CN117850059A - Device and method for reducing speckle contrast of laser source - Google Patents

Abstract

The invention discloses a device and a method for reducing speckle contrast of a laser source, which relate to the field of laser display. After being collimated by a lens, light emitted by the semiconductor laser is incident into the grating to generate diffracted light, part of the diffracted light is fed back to the laser, the other part of the diffracted light is used as emergent light, a modulating signal is applied to the laser and the piezoelectric ceramic, the piezoelectric ceramic drives the grating to change the angle, so that the wavelength of the diffracted light is changed, a plurality of different speckle patterns are superimposed within a certain time, and the best speckle suppression effect is achieved in terms of time and space coherence.

Description

Device and method for reducing speckle contrast of laser source

Technical Field

The invention relates to a technology for reducing speckle contrast of a laser source, in particular to a device and a method for reducing speckle contrast by mechanically and electronically modulating a semiconductor laser based on a grating external cavity optical feedback structure.

Background

The laser display technology is the new generation display technology with the most development potential because of the characteristics of large color gamut, high brightness, low maintenance cost and the like. But the problem of speckle due to the high coherence of the laser itself prevents further application of laser display technology.

The laser speckle is a random intensity pattern caused by random interference of laser beams on a display screen, so that the resolution and contrast of images can be reduced for display, image information is lost, eye strain and eye fatigue are caused, and the laser speckle is one of key problems to be solved in the laser display technology.

There are two main approaches to inhibiting laser speckle: one is to change the coherence of the laser beam emitted by the laser through various optical-mechanical components, such as means of vibration light homogenizing sheet, light pipe, multimode fiber, depolarization screen and the like; and secondly, the coherence of laser is reduced at the source of the laser, such as the technology of developing a broadband laser source, multi-wavelength mixing, a degenerate cavity laser and the like.

Among other things, the approach of optomechanical structures tends to require multiple optomechanical components to mate, generally increasing the volume and complexity of the system. If the light source is improved, the basic output power, efficiency, volume and cost of the laser are ensured, and the practical production and application scene can be met.

Currently, the development of low-speckle laser sources is mainly based on the following approaches: firstly, changing the spectral characteristics of laser beams, namely increasing the spectral width of output laser beams, or combining the laser beams with a plurality of wavelengths; or in the time domain and in the spatial mode of the laser beam in order to produce appropriate amplitude fluctuations or a more uniform spatial mode distribution to reduce the temporal and spatial coherence of the laser. However, the foregoing technical means often cannot fully compromise volume, cost, power and energy conversion efficiency. The broadband laser light source commercialized at present is mainly a traditional semiconductor laser, but an additional optical mechanical structure is still needed to further inhibit speckles (appl. Opt. 55.6:1267-1274); other studies on broadband lasers often use special gain media or laser structures (appl. Phys. B117:1117-1121), which are less than ideal in cost and efficiency. While lasers based on random lasers and degenerate cavity structures can effectively suppress laser speckle noise by reducing the temporal or spatial coherence of the laser, they also face limitations in terms of output power, energy conversion efficiency, volume, and cost (opt. Lett. 45:3816-3819), which are still a distance from practical use.

In recent years, various types of lasers based on optical feedback have been used in communication, sensing, and various experimental applications. The chaotic output of the laser can be realized by reasonably designing the optical feedback structure, and the chaotic waveform which is an irregular signal in the time domain is beneficial to inhibiting the speckle of laser. On the other hand, a semiconductor laser based on a grating external cavity optical feedback structure has been used for generating a visible light laser source with a narrow linewidth, and has the characteristics of simple structure, high power and efficiency, low cost and the like, so that the semiconductor laser has been receiving a great deal of attention. Therefore, the laser light source designed and realized based on the grating external cavity optical feedback structure has unique advantages in economy and practicality.

Disclosure of Invention

The invention provides a device and a method for reducing speckle contrast of a laser source aiming at the defects of the prior art.

In a first aspect, the present invention provides an apparatus for reducing speckle contrast of a laser source, comprising a semiconductor laser, a first lens, a diffraction grating, a second lens, a multimode fiber, and a light pipe.

The output light of the semiconductor laser is collimated by the first lens and then enters the diffraction grating, and the diffraction grating is used for periodically adjusting the output wavelength and mode of laser, so that the speckle of the laser is reduced in a spectrum level;

the second lens is used for converging the grating diffraction light beams, and the converged light beams are incident to the light guide pipe.

In a second aspect, the present invention provides a testing device for reducing speckle contrast of a laser source, which includes the device for reducing speckle contrast of a laser source, and further includes a projection lens, a screen, and a camera; the outgoing light of the light pipe is projected on the screen through the projection lens; the camera performs speckle sampling to simulate the human eye.

