (3). The DC drive power supply (3) provides DC power supply current to the semiconductor laser (8) through the radio frequency bias (5). Proceed to step 4. Step 4: Adjust the pitch and tilt of the external cavity mirror (9) to provide optical feedback to the semiconductor laser (8) and form an external cavity laser. The feedback intensity changes with the reflection coupling coefficient of the external cavity mirror (9). Proceed to step 5. Step 5: The noise source (1) outputs high-intensity, wide-spectrum noise, which, together with the DC drive current, is injected into the semiconductor laser (8) through the radio frequency biaser (5), causing the laser spectrum output by the semiconductor laser (8) to be modulated and broadened, achieving the effect of a short coherent light source, and then proceeding to step 6. Step 6: Align the modulated external cavity laser with the fiber coupler (11) by rotating the half-wave plate (10) on the slow axis, and then couple it into the polarization-maintaining fiber for output.

2. A short-coherence light source based on a noise-modulated external cavity feedback LD according to claim 1, characterized in that: The semiconductor laser (8) is a butterfly Fabry-Perot laser with an antireflection coating on the output end of the gain chip. Its reflectivity is less than 0.5%, the output center wavelength is 630 nm, and the output spectral linewidth is 0.1 nm.

3. A short-coherence light source based on a noise-modulated external cavity feedback LD according to claim 2, characterized in that: The external cavity reflector (9) is a wedge or plate made of fused silica. The reflector does not require coating treatment and has a wide reflection band ranging from 200 nm to 2.2 µm and a single-sided reflectivity of 4%.

4. A short-coherence light source based on a noise-modulated external cavity feedback LD according to claim 3, characterized in that: The half-wave plate (10) uses a zero-order wave plate with a center wavelength of 630 nm. The wave plate is coated with an anti-reflection and anti-dispersion film to reduce dispersion and backlighting.

5. A short-coherence light source based on a noise-modulated external cavity feedback LD according to claim 4, characterized in that: A noise signal and a driving DC signal are superimposed in the radio frequency biaser (5). The noise signal has a power of 30.5 dBm and a frequency bandwidth of 10 GHz. The bias current of the driving DC signal is 85 mA. The noise current is used to directly modulate the semiconductor laser (8). According to the multi-beam interference theory, the cavity gain spectrum of the semiconductor laser (8) is... Represented as: ， In the formula, R1 is the reflectivity of the high-reflectivity end of the semiconductor laser (8), R2 is the reflectivity of the low-reflectivity end of the semiconductor laser (8), v is the optical frequency, and Δv is the longitudinal mode spacing of the cavity. q1 = c / (2nL), where c is the speed of light, n is the refractive index of the resonant cavity, and L is the cavity length; v c (t) represents the center frequency of the laser, which varies linearly with the noise current, v c (t) = v0 [1 + MN(t)], where v0 is the center frequency at the bias DC, M is the frequency response coefficient of the noise power, N(t) is the normalized noise, and S(v,t) is the single-pass gain spectrum of the LD. S(v,t) is represented as: ， In the formula, the center frequency v of the gain spectrum m (t) = v0[1 + m MN(t)], which varies linearly with the noise current, where m is the gain response coefficient of the noise power and Δv is the full width at half maximum (FWHM) of the gain spectrum; S is the maximum value of the single-pass gain, S = S0 exp [- (S -1) P out / (SP sat )], where S0 is the small-signal gain, P out For output power, P sat The saturation power is reached; when the saturation power is reached, the single-pass gain decreases as the output power increases.

6. A short-coherence light source based on a noise-modulated external cavity feedback LD according to claim 5, characterized in that: External cavity gain spectrum of short coherent light source Represented as: ， In the formula, R3 is the reflectivity of the external cavity mirror (9), and Δv q2 The longitudinal mode spacing of the external cavity laser. The external cavity's reflection coupling coefficient is denoted as .

7. A short-coherence light source based on a noise-modulated external cavity feedback LD according to claim 6, characterized in that: Combined gain spectrum of short coherent light source output The integral of the product of the semiconductor laser (8) gain spectrum and the external cavity gain spectrum over the spectral detection time T is as follows: ， Complex coherence of short coherent light sources The inverse Fourier transform of its spectral distribution is as follows: ， In the formula, the time delay τ between the two interfering beams is ∆. OPD / c, optical path difference ∆ OPD = 2h, where h is the length difference between the two interference arms; j represents the imaginary part; Assuming both beams of light have the same intensity I0, the interference fringe intensity I(h) is expressed as: ， Where Δφ is the additional phase and |γ| is the mode of complex coherence. In interferometry, |γ| is measured by the contrast as a function of optical path difference. As can be seen from the above formula, the high-frequency components of the spectrum will cause coherent sidelobes; under the high-speed modulation of the noise source (1), the more continuous the optical spectrum output by the semiconductor laser (8) is modulated, the higher its coherent sidelobe suppression ratio is.

8. An orthogonal polarization system (12) using a short-coherent light source based on a noise-modulated external cavity feedback LD, characterized in that: It includes a polarization-maintaining fiber bundler (16), an optical fiber adjustable attenuator (17), an optical fiber delay line (19), a polarization-maintaining fiber bundler (18), and a short coherent light source as described in any one of claims 1 to 7; The short coherent light source is connected to the orthogonal polarization system (12) through the first polarization-maintaining flange (15-1). The output short coherent light enters the polarization-maintaining fiber beam splitter (16) through the polarization-maintaining fiber. The split reference light P light enters port 1 of the polarization-maintaining fiber combiner (18) through the fiber adjustable attenuator (17). The test light S light enters port 2 of the polarization-maintaining fiber combiner (18) through the fiber delay line (19). The reference light and the test light are combined by the polarization-maintaining fiber combiner (18) and output as a beam of orthogonal polarized light through port 3 through the polarization-maintaining fiber. The various devices in the system are connected through the polarization-maintaining fiber. Optical path matching is achieved by controlling the optical delay line (19) to compensate for the optical path difference between the reference light and the test light; optical intensity matching is achieved by using the optical fiber adjustable attenuator (17) to adjust the intensity difference between the two orthogonally polarized lights, so as to obtain clear and stable interference fringes.

9. An interferometer (13) using a short-coherent light source based on a noise-modulated external cavity feedback LD, characterized in that: The system includes a short coherent light source as described in any one of claims 1 to 7 and an orthogonal polarization system (12) as described in claim 8, as well as a collimating mirror (20), a polarizing beam splitter (21), a first quarter wave plate (22-1), a second quarter wave plate (22-2), a third quarter wave plate (22-3), a standard mirror (23), a polarization camera (24), and a mirror under test (14). Compared with a short coherent light source interferometer that only uses noise current modulation, the short coherent light source interferometer with external cavity feedback completely suppresses the coherent side lobes caused by the internal cavity. The introduction of the external cavity also brings the influence of the external cavity coherent side lobe peak. When the thickness of the mirror under test matches the position of the external cavity side lobe peak, the coherent side lobes caused by the external cavity can be freely controlled by controlling the cavity length, thus solving the problem of stray fringe crosstalk caused by coherent side lobes and improving the accuracy of interferometric measurement.

10. The interferometer based on a noise-modulated external cavity feedback LD for short-coherent light sources according to claim 9, characterized in that: The short coherent light source achieves a wide range of free optical path difference and a scanning resolution of up to 3 μm by an orthogonal polarization system (12) to meet the requirements of the interferometer for measurement under different cavity lengths.

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