CN103605248B - Based on the Enhancement Method of the frequency multiplication of periodic polarized lithium niobate - Google Patents

Abstract

A frequency-doubling enhancement method based on periodically polarized lithium niobate in the field of optical information processing technology. Firstly, the lithium niobate crystal is subjected to electric field polarization at room temperature, and the electric domain polarization is changed in the negative domain region on the +Z surface of the crystal. direction, vacuum coating sputtering electrodes are carried out on both sides of the Y direction of the crystal; then the crystal is irradiated with ordinary light and at the same time, a high voltage source is used to apply voltage to both sides of the Y direction of the lithium niobate crystal, and the ordinary light is realized through the slow light effect generated. Multiplier enhancement. The present invention is the first to combine the technology of quasi-phase matching (QPM) with the increase of effective optical power caused by the slow light effect.

Description

Enhancement method based on frequency doubling of periodically poled lithium niobate

technical field

The invention relates to a method in the technical field of optical information processing, in particular to a frequency doubling enhancement method based on periodically polarized lithium niobate.

Background technique

Since the birth of laser technology, nonlinear frequency conversion technology and slow light technology have been research hotspots. The quasi-phase matching technique (QPM) based on periodically poled nonlinear crystals is one of the most effective and commonly used methods to broaden the tunable wavelength of lasers. Commonly used periodically poled nonlinear crystals are: LiNbO ₃ (PPLN), LiTaO ₃ and KTP. The quasi-phase-matching technology artificially prepares a periodically polarized inversion grating in a nonlinear crystal, and periodically modulates the nonlinear coefficient of the crystal to meet the conditions of phase matching. Among frequency conversion technologies, frequency multiplication (SHG) technology is most widely used. LiNbO ₃ crystal is a negative uniaxial crystal, and there are two frequency doubling methods, 0-type frequency doubling and I-type frequency doubling. Type 0 frequency doubling means that both the fundamental frequency light and the frequency doubled light participating in the frequency doubling process are incident or propagated in the crystal as extraordinary light (E light, Extraordinary Light); type I frequency doubling refers to the fundamental frequency light participating in the frequency doubling process. Frequency light is incident or propagated by ordinary light (O light, Ordinary Light), and double frequency light is incident or transmitted by extraordinary light. The non-linear coefficient used by type 0 multiplier is d ₃₁ , which has high multiplication efficiency but narrow bandwidth; the non-linear coefficient used by type I multiplier is d ₃₃ , which has wide multiplication bandwidth but low efficiency. Both frequency doublings produce extraordinary light. As a very important characteristic of photonic crystals, the slow light effect can be used to realize time delay, increase phase shift and enhance nonlinear effects, etc.

After searching the existing technology, it was found that in 2007, F.LU et al. published "Broadcast wavelength conversion based on cascaded? (2) nonlinearity in MgO-doped periodically poled LiNbO3" in "Electronics Letters" (43, 2007) ("Second-order based on periodically poled lithium niobate crystal Nonlinear Coefficient Multiwavelength Converter"), which uses a nonlinear frequency doubling process. Although the quasi-phase matching technology (QPM) is adopted, at the same time, due to the small nonlinear coefficient of frequency doubling, a high pump optical power is required to obtain considerable conversion efficiency.

In 2010, Kun Liu et al. published the article "Active control of group velocity by use off folded dielectric axes structures" ("Group Velocity Control Using Stacked Dielectric Axis Structure") in "Applied Physics Letters" (97, 2010), which introduced the use of periodic domain inversion lithium niobate In the crystal PPLN, the optical axes of the positive and negative domains of the crystal form a periodic swing by applying a transverse Y-direction electric field, so that the polarization direction of the outgoing light after passing through each domain changes. After passing through the appropriate N domains, the polarization direction of the outgoing light is perpendicular to that of the incident light, thus forming a forbidden band. It is also shown in the article that by utilizing this forbidden band, slower group velocities can be formed.

Compared with the above references of quasi-phase matching frequency conversion technology and group velocity control method, this patent proposes to make a periodic structure and apply a voltage to construct a one-dimensional electro-optic photonic crystal, and realize the above two frequency doubling methods and group velocity based on quasi-phase matching at the same time. The slow light effect formed by speed regulation is designed to enhance the second-order nonlinear effect, and solve the problems of low frequency doubling efficiency of O light and narrow bandwidth of E light frequency doubling.

