JPH04263231A - Second higher harmonic generating element - Google Patents

Abstract

PURPOSE:To obtain high efficiency with small-sized, simple constitution which obtains the same effect without manufacturing a domain. CONSTITUTION:When a fundamental wave entering a ring resonator 5 is made incident on a nonlinear optical medium 6, a 2nd higher harmonic is generated gradually. The strength of the 2nd higher harmonic is varied periodically unless the phase is matched completely, but the length of the nonlinear optical medium 6 is set nearly to coherent length, thereby outputing the wave in a state deviated by pi in a phase. This is deviated further in phase by pi through a phase shifter 7 to match the phases of the both with each other and the wave is made incident again on the nonlinear optical medium 6 through the ring resonator 5, thus realizing high-efficiency amplifying operation for 2nd higher harmonic generation. Namely, a medium which is nearly as long as the coherent length and can not be developed greatly is usable as the nonlinear optical medium 6 and a medium which is not matched in phase is also usable.

Description

[Detailed description of the invention]

0001

[Industrial Field of Application] The present invention relates to a second
Related to harmonic generation elements.

0002

2. Description of the Related Art Generally, in nonlinear interactions between different wavelengths, phase matching under energy conservation is important. If this condition is not satisfied, interference will occur between the nonlinear polarization waves generated by the interaction and the light waves, and high conversion efficiency will not be obtained.

[0003] Second harmonic generation element (Second Ha
rmonic Generation (SHG), this situation is illustrated in FIG. 2. This is based on the document "Optics" (Vol. 19 No. 6 1990)
This is shown in "Second harmonic generation by periodic domain inverted optical waveguide" (p. 373) in June 2013), and the harmonic (SH) output is as shown in (c) in the same figure. Along with propagation, Lc=λF/4(|nF-nSH|)
………………………(1) The maximum and minimum values are repeated for each interference distance (coherent length) Lc given by: Here, subscripts F and SH indicate the fundamental wave and SH wave components, respectively, and λ and n indicate the wavelength and refractive index.

[0004] Therefore, if the sign of the polarization waves generated for each Lc can be alternately inverted by some method, there will be no cancellation in the SH output, and conversely, as shown in (a) and (b) in the same figure, there will be no cancellation in the SH output. Such a superposition occurs. If this can be done, apparent phase matching can be achieved by adjusting the domain period using a material that is optically isotropic or has a large tensor component that cannot be phase matched as a bulk material due to its dispersion characteristics. can be taken. This method uses quasi phase matching (quasi phase matching).
-phase matching (QPM)
It was proposed in 1962.

As is clear from FIG. 2, the half period Λ of the domain can be an odd multiple of Lc, so Λ
=(2m+1)Lc =(2m+1)・λF/4(|nF−nSH|
) ......(2) is given by. Here, m is the order of the periodic domain, and (a) and (b) in FIG. 2 show the situations when m=0 and m=1. If Δn=|nF−nSH|=0.025,
When m=0, the QPM pitch is 10 times the fundamental wavelength,
When Δn=0.1, it becomes 2.5 times.

Note that (d) in FIG. 2 is used to compare the optical output when the perfect phase matching condition is satisfied under the same nonlinear optical coefficient (Lc=∞: cannot be realized). It shows.

[0007]

[Problems to be Solved by the Invention] However, in order to obtain high efficiency, it is necessary to create domains homogeneously over ~103 periods, but the length of the domains is approximately 10 times the fundamental wavelength, which is extremely long. It is extremely difficult to manufacture something like this in the current small size. Even if it could be manufactured, it would be necessary to adjust its temperature because it is large.

[0008]

Means for Solving the Problems A nonlinear optical medium and a phase shifter for shifting the phase difference between the fundamental wave and the second harmonic by approximately π are provided on the optical path within the ring resonator.

[0009]

[Operation] When the fundamental wave entering the ring resonator enters the nonlinear optical medium, a second harmonic is gradually generated. At this time, if the phase matching is not completely achieved, the intensity of the second harmonic changes periodically, but by setting the length of the nonlinear optical medium to about the coherent length, the phase is shifted by π. is output. By further shifting the phase by π using a phase shifter, the phases of the two match, and by passing through the ring resonator and entering the nonlinear optical medium again, a highly efficient amplification effect is achieved for second harmonic generation. receive. That is, since the nonlinear optical medium only needs to have a coherent length, it is very small, does not require temperature adjustment, and also allows the use of a medium that cannot be grown to a large size. Furthermore, it is possible to use materials that are not phase matched, and materials made of various inexpensive materials can be used.

0010

[Embodiment] An embodiment of the present invention will be explained based on FIG. First, a ring resonator 5 in which four mirrors 1, 2, 3, and 4 are arranged in a ring shape is provided. Further, mirror 1 is located at the incident position of the input light due to the fundamental wave, and mirror 2
is located at the SH light emission position, mirror 1
, 2 are provided with a crystal 6, which is a nonlinear optical medium, and a π-type phase shifter 7. Various materials can be used as the phase shifter 7. For example, it may be made of a transparent material with dispersion characteristics, such as glass, or a birefringent crystal.

[0011] In such a configuration, the input light of the fundamental wave passes through the mirror 1, enters the ring resonator 5, and enters the crystal 6 through an optical path length L0. Here, SH light is gradually generated. At this time, if phase matching is not completely achieved, S
The intensity of the H light changes periodically. Now, if the length of the crystal 6 is L, then L=(2m+
1) Lc (3) is set. Here, m is an integer of 0 or more. That is, the crystal 6 may have a coherence length of approximately Lc.

[0012] Then, the fundamental wave and the SH wave exiting the crystal 6 have a phase shift of exactly π. These two lights enter the phase shifter 7, and the phases of the fundamental wave and the SH wave are further shifted by π, and eventually they are brought into the same phase.

Next, when the wave goes around the ring resonator 5 once and returns to the crystal 6, the fundamental wave and the SH wave are in the same phase, so amplification begins. In this way, S
H light amplification is performed.

[0014] In addition to the various optical crystals 6, the nonlinear optical medium may be made of various materials, whether organic or inorganic, and in particular may be one that is not phase matched.

[0015]

[Effects of the Invention] As described above, the present invention includes a nonlinear optical medium and a phase shifter that shifts the phase difference between the fundamental wave and the second harmonic by approximately π on the optical path within the ring resonator. So,
Even in cases where phase matching is not completely achieved and the intensity of the second harmonic changes periodically, the length of the nonlinear optical medium is set to about the coherent length, and the output is output with a phase shift of π. It is possible to further shift the phase by π using a phase shifter, match the phases of the two, pass the ring resonator, and re-enter the nonlinear optical medium to perform a highly efficient amplification action for second harmonic generation. Therefore, since the nonlinear optical medium only needs to have a coherent length, it is extremely compact, does not require temperature adjustment, and allows the use of a medium that cannot be grown to a large size. It is possible to use even those made of a variety of inexpensive materials.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of an optical system showing an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing how the AHG output changes due to QPM.

[Explanation of symbols]

5 Ring resonator 6 Nonlinear optical medium 7 Phase shifter

Claims (1)

[Claims] Claim 1: A second harmonic, characterized in that a nonlinear optical medium and a phase shifter for shifting the phase difference between the fundamental wave and the second harmonic by approximately π are provided on the optical path within the ring resonator. Wave generating element.