In a third aspect, the present invention provides a method for reducing speckle contrast of a laser source, where the foregoing device for reducing speckle contrast of a laser source specifically includes:

setting a modulation signal for the current of the laser to widen the output spectrum of the laser;

applying a periodic square wave modulation signal to the piezoelectric ceramic, so as to drive the diffraction grating to realize angle change, and periodically changing the diffraction angle of the emergent laser beam and the grating between 10.6-10.8 degrees, thereby realizing multi-wavelength oscillation output;

the second lens is arranged on the light path of the 1 st-order diffraction light of the diffraction grating, and the incidence section of the multimode optical fiber is adjusted horizontally/vertically so that the emergent light spots of the multimode optical fiber are uniform circular light spots;

the multi-mode optical fiber emergent end is horizontally/vertically adjusted, so that the light beam emergent from the light pipe presents a plurality of hexagonal light spot patterns symmetrically distributed on the X/Y axis; the speckle contrast is further reduced by homogenizing light from the output optical path through the multimode optical fiber and the light pipe.

The invention has the beneficial effects that:

firstly, the invention can modulate the output wavelength, effectively broaden the frequency spectrum of the output wavelength or realize the oscillation output of multiple wavelengths by a method of adjusting the diffraction grating of the external cavity laser through vibration, and then reduce the speckle of the laser on the spectrum level; meanwhile, the periodic current modulation is applied to the laser, so that the stability of laser feedback is further destroyed, and further fluctuation of output amplitude is formed in a time domain.

Secondly, the laser mode fed back to the laser is quickly adjusted through grating vibration, then the laser mode fed back and output is modulated in time domain and space, the time and space coherence of the laser beam is reduced, and speckle contrast is further suppressed.

Thirdly, the grating element modulated by the invention is a part of the external cavity laser, and the laser beam is often required to be turned back and collimated in the optical path of the practical application case, so the invention does not obviously increase the whole volume of the system; and the energy loss of 10-20% can be generated after the collimation and the foldback of the common multimode semiconductor laser, and the actual obtained power is similar to that of the invention.

Drawings

The following describes the embodiments of the present invention in further detail with reference to the drawings;

FIG. 1 shows a diagram of an apparatus and test apparatus for reducing speckle contrast of a laser source provided in example 1;

FIG. 2 shows a speckle image without the speckle removing device added;

fig. 3 shows a speckle image under the conditions of example 1.

Detailed Description

The invention provides a method for reducing laser speckle contrast, which is mainly based on a semiconductor laser light source based on a grating external cavity optical feedback structure under mechanical and electrical modulation, wherein a grating is subjected to vibration with certain frequency and amplitude, then the output wavelength and mode of laser are periodically regulated, the loading current of the laser source is periodically modulated, the basic output power is ensured, the speckle diversity is increased within a certain exposure time, and the time and spatial coherence of the laser beam are reduced, so that the low speckle laser output is realized.

The embodiment of the application provides a device for reducing speckle contrast of a laser source, which comprises a semiconductor laser, a first lens, a diffraction grating, a second lens, a multimode fiber and a light pipe.

The output light of the semiconductor laser is collimated by the first lens and then enters the diffraction grating, and the diffraction grating is used for periodically adjusting the output wavelength and mode of laser, so that the speckle of the laser is reduced in a spectrum level;

the second lens is used for converging the grating diffraction light beams, and the converged light beams are incident to the light guide pipe.

Further, the center wavelength of the semiconductor laser is 400-700 nm, and the semiconductor laser comprises a blue light GaN semiconductor laser, a green light GaN semiconductor laser and a red light InGaAs laser; the semiconductor laser is controlled and driven by a laser driver.

The laser driver outputs a modulated current at a frequency of 2000 Hz.

Furthermore, the collimating lens adopts an aspheric lens to optimize spherical aberration to obtain better collimating effect, the focal length f=5mm, and the mirror surface is plated with an AR film, so that the reflectivity of the wave band of 400-700 nm is lower than 0.5%.

Further, the diffraction grating is a reflective diffraction grating, including a reflective reticle grating and a reflective holographic grating.

In one example, the grating line density comprises 600-1800 lines/mm with a blaze wavelength located near the center wavelength of the laser.

Further, the outgoing light included angle between the grating and the semiconductor laser satisfies the formula 2d.sinθ=λ, where d is the grating line density, θ is the included angle between the grating and the outgoing light, and λ is the center wavelength of the diffracted light.

In one example, the diffraction grating is fixed to the piezoelectric ceramic by a custom fixture; the front and the back of the clamp are respectively provided with a grating and a mounting groove of piezoelectric ceramics; the piezoelectric ceramics adopt co-fired stacked piezoelectric ceramics to drive the grating to deflect, and the custom fixture can be matched to bring displacement of 0-28 mu m to the grating.