After searching the existing technology, it is found that the Chinese patent document number CN102338966A, the publication date is 2012-02-01, discloses a polarization-independent quasi-phase matching frequency multiplier, which includes depolarizers placed in sequence in the optical path and the first periodically poled lithium niobate crystal plate and the second periodically poled lithium niobate crystal plate; wherein, the first periodically poled lithium niobate crystal plate and the second periodically poled lithium niobate crystal plate The lithium niobate crystal sheets have the same optical properties, and are respectively made of Z-cut lithium niobate crystals subjected to electric field polarization at room temperature. Polarization direction; the direction of the optical path is the x direction in the x-y-z coordinate system; the c-axis of the first periodically poled lithium niobate crystal sheet is along the z direction, and the second periodically poled lithium niobate crystal The c-axis of the slice is along the y-direction. But the defect of this technology is that its structure is complex, and it must be realized through two PPLN chips. And only 0-type frequency doubling can be achieved, because the 0-type frequency doubling bandwidth is narrow and the pump wavelength is fixed, so the wavelength of the frequency doubling light cannot be adjusted, which is difficult to meet the needs of existing industries.

Contents of the invention

Aiming at the above-mentioned deficiencies in the prior art, the present invention proposes a frequency doubling enhancement method based on periodically polarized lithium niobate. Based on two frequency doubling effects, O light will be converted to E light after voltage is applied, and E light The frequency doubling efficiency is much higher than that of O light, and the mutual coupling of O light and E light can generate slow light, which can further enhance frequency doubling. Therefore, the frequency doubling efficiency of the O light can be greatly enhanced. Furthermore, by changing the period and temperature of the periodically poled lithium niobate crystal, the wavelength position of simultaneously realizing O, E light and forbidden band can be changed, and the position of frequency doubling enhancement can be adjusted in broadband frequency doubling.

The present invention is achieved through the following technical solutions. In the present invention, the room temperature electric field polarization is carried out on the lithium niobate crystal first, and the electric domain polarization direction is changed in the negative domain region on the +Z surface of the crystal, and the vacuum is carried out on both sides of the crystal Y direction. Coating sputtering electrodes; then, while irradiating the crystal with ordinary light, a high-voltage source is used to apply voltage to both sides of the Y side of the lithium niobate crystal, and the frequency multiplication of ordinary light is enhanced through the slow light effect generated.

The cycle adopted by the electric field polarization is obtained in the following way:

1) Determine the polarization period Λ required to satisfy the quasi-phase matching strip at the same time, that is, satisfy: ${\mathrm{\Λ}}_1=\frac{\mathrm{\λ}}{2({\mathrm{no}}_z^{2\mathrm{\ω}}-{\mathrm{no}}_{\mathrm{the\; y}}^{\mathrm{\ω}})};{\mathrm{\Λ}}_2=\frac{\mathrm{\λ}}{2({\mathrm{no}}_{\mathrm{the\; y}}^{\mathrm{\ω}}-{\mathrm{no}}_z^{\mathrm{\ω}})};$ Where: λ is the wavelength of the fundamental light, is the refractive index of the fundamental wave E light, is the refractive index of fundamental O light, Be the refractive index of frequency doubled E light, Λ ₀ is the cycle of I type frequency double, Λ ₁ is the cycle of 0 type frequency double, Λ ₂ is the cycle of forbidden band.

2) Since O optical broadband frequency multiplication requires not only quasi-phase matching, but also group velocity matching, that is, the required period is at the tangent point of the Λ-λ relationship curve; therefore, use matlab software to simulate Λ ₀ , Λ ₁ and Λ ₂ and The relationship diagram of λ can obtain the intersection point of three periods, and the period corresponding to the intersection point can realize simultaneous frequency multiplication and band gap of O and E light; by changing the temperature, different intersection points can be obtained; and at a specific temperature The intersection point can be made to fall on the tangent point of the Λ ₁ -λ curve, and the temperature and period corresponding to this point can simultaneously realize the broadband doubling of the O light, the frequency doubling of the E light and the forbidden band, which are the required period and period of the present invention. temperature.

