Speckle-free imaging light source based on random fiber laser using strong-coupling multi-core optical fiber

Abstract

A speckle-free imaging light source based on a random fiber laser (RFL) using a strong-coupling multi-core optical fiber, relating to a field of optical fiber laser illumination light source, is provided, mainly including a pumping source and an optical fiber loop mirror, and further including the strong-coupling multi-core optical fiber with/without a single-mode optical fiber. Through directly adopting the strong-coupling multi-core optical fiber or combining the single-mode optical fiber with the strong-coupling multi-core optical fiber to serve as a main device in the RFL-based illumination light source, the generated RFL has multiple transvers modes and low spatial coherence which prevent speckle formation during illumination, which provides an ideal illumination light source for high-speed full-field speckle-free imaging technology.

Claims

1. A speckle-free imaging light source based on a random fiber laser using a strong-coupling multi-core optical fiber, mainly comprising a pumping source and an optical fiber loop mirror, and further comprising a strong-coupling multi-core optical fiber connected with the pumping source and the optical fiber loop mirror, wherein: a long-period optical fiber grating is written in the strong-coupling multi-core optical fiber; the strong-coupling multi-core optical fiber comprises a main optical fiber core, multiple secondary optical fiber cores and a cladding; the main optical fiber core is a multi-mode optical fiber core with a large core diameter; the multiple secondary optical fiber cores are uniformly distributed around the main optical fiber core; the secondary optical fiber cores cling to the main optical fiber core, so as to stimulate strong coupling between the main optical fiber core and the secondary optical fiber cores; and strong coupling also exists among the secondary optical fiber cores.

2. The speckle-free imaging light source based on the random fiber laser using the strong-coupling multi-core optical fiber, as recited in claim 1, further comprising a single-mode optical fiber respectively connected with the pumping source, the optical fiber loop mirror, and the strong-coupling multi-core optical fiber.

3. The speckle-free imaging light source based on the random fiber laser using the strong-coupling multi-core optical fiber, as recited in claim 2, wherein the single-mode optical fiber is one of a dispersion compensating optical fiber, a dispersion-shifted fiber and a highly-nonlinear fiber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of a speckle-free imaging illumination light source and an imaging test device thereof according to a preferred embodiment of the present invention.

(2) FIG. 2 is a structural diagram of a strong-coupling multi-core optical fiber according to the present invention.

(3) FIG. 3 is a schematic diagram of spatial coherence modulation of a long-period optical fiber grating written in the strong-coupling multi-core optical fiber according to the present invention.

(4) FIG. 4 is an optical spectrum of a random fiber laser (RFL) according to the preferred embodiment of the present invention.

(5) FIG. 5 shows imaging results of speckle-free imaging using multi-transverse-mode illumination according to the preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(6) In order to make objects, technical solutions and advantages of the present invention more clear and understandable, the present invention is further described in detail with the accompanying drawings and the preferred embodiment. It should be understood that the described preferred embodiment is merely for explaining the present invention, not for limiting the present invention.

(7) A speckle-free imaging illumination light source and an imaging test device thereof are showed in FIG. 1. The speckle-free imaging illumination light source mainly comprises an optical fiber loop mirror 1, a pumping source 2, a single-mode optical fiber 3, and a strong-coupling multi-core optical fiber 4. Alternatively, the combination of the single-mode optical fiber 3 and the strong-coupling multi-core optical fiber 4 is replaced by the strong-coupling multi-core optical fiber 4.

(8) An input end of the single-mode optical fiber 3 is connected with the optical fiber loop mirror 1. Point feedback is provided by the optical fiber loop mirror 1. The pumping source 2 is injected into the sing-mode optical fiber 3 using a wavelength division multiplexer. A specific wavelength of random fiber laser (RFL) can be generated through adjusting a wavelength of the pumping source 2, so that the RFL has a flexible and adjustable lasing wavelength, thereby satisfying a requirement for specified speckle-free imaging and eliminating a noise background from an ambient light.

(9) An output end of the single-mode optical fiber 3 is connected with the strong-coupling multi-core optical fiber 4. Amplification and distributed feedback for a random lasing process are provided by both the two optical fibers through stimulated Raman scattering and distributed Rayleigh scattering.

(10) The single-mode optical fiber 3 can be one of a communication standard single-mode optical fiber, a dispersion compensating fiber, a dispersion-shifted fiber and a highly-nonlinear fiber. Through combining with the specified single-mode optical fiber 3, a bandwidth characteristic of the random lasing process can be adjusted and controlled.

