DUAL FEMTOSECOND OPTICAL FREQUENCY COMB GENERATION DEVICE

Abstract

A dual femtosecond optical frequency comb generation device is provided. The device includes a pump source, a wavelength division multiplexer, a piezoelectric ceramic, an erbium doped fiber, a single mode fiber, a first fiber collimating mirror, a second fiber collimating mirror, and spatial optical path elements, a first quarter-wave plate, a first half-wave plate, a polarization beam splitting prism, an optical isolator, a second half-wave plate and a second quarter-wave plate, and further including a grating pair, the grating pair being composed of a first grating and a second grating and being provided between the polarization beam splitting prism and the optical isolator. The device introduces the light distance difference by using the grating pair, so as to generate dual femtosecond optical frequency combs with a difference in repetition frequency, and the repetition frequency difference can be adjusted by the pitch of the grating pair.

Claims

1. A dual femtosecond optical frequency comb generation device having a ring resonant cavity structure composed of optical fibers and a spatial optical path, and comprising: a pump source (1), a wavelength division multiplexer (2), a piezoelectric ceramic (3), an erbium doped fiber (4), a single mode fiber (5), a first fiber collimating mirror (6), a second fiber collimating mirror (14), and spatial optical path elements, a first quarter-wave plate (7), a first half-wave plate (8), a polarization beam splitting prism (9), an optical isolator (11), a second half-wave plate (12) and a second quarter-wave plate (13), and characterized by further comprising a grating pair (10), the grating pair (10) being composed of a first grating (15) and a second grating (16) and being provided between the polarization beam splitting prism (9) and the optical isolator (11).

2. The dual femtosecond optical frequency comb generation device according to claim 1, characterized in that the first grating (15) and the second grating (16) are two identical high-density transmissive quartz gratings.

3. The dual femtosecond optical frequency comb generation device according to claim 2, characterized in that the grating duty ratio is 0.5, a grating period a has a range of values of 2<a<2.15 m, and a grating depth h has a range of values of 2.65<h<2.72 m.

4. The dual femtosecond optical frequency comb generation device according to claim 1, characterized in that the grating pair (10) is perpendicular to the direction of light, the first grating (15) and the second grating (16) are parallel, the grooved surfaces are opposite to each other and are completely symmetrically placed; the first grating (15) or the second grating (16) is fixed onto a precision nano-displacement device.

5. The dual femtosecond optical frequency comb generation device according to claim 1, characterized in that the pitch of the grating pair (10) is less than or equal to 200 m.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 is a schematic structural view of a dual femtosecond optical frequency comb generation device;

[0021] In the drawings:

[0022] 1pump source, 2wavelength division multiplexer, 3piezoelectric ceramic, 4erbium doped fiber, 5single mode fiber, 6first fiber collimating mirror, 7first quarter-wave plate, 8first half-wave plate, 9polarization beam splitting prism, 10grating pair, 11optical isolator, 12second half-wave plate, 13second quarter-wave plate, 14second fiber collimating mirror, 15first grating, 16second grating.

[0023] FIG. 2 is a schematic view showing the propagation of laser light through a grating pair;

[0024] In the drawing, A is the incident beam, B, C, and D are respectively +1 level diffraction, 0 level diffraction, and 1 level diffraction of A passing through the first grating 15, is the diffraction angle, E is 1 level diffraction of B passing through the second grating 16, F is the 0 level diffraction of C passing through the second grating 16, G is the +1 level diffraction of D passing through the second grating 16, a represents the grating period, and h represents the grating depth.

[0025] FIG. 3 is a graph showing the relationship between the grating diffraction efficiency and the grating depth.

[0026] FIG. 4 is a graph showing the relationship between the frequency difference of the dual femtosecond optical frequency combs and the pitch of the grating pair.

DETAILED DESCRIPTION

[0027] The invention is further illustrated in combination with the following embodiments and drawings, which however should not be construed as limiting the scope of protection of the invention.

Embodiment 1

[0028] The dual femtosecond optical frequency comb generation device disclosed in this embodiment, as shown in FIG. 1, has a ring resonant cavity structure composed of optical fibers and a spatial optical path, including a pump source 1, a wavelength division multiplexer 2, a piezoelectric ceramic 3, an erbium doped fiber 4, a single mode fiber 5, a first fiber collimating mirror 6, a second fiber collimating mirror 14, and spatial optical path elements, a first quarter-wave plate 7, a first half-wave plate 8, a polarization beam splitting prism 9, an optical isolator 11, a second half-wave plate 12 and a second quarter-wave plate 13, and further including a grating pair 10, the grating pair 10 being composed of a first grating 15 and a second grating 16 and being provided between the polarization beam splitting prism 9 and the optical isolator 11.