The room temperature electric field poling refers to: using room temperature poling technology, realizing periodic reversal according to the obtained period, and obtaining periodically poled lithium niobate crystal (PPLN).

The vacuum coating sputtering electrode refers to: using a vacuum coating machine to plate electrodes on both sides of the Y direction of the periodically polarized lithium niobate crystal by sputtering.

The ordinary light irradiation refers to: through the precise control of the ambient temperature, the fundamental frequency light is emitted from the continuously adjustable laser, and the ordinary light is incident on the crystal surface through the polarization beam splitter (PBS), and the lithium niobate crystal is fed to the lithium niobate crystal by the high voltage source. While voltage is applied to both sides of Y, the intensity of the outgoing frequency-doubled light is measured by an optical power meter.

The fundamental frequency light wavelength of the laser is 1518-1627nm, and the light passing direction is the x direction.

The enhancement of frequency doubling of ordinary light refers to: using a high voltage source with an output voltage not exceeding 10KV to apply voltage in the Y direction of lithium niobate crystal, and to enhance the frequency doubling of O light through the slow light effect generated, and at the same time, O light will also Partially converted into E light, the frequency doubling efficiency of E light is much greater than that of O light, so the frequency doubling efficiency of O light will be further improved, specifically: when the conversion loss from the fundamental frequency light wave to the frequency doubling light wave is ignored, the frequency doubling enhanced The maximum multiple is: Among them: S is the multiple of slow light enhancement, d ₃₃ is the nonlinear coefficient used for type 0 frequency multiplication, and d ₃₁ is the nonlinear coefficient used for type I frequency multiplication.

technical effect

Compared with the prior art, the technical effects of the present invention include:

1. On the premise of not changing the wavelength conversion output spectrum and bandwidth, it can achieve higher conversion efficiency than the original frequency doubling scheme near a certain wavelength in the broadband.

2. With the temperature control of the temperature-controlled furnace and the magnitude of the applied voltage, the frequency doubling process at a slow group velocity can be well combined with the O-light broadband frequency doubling and E-light height of the lithium niobate crystal itself. efficiency multiplier combined.

3. Changing the period and temperature of the periodically polarized lithium niobate crystal can change the wavelength position of simultaneously realizing O, E light and band gap, and can adjust the position of frequency doubling enhancement in broadband frequency doubling, which can make O light The broadband frequency doubling and E-light high-efficiency frequency doubling are well combined.

4. It can achieve frequency multiplication enhancement under low light conditions.

5. Slow light enhanced frequency doubling can be extended to nonlinear processes such as enhanced sum frequency, difference frequency, and parametric oscillation.

6. The implementation structure is simple, only one PPLN chip is needed to realize the frequency multiplication enhancement, and the magnitude of the frequency multiplication enhancement can be changed by adjusting the voltage.

Description of drawings

Fig. 1 is the structural representation of the prototype device of the one-dimensional electro-optic photonic crystal that realizes the enhanced second-order nonlinearity of the present invention;

Figure 2 is a schematic diagram of the working principle of the device shown;

In the figure: the fundamental frequency light is output from an adjustable laser whose wavelength range can be adjusted within the range of 1518-1627nm, and is incident on the polarization beam splitter through the collimator (Collimator), forming ordinary light incident on the surface of the PPLN, and the Y direction of the PPLN Adding voltage and precisely controlling the temperature of the environment can realize E-light frequency doubling and forbidden band within the broadband frequency doubling of O-light, and achieve the effect of enhancing O-light frequency doubling. The output power of the frequency doubled light is measured with an optical power meter.

detailed description

The embodiments of the present invention are described in detail below. This embodiment is implemented on the premise of the technical solution of the present invention, and detailed implementation methods and specific operating procedures are provided, but the protection scope of the present invention is not limited to the following implementation example.

Example 1

Embodiments of the present invention will be described in detail below with reference to FIG. 1 and FIG. 2 .