(11) As shown in FIG. 2, the strong-coupling multi-core optical fiber 4 comprises multiple secondary optical fiber cores 11, a main optical fiber core 12 and a cladding 13. The main optical fiber core 12 is a multi-mode optical fiber core with a large core diameter; the multiple secondary optical fiber cores 11 are uniformly distributed around the main optical fiber core 12; the secondary optical fiber cores 11 cling to the main optical fiber core 12, so as to stimulate strong coupling between the main optical fiber core 12 and the secondary optical fiber cores 11. Strong coupling also exists among the secondary optical fiber cores 11, for effectively stimulating more transverse high-order modes and realizing a multi-transverse-mode RFL with low spatial coherence.

(12) The strong-coupling multi-core optical fiber 4 can be applied in generating the multi-transverse-mode RFL. The strong-coupling multi-core optical fiber can be replaced by a multi-mode optical fiber. The multi-mode optical fiber can be one of a step index multi-mode optical fiber with a large core diameter, a graded index multi-mode optical fiber with a large core diameter, and a hollow-core optical fiber with a large core diameter. However, with adopting the strong-coupling multi-core optical fiber 4, the required optical fiber length is shorter, that is to say the strong-coupling multi-core optical fiber is more beneficial to obtaining the low spatial coherence.

(13) A long-period optical fiber grating is written in the strong-coupling multi-core optical fiber 4 or the multi-mode optical fiber. Through imposing a tensile force on the long-period optical fiber grating and bending the long-period optical fiber grating, the spatial coherence of the multi-mode RFL is adjusted and controlled, so as to further optimize the spatial coherence of the multi-mode RFL.

(14) A schematic diagram of spatial coherence modulation of the long-period optical fiber grating written in the strong-coupling multi-core optical fiber is showed in FIG. 3. An input light 14, a grating 15 written in the strong-coupling multi-core optical fiber or the multi-mode optical fiber, and an output light 16 are showed in FIG. 3. The grating 15 written in the strong-coupling multi-core optical fiber or the multi-mode optical fiber is to further convert lower-order transverse modes into higher-order transverse modes and stimulate enough transverse modes. The spatial coherence of the RFL can be adjusted and controlled, so as to realize the low spatial coherence for speckle-free imaging.

Preferred Embodiment

(15) A speckle-free imaging illumination light source and an imaging test device thereof are showed in FIG. 1. The speckle-free imaging illumination light source comprises an optical fiber loop mirror 1, a pumping source 2, a single-mode optical fiber 3, and a step index multi-mode optical fiber 4 with a large core diameter. The imaging test device comprises a first lens 5 (with a focal length of a mm), a second lens 6 (with a focal length of b mm), a ground glass 7, a US Air Force resolution chart 8 (USAF 1951), a microscope objective 9 and a camera 10 (CCD), wherein: the first lens 5 (with the focal length of a mm) and the second lens 6 (with the focal length of b mm) form a classic Kohler illumination system; the ground glass 7 is for adding a random phase modulation to the illumination light source; the US Air Force resolution chart 8 (USAF 1951) is used as an imaging object after the illumination light is modulated, so as to assess an imaging quality; the microscope objective 9 and the camera 10 (CCD) are used for imaging of the US Air Force resolution chart 8 (USAF 1951).

(16) A central wavelength of the RFL generated by the pumping source 2 is 1555 nm and a bandwidth is 1 nm.

(17) The core diameter of the step index multi-mode cal fiber 4 is 105 m, while a numerical aperture is 0.24 and a length is 50 m.

(18) FIG. 4 shows an optical spectrum of the RFL according to the preferred embodiment of the present invention. The central wavelength of the RFL is 1550 nm, while a spectral density of the RFL is much higher than that of amplified spontaneous emission sources (ASEs).

(19) FIG. 5 shows an imaging result of the speckle-free imaging according to preferred embodiment of the present invention, showing the USAF resolution chart 8 is illuminated by the RFL through the ground glass 7. In FIG. 5, the USAF resolution chart 8 has clear stripes and no speckles.

(20) The above-mentioned is merely a preferred embodiment of the present invention, not for limiting the present invention. Modifications, equivalents and improvements made within the spirit and principles of the present invention are all included in the protection scope of the present invention.