JP2766998B2 - Measurement device for third-order nonlinear optical constant - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for measuring a third-order nonlinear optical constant, and in particular, to a nonlinear optical material used for realizing an optical gate optical switch, an optical bistable element, and the like. The present invention relates to a measuring device for measuring a third-order nonlinear optical characteristic, that is, a nonlinear refractive index or a third-order nonlinear susceptibility.

[Prior Art] The nonlinear optical effect means that the electric polarization P in a substance is E ² , E ³ other than a term proportional to the electric field E of light as shown in the following equation (1).
This is an effect caused by having a higher-order term proportional to.

P = X ⁽¹⁾ E + X ⁽²⁾ E ² + X ⁽³⁾ E ³ + (1) where X ⁽¹⁾ is the linear susceptibility, X ⁽²⁾ and X ⁽³⁾ are quadratic and 3 respectively. It is called the following non-linear susceptibility. The third term causes a change in the refractive index (referred to as a nonlinear refractive index effect) depending on the light intensity I shown in the following equation (2).

n = n ₀ + n ₂ I (2) The nonlinear refractive index constant n ₂ (unit: cm ² / W) in the equation (2) is 3
For the non-linear susceptibility X ⁽³⁾ (unit: esu), Given by Where n ₀ is the normal (low excitation) refractive index,
c is the speed of light.

An optical medium having this nonlinear refractive index effect, a polarizer,
When combined with other optical elements such as optical resonators or reflecting mirrors, optical gated optical switches, optical bistable elements, or phase conjugate wave generators can be used in optical information processing and optical communication systems in the future. Devices can be built.

Optical materials exhibiting a non-linear refractive index include semiconductor materials such as InSb and GaAs (the mechanism (action) is based on the band-filling effect), semiconductor materials such as ZnS and ZnSe (by the thermal effect), and semiconductor superlattices such as GaAs / AIGaAs. material (due to absorption saturation effect of exciton spectrum), a liquid crystal material (by the liquid crystal orientation effect), (due to molecular orientation effect) molecular liquids such as CS _2, (due to the nonlinear polarization effect) organic materials, photorefractive dielectric material ( Semiconductor-doped glass material containing fine particles such as CdSSe in glass (due to band-filling effect)
And the like.

By the way, since the above-mentioned nonlinear refractive index materials have different mechanisms (actions), the response speeds of the materials are significantly different. For example, a liquid crystal material or a photorefractive material has a large nonlinear refractive index constant n ₂ (n ₂ 10 ⁻³ cm ² / W) and is therefore efficient, but has a slow response speed of about 1 ms. On the other hand, semiconductor-doped glass and organic materials have a poor efficiency of n ₂ 10 ⁻¹¹ cm ² / W, but have a very fast response speed of about 10 ps or less. Therefore, in order to realize an optical nonlinear element using the nonlinear refractive index effect,
It is important to select a material whose response speed is sufficiently higher than the required speed, and to precisely measure the constant n ₂ value to design a device.

The present invention, among these optical nonlinear element, particularly in those high speed is required, precise values of the nonlinear refractive index n ₂ of the high-speed non-linear materials such as conventional measurement difficult semiconductor doped glass or organic material The present invention relates to an apparatus for performing measurement at a time.

Hereinafter, a conventional example of a measuring apparatus for measuring the nonlinear refractive index of a semiconductor-doped glass which is a high-speed nonlinear material will be described.

FIG. 3 shows the principle of a conventional measuring device using an interferometer developed at the University of Arizona (Reference: GROlbright an
d N. Peyghambarian; Appl. Phys. Lett. Vol. 48, (1986) p.
48). The laser light of the input light is
The light is branched into two optical paths, and enters the total reflection mirrors 13 and 14 almost perpendicularly, and is reflected. This reflected light is recombined by the semi-transmissive mirror 12 to become output light. Interference fringes appear in this output light. For observing the interference fringes, a pattern observer such as a television camera is used. This optical system is called a Twyman-Green interferometer.

The sample 1 to be measured made of semiconductor-doped glass is placed immediately before one total reflection mirror (for example, 14). One of the optical path lengths (refractive index x length) changes by placing sample 1,
As a result, the positions and intervals of the interference fringes of the output light change. When the sample 1 has a nonlinear refractive index, this change depends on the input light intensity. Conversely, when the spatial shift amount of the interference fringes is observed, the nonlinear refractive index is measured.

In the example reported in the above document, the semiconductor doped glass sample has n _{2 at} an input light wavelength of 0.49 to 0.52 μm.
A value of 2 × 10 ⁻¹¹ cm ² / W was determined.

FIG. 4 is a second conventional example reported by the inventors of the present invention of Nippon Telegraph and Telephone Corporation, and shows the principle of a measuring device using an optical resonator (literature: J. Yumoto, S. Fukushima, and K.Ku
Bodera; Opt. Lett. Vol. 12, (1987) p. 832). The laser light of the input light is incident on an optical resonator composed of two reflecting mirrors 2 and 3 facing each other (having a reflectivity of about 90%), and is emitted as output light by repeating multiple reflections. At this time, the transmittance (output light intensity / input light intensity) of the optical resonator is two reflecting mirrors 2,
It depends very sensitively on the spacing l _cav of 3 when the spacing (resonator length) of this mirror is just an integral multiple of 1/2 of the incident light wavelength λ (l
_cav = mλ / 2: m is an integer), and is known to exhibit high transmittance. This is shown in FIG. 4 in FIG. 4 is a piezo element whose thickness is minutely changed by applying a voltage.
The reflecting mirror 3 is attached and fixed to the piezo element. That is, by adjusting the voltage applied to the piezo element 4, the cavity length _{lcav in} FIG. 5 can be arbitrarily changed.

The sample 1 to be measured made of semiconductor-doped glass is placed in the above-described optical resonator. Cavity length in this case is l _cav = nl _s + l
_a (where n is the refractive index of the sample 1, l _s, and l _a sample 1 and the thickness of the air layer) is represented by. When the refractive index of the sample 1 changes depending on the intensity I of the incident light as in the above equation (2), the value on the horizontal axis in FIG.
_If the initial value of the _cav (the value when the intensity I of the incident light is 0) is properly set, a slight change in the refractive index can be measured with high sensitivity.

In the example reported in the above-mentioned document, the semiconductor doped glass sample has n ₂ 3 × at an incident light wavelength of −0.53 μm.
A value of 10 ^-11 cm ² / W was determined. This measuring device is characterized in that it can be measured with a light source having a lower output than the conventional example shown in FIG.

[Problems to be Solved by the Invention] However, the conventional measuring device shown in FIG. 3 requires an extremely large high-precision interferometer, and the shift amount of interference fringes is actually small. However, extremely high-power lasers are required as input light, and if a simple pulsed laser is used as the high-power laser, a large pattern observation for detecting the instantaneous shift amount of interference fringes is required. There were drawbacks such as the need for a vessel.

On the other hand, the conventional measuring device shown in FIG. 4 has a feature that it can be measured with a low-power light source with a simple configuration, but it is difficult to accurately estimate multiple reflections, and as a result, the measurement accuracy is greatly reduced. There was a problem.

The present invention has been made in view of these points, and an object of the present invention is to provide a measuring apparatus which can use a low-power laser light source with a simple configuration and can determine a highly accurate nonlinear refractive index constant. It is in.

[Means for Solving the Problems] In order to achieve the above object, the present invention has a high reflectivity of about 90% at a wavelength λprobe and the wavelength λprob
e, an optical resonator in which two mirrors having sufficiently low reflectivity at a wavelength λgate opposite to each other are configured to face each other, and a sample to be measured is placed inside the optical resonator. The non-linear refractive index of the sample to be measured is determined from an optical system for injecting the probe light of the wavelength λprobe, the pulsed gate light of the wavelength λgate, and the time change of the amount of light transmitted from the optical resonator of the probe light. And measuring means for performing the measurement.

[Operation] In the conventional example shown in FIG. 4, the input light is a single beam, whereas in the present invention, light (called gate light) for causing a change in the refractive index of the sample to be measured and its refraction are generated. Since the light for detecting the rate change (called probe light) is incident on the optical resonator as separate light, and this optical resonator is configured to be used as an optical resonator for only the probe light,
Unlike the conventional example, there is no need to consider the influence of multiple reflections of the gate light, and therefore, a highly