Furthermore, the piezoelectric ceramic is connected with a signal generator, and the signal generator applies a square wave signal of 50-100 Hz and 0-5 Vpp to the piezoelectric ceramic to modulate the vibration frequency and deflection angle of the piezoelectric ceramic plate.

The second lens converges the grating diffraction light beams and then enters the light guide pipe; the second lens is an aspheric lens, an AR film is plated on the mirror surface, and the focal length is 18mm;

further, the light pipe structure parameter is determined by the following formula,where θ is the divergence angle of the incident light, L is the length of the light pipe, and D is the cross-sectional diameter of the light pipe.

In one example, the screen is a non-smooth white opaque plane, including most projection screens, white walls, white printing papers, and the like.

In a certain example, the camera is a black-and-white scientific research camera, including a CCD camera and a CMOS camera, and is matched with a lens to sample speckle images on a screen.

Embodiment one:

the device for reducing speckle contrast of the laser source comprises a semiconductor laser 1, a semiconductor laser driver 11, wherein a collimating lens 2, a grating 3, a clamp 31, piezoelectric ceramics 32, a signal generator 33, a lens 4, a multimode fiber 5, a light pipe 6, a projection lens 7, a screen 8 and a camera 9 are sequentially arranged on one side of the semiconductor laser 1.

In the present embodiment, the center wavelength of the semiconductor laser 1 is 445nm; the laser driver 11 outputs a current 1A with a modulation frequency of 2000Hz and a pulse width of 400000ns;

in this embodiment, the collimating lens 2 is a 1/2 inch AR film coated aspheric lens with a reflectance of <0.25% at a center wavelength of 450nm, a focal length of 8mm, and a numerical aperture na=0.5; the collimating lens 2 collimates the emergent light of the semiconductor laser 1, and the diameter of a collimated laser spot is about 7mm;

in this embodiment, the diffraction grating 3 is a reflective grating line grating, the size is 12.7x12.7mm, the number of grating lines is 600/nm, and the blaze wavelength is 400nm; the distance between the diffraction grating 3 and the collimating lens 2 is 50mm, and the included angle between the diffraction grating and the laser beam emitted by the semiconductor laser 1 is 10.6 degrees; the grating 3 is fixed on the front surface of the clamp 31, and the grating surface faces the semiconductor laser 1; the piezoelectric ceramic 32 is fixed on the back of the clamp 31; the piezoelectric ceramic 32 is a piezoelectric ceramic stack having a size of 4.5 (W) ×4.5 (L) ×20 (H) mm; a signal generator 33 applies a modulation signal to the piezoelectric ceramic 32; the modulating signal is a square wave signal with the frequency of 80Hz, 5Vpp and the duty ratio of 50 percent, and is used for periodically and rapidly modulating the angle of the grating 3 so as to change the output wavelength of the laser;

in this embodiment, the lens 4 is a 1 inch aspheric lens, the mirror surface is coated with an AR film, and the focal length is 18mm, so as to couple out laser light into the multimode optical fiber 5; the distance between the lens 4 and the multimode optical fiber 5 is 18mm; the multimode optical fiber 5 has a core diameter of 105 μm, a numerical aperture of 0.22 and a length of 100mm; the emergent light of the multimode optical fiber 5 enters the light guide 6 to further enhance the light homogenizing effect, and the distance between the multimode optical fiber 5 and the light guide 6 is 2mm;

in this embodiment, the light guide 6 is a hexagonal light guide, the distance between two parallel edges is 4mm, the length is 100mm, and two ends are plated with 400-1000 nm AR films; the other end of the light pipe 6 emits light to be projected on the screen 8 through the projection lens 7; the projection lens 7 is a lens with a projection ratio of 1.2; camera 9 is a black and white CMOS camera with f=50 mm, f/16 lens to simulate a human eye for speckle sampling.

Embodiment two:

the embodiment is a device for reducing speckle contrast of a laser source and an operation method of a testing device, which includes the following steps: adjusting the distance between the semiconductor laser 1 and the collimating lens 2 to obtain the diameter of the collimated light of 7mm; setting a modulation signal with 2000Hz and 400000ns pulse width for the current of the laser, so that the output spectrum of the laser is widened, providing a periodic square wave modulation signal with 80Hz and 5Vpp for the piezoelectric ceramic 32 by the signal generator 33, periodically changing the elongation of the piezoelectric ceramic 32 under the drive of the modulation signal to drive the grating 3 to realize angle change, periodically changing the diffraction angle of the emergent laser beam and the grating between 10.6 degrees and 10.8 degrees, and realizing multi-wavelength oscillation output; laser speckle noise is suppressed in terms of temporal and spatial coherence for lasers by spectral broadening of the laser and grating angle variation.