1) Select a piece of 5%mol Mg-doped Z-cut lithium niobate crystal with a size of 30×10×0.5mm, that is, a thickness of 0.5mm. Both +/-Z of the crystal are polished, where the +Z plane is parallel to the horizontal plane and faces the positive direction of the Z-axis, and the -Z plane is parallel to the horizontal plane and faces the negative direction of the Z-axis.

2) Use Matlab software to simulate the period Λ required to satisfy the quasi-phase matching condition: ${\mathrm{\Λ}}_1=\frac{\mathrm{\λ}}{2({\mathrm{no}}_z^{2\mathrm{\ω}}-{\mathrm{no}}_{\mathrm{the\; y}}^{\mathrm{\ω}})},{\mathrm{\Λ}}_2=\frac{\mathrm{\λ}}{2({\mathrm{no}}_{\mathrm{the\; y}}^{\mathrm{\ω}}-{\mathrm{no}}_z^{\mathrm{\ω}})},$ Among them: the range of λ is 1518-1627nm, and the refractive index can be obtained by the Sellmeier equation.

It is obtained that when the temperature of the wafer is 15.15°C, the E light can be frequency-multiplied at the O-light broadband frequency doubling center, and at the same time, the O-light and E light coupling can be realized at this position, and the period of slow light generation is 19.5 μm, wherein: the O-light broadband frequency multiplication The range is: 1.537-1.558μm, the frequency doubling of E light and the center wavelength of forbidden band is 1.549μm.

3) Polarize the crystal at room temperature according to the obtained period, as follows:

First, make a periodic electrode mask according to the calculated period; orient the crystal through the pyroelectric effect to determine the +Z surface of the wafer; clean the wafer to ensure that there are no impurities on the surface; use ion beam sputtering on the wafer The +Z plated electrode; on the metal film on the crystal surface, etch the geometric figure that completely corresponds to the mask to achieve the purpose of selective diffusion and metal film wiring. There are seven main processes in photolithography: Coating , pre-baking, exposure, development, corrosion and degumming; design polarization circuit, the electrode configuration of electric field polarization usually adopts the method of liquid electrode contact; finally use pulse high voltage electric field for polarization, in the domain area with electrodes, use high voltage The electric field overcomes the coercive field inside the crystal to reverse the spontaneous polarization direction of the electric domain in the domain region; in the domain region without electrodes, the polarization direction of the electric domain remains in the original direction, where the +Z plane is parallel to the horizontal plane and face the positive z-axis. The external electric field used is a pulsed high-voltage electric field. Since the coercive field of lithium niobate crystal is 21kv/mm, the pulse peak voltage applied to a lithium niobate wafer with a thickness of 0.5mm must be greater than 10.5kv/mm. The length of the pulse period is related to The number of times is related to the actual surface area of the electrode, which can be calculated by the following formula: $I_{\mathrm{pol}}=\frac{V_1-V_c}{R_{\mathrm{the\; s}}}-\frac{V_c}{R_{\mathrm{vm}}},Q={2P}_{\mathrm{the\; s}}A,t_{\mathrm{pol}}=\frac{Q}{I_{\mathrm{pol}}},$ Among them: I _pol is the polarization current, Q is the transport charge on the crystal surface, and t _pol is the polarization time.

4) Plating Y-direction electrodes on the polarized wafers, as follows:

First coat a layer of photoresist with a thickness of 1 μm on the +/-Z surface and +/-X surface of the polarized crystal, and put the sample into the pad in the center of the cavity of the vacuum sputtering machine (the target used is silver) On-chip; vacuumize, first use a mechanical pump, and reduce the pressure of the sample chamber to 5Pa; pre-sputter the substrate (lithium niobate) at a power of 200W for 4 minutes; turn off the sputtering power supply, cool for 5 minutes, open the vacuum chamber and take out the PPLN , that is, the PPLN crystal on the Y-plated electrode can be obtained.

5) As shown in Figure 2, the room temperature is controlled at 15.15°C. A polarizing beamsplitter prism is placed in front of the PPLN to make the O light incident. The tunable laser of 1520-1620nm is used as the fundamental wave light input. The Y direction of the PPLN is equipped with electrodes, and the optical power is used behind the PPLN. The meter measures the power of frequency-doubled light.

6) When a voltage not exceeding 400V/mm is applied, the power of the frequency-doubled light near the fundamental wavelength of 1549nm will be greater than that without voltage. When the voltage is 80V/mm, the power of the frequency-doubled light is the largest. It shows that here, frequency-doubled light is enhanced.

In other embodiments, other periodic or other non-periodically polarized nonlinear crystals can also be used to achieve this purpose; at the same time, the smaller implementation equipment as shown in Figure 2 takes up a volume to realize changing the period of the PPLN And the temperature during the experiment, adjust the position of frequency multiplication enhancement in the broadband frequency multiplication of O light.

Claims (7)

1. the Enhancement Method based on the frequency multiplication of periodic polarized lithium niobate, first room temperature electrical-poling is carried out to lithium columbate crystal, on+Z the face of crystal, negative farmland area change electricdomain polarised direction, is characterized in that, carry out vacuum coating sputtering electrode in the Y-direction both sides of crystal; Then to crystal carry out ordinary light-struck simultaneously with high-voltage power supply to the Y-direction both sides making alive of lithium columbate crystal, realize the enhancing of frequency multiplication by the slow light effect produced.

2. method according to claim 1, is characterized in that, the cycle that described electric field polarization adopts obtains in the following manner:

1) determine the polarization cycle Λ simultaneously met required for quasi-phase matched bar, namely meet: wherein: λ is the wavelength of fundamental wave, for the refractive index of first-harmonic E light, for the refractive index of first-harmonic O light, for the refractive index of frequency multiplication E light, Λ ₀for the cycle of I type frequency multiplication, Λ ₁be the cycle of 0 type frequency multiplication, Λ ₂for the cycle in forbidden band;

2) because O light broadband SHG not only requires quasi-phase matched, also require Group-velocity Matching, namely required cycle Λ ?the point of contact place of λ relation curve; Therefore matlab software simulation Λ is used ₀, Λ ₁and Λ ₂with the graph of a relation of λ, try to achieve the intersection point in three cycles, the cycle that intersection point is corresponding then can realize O, E light frequency multiplication and forbidden band simultaneously; By changing temperature, different intersection points can be obtained; And intersection point can be allowed to drop on Λ at a certain specific temperature ₁?the point of contact place of λ curve, the temperature that this point is corresponding and cycle, then can realize simultaneously the broadband of O light doubly, E optical sccond-harmonic generation and forbidden band, be required cycle and temperature.

3. method according to claim 2, is characterized in that, described room temperature electrical-poling refers to: adopt room temperature Polarization technique, according to sex reversal performance period in cycle obtained, obtain periodic polarized lithium columbate crystal.

4. method according to claim 1, is characterized in that, described vacuum coating sputtering electrode refers to: adopt vacuum coating equipment to adopt the method for sputtering to plate electrode in the Y-direction both sides of periodic polarized lithium columbate crystal.

5. method according to claim 1, it is characterized in that, described ordinary light irradiates and refers to: by the temperature that accurately controls environment, adopt outgoing fundamental frequency light in continuously adjustable laser instrument, produce ordinary light through polarization splitting prism and incide plane of crystal, high-voltage power supply is alive to the Y-direction both sides of lithium columbate crystal while, the frequency doubled light intensity of outgoing is measured by light power meter.

6. method according to claim 5, is characterized in that, the fundamental frequency light wavelength of described laser instrument is 1518 ?1627nm, and optical direction is x direction.

7. method according to claim 1, it is characterized in that, the enhancing of described frequency multiplication refers to: adopt output voltage to be no more than the Y-direction making alive of high-voltage power supply at lithium columbate crystal of 10KV, O optical sccond-harmonic generation is strengthened by the slow light effect produced, O light can portions turn be also E light simultaneously, the shg efficiency of E light is much larger than O light, and when ignoring the transition loss of fundamental light wave to frequency multiplication light wave, the maximum multiple can deriving frequency multiplication enhancing is: wherein: S is the multiple that slower rays strengthens, d ₃₃be the nonlinear factor that 0 type frequency multiplication is, d ₃₁for the nonlinear factor that I type frequency multiplication uses.