[0029] The wavelength division multiplexer 2, the erbium doped fiber 4, the single mode fiber 5, the first fiber collimating mirror 6, the second fiber collimating mirror 14, and the spatial optical path elements, the first quarter-wave plate 7, the first half-wave plate 8, the polarization beam splitting prism 9, the grating pair 10, the optical isolator 11, the second half-wave plate 12, and the second quarter-wave plate 13, collectively constitute a resonant cavity of the dual femtosecond optical frequency comb generation device; the erbium doped fiber 4 is welded onto the single mode fiber 5 as a gain medium, and except for this, all other fibers in the device are single mode fibers; the optical isolator 11 ensures one-way laser operation within the cavity, and the first quarter-wave plate 7, the first half-wave plate 8, the polarization beam splitting prism 9, the grating pair 10, the optical isolator 11, the second half-wave plate 12, and the second quarter-wave plate 13 are sequentially arranged in a spatial optical path along a light forwarding direction; the pump source 1 is connected to the wavelength division multiplexer 2 for supplying pumping light to the dual femtosecond optical frequency comb generation device; the piezoelectric ceramic 3 is wound with a length of single mode fiber 5 for adjusting a cavity length; the first quarter-wave plate 7, the half-wave plate 8, the second half-wave plate 12, and the second quarter-wave plate 13 realize the mode locking of femtosecond laser pulses of the dual femtosecond optical frequency comb generation device by using a nonlinear polarization rotary mode locking principle; the reflecting port of the polarization beam splitting prism 9 is used for the output of the dual femtosecond optical frequency combs.

[0030] The grating pair 10 is perpendicular to the light forwarding direction, the first grating 15 and the second grating 16 thereof are parallel, the grooved surfaces are opposite to each other and are completely symmetrically placed; the second grating 16 is fixed onto a precision nano-displacement device, and can move linearly along the light direction to accurately control the distance between the grating pair, as shown in FIG. 2.

[0031] The first grating 15 and the second grating 16 are high-density transmissive quartz gratings having a same period. A quartz grating is fabricated by a microelectronic etching process, and the grating etching depth affects the diffraction efficiency of different levels. The grating duty ratio used in this example is 0.5, and the period a is 2.10 m. FIG. 3 shows the relationship between the diffraction efficiency and the grating depth. In order to make the two components of the dual femtosecond optical frequency comb have the same intensity, the grating depths h of the first grating 15 and the second grating 16 are both 2.68 m; their +1 level (1 level) diffraction efficiency is 28.5%, the diffraction angle is 47.6, and the 0 level diffraction efficiency is 40.3%.

[0032] The pitch of the grating pair 10 is controlled to be less than or equal to 200 m.

[0033] The specific principle of the device of the present invention for generating dual femtosecond optical frequency combs is that an incident beam A produces +1 level, 0 level, and 1 level diffraction B, C, D through the first grating 15, B, D and C have light distance difference therebetween, the light distance difference l=d/cos d, d is the pitch of the grating pair 10; after passing through the second grating 16, the 1 level diffraction E of B and the +1 level diffraction G of D are parallel with the 0 level diffraction F of C again, as shown in the FIG. 2. Since the pitch of the grating pair is very small, the lateral deviation between E, F, and G is much smaller than the effective aperture of subsequent optical elements, and E, F, and G all participate in the intracavity laser cycle. The beams E and G are in phase, A-B-E and A-D-G collectively form a laser transmission path, and A-C-F form another laser transmission path. Therefore, two paths with different light distances exist within the laser cavity of the dual femtosecond optical frequency comb generation device and can output dual femtosecond optical frequency combs with different repetition frequencies. The relationship between the repetition frequency difference and the light distance difference is f=(f.sub.r.sup.2/c)l, where f.sub.r is the repetition frequency when the pitch of the grating pair 10 is 0.

[0034] In the dual femtosecond optical frequency comb generation device of the present example, the length of the erbium doped fiber 4 is 50 cm, the total length of the single mode fiber 5 is 130 cm, and the spatial optical path is 30 cm. The device can generate dual femtosecond optical frequency combs of 1550 nm having a repetition frequency near 100 MHz. By turning on the pump source 1, adjusting the orientation of the first quarter-wave plate 7, the first half-wave plate 8, the second half-wave plate 12 and the second quarter-wave plate 13, and using nonlinear polarization rotary mode-locking principle to achieve the mode-locking of femtosecond laser pulses, the reflection port of the polarization beam splitting prism 9 can output dual femtosecond optical frequency combs; a photodetector can be used at the reflection port of the polarization beam splitting prism 9 to detect the repetition frequency difference of the dual femtosecond optical frequency combs.

[0035] The repetition frequency difference of the dual femtosecond optical frequency combs can be conveniently adjusted by the pitch of the grating pairs 10. When the pitch of the grating pair 10 is varied between 0 and 200 m, the repetition frequency difference of the dual femtosecond optical frequency combs generated by the device is varied between 0 and 3.2 kHz, and the relationship between the repetition frequency and the pitch of the grating pair is linear, as shown in FIG. 4.

[0036] The device of the invention introduces the light distance difference by using the grating pair, and generates dual femtosecond optical frequency combs with a slight difference in repetition frequency through a single device, and has the advantages of small volume, simple and compact structure, and convenient operation. the external environment has the same influence on the dual femtosecond optical frequency combs generated by the device, the system is stable and reliable, and the coherence between the beams is good. The device can be used in frontier fields such as dual optical comb spectral measurements, and has a strong application value.

[0037] The above specific description of the present invention further illustrates the object, technical solution and beneficial effects of the invention in detail, and it should be appreciated that the foregoing is merely pertaining to the specific embodiments of the invention and is not intended to limit the scope of protection of the invention. Any modification, equivalent substitution and improvement within the spirit and principle of the invention shall be included within the scope of protection of the invention.