The distance between the grating 3 and the collimating lens 2 is 50mm; the lens 4 is arranged on the light path of the 1 st-order diffraction light of the grating, the multimode optical fiber 5 is arranged at the 18mm position behind the lens 4, and the incidence section of the multimode optical fiber is horizontally/vertically adjusted to enable the emergent light spot of the multimode optical fiber to be a uniform circular light spot; the light guide pipe is placed at the position 1mm behind the emergent end of the multimode optical fiber, and then the emergent end of the multimode optical fiber is horizontally/vertically adjusted, so that the light beam emergent from the light guide pipe presents a plurality of hexagonal light spot patterns symmetrically distributed along the X/Y axis; the speckle contrast is further reduced by the uniform light of the output light path through the multimode optical fiber and the light guide pipe; the projection lens 7 is arranged behind the light pipe 6 for 10cm to project, and the screen 8 is a projection curtain 4m away from the projection lens; the camera 9 is arranged at the position of 2m in front of the curtain, the exposure time is set to be 30ms, the lens parameter f=50mm is matched, F/16 simulates human eyes, and speckle image sampling is carried out on the hexagonal light spot pattern on the projection curtain.

Claims (9)

1. An apparatus for reducing speckle contrast of a laser source, comprising a semiconductor laser, a first lens, a diffraction grating, a second lens, a multimode fiber and a light pipe, characterized in that:

the output light of the semiconductor laser is collimated by the first lens and then enters the diffraction grating, and the diffraction grating is used for periodically adjusting the output wavelength and mode of laser, so that the speckle of the laser is reduced in a spectrum level;

the second lens is used for converging the grating diffraction light beams, and the converged light beams are incident to the light guide pipe.

2. A device for reducing speckle contrast of a laser source as set forth in claim 1, wherein: the laser device is also provided with a laser driver, which is used for applying periodic current modulation to the laser device to further destroy the stability of laser feedback, thereby forming fluctuation of output amplitude in the time domain.

3. A device for reducing speckle contrast of a laser source according to claim 1 or 2, characterized in that: the diffraction grating is a reflection diffraction grating and is fixed on the piezoelectric ceramics through a custom fixture.

4. A device for reducing speckle contrast of a laser source as set forth in claim 3, wherein:

the piezoelectric ceramics adopt co-fired stacked piezoelectric ceramics to drive the diffraction grating to deflect, and the piezoelectric ceramics are matched with the custom fixture to bring 0-28 mu m of displacement to the diffraction grating.

5. A device for reducing speckle contrast of a laser source as set forth in claim 1, wherein: the first lens adopts an aspheric lens to optimize spherical aberration to obtain better collimation effect, the focal length f=5mm, and the mirror surface is plated with an AR film, so that the reflectivity of the wave band of 400-700 nm is lower than 0.5%.

6. A device for reducing speckle contrast of a laser source as set forth in claim 1, wherein: the emergent light included angle between the grating and the semiconductor laser meets the formula 2d.sinθ=λ, wherein d is grating line density, θ is the included angle between the grating and the emergent light, and λ is the center wavelength of the diffracted light.

7. A test device for reducing speckle contrast of a laser source, comprising the device for reducing speckle contrast of a laser source of any one of claims 1 to 6, further comprising a projection lens, a screen, and a camera; the method is characterized in that:

the outgoing light of the light pipe is projected on the screen through the projection lens;

the camera performs speckle sampling to simulate the human eye.

8. A test device for reducing speckle contrast of a laser source as set forth in claim 7, wherein: the projection lens is a lens with the projection ratio of 1.2; the camera was a black and white CMOS camera with f=50 mm, f/16 lens to simulate a human eye for speckle sampling.

9. A method for reducing speckle contrast of a laser source using the device for reducing speckle contrast of a laser source according to any one of claims 1 to 6, characterized in that:

setting a modulation signal for the current of the laser to widen the output spectrum of the laser;

applying a periodic square wave modulation signal to the piezoelectric ceramic, so as to drive the diffraction grating to realize angle change, and periodically changing the diffraction angle of the emergent laser beam and the grating between 10.6-10.8 degrees, thereby realizing multi-wavelength oscillation output;

the second lens is arranged on the light path of the 1 st-order diffraction light of the diffraction grating, and the incidence section of the multimode optical fiber is adjusted horizontally/vertically so that the emergent light spots of the multimode optical fiber are uniform circular light spots;

the multi-mode optical fiber emergent end is horizontally/vertically adjusted, so that the light beam emergent from the light pipe presents a plurality of hexagonal light spot patterns symmetrically distributed on the X/Y axis; the speckle contrast is further reduced by homogenizing light from the output optical path through the multimode optical fiber and the light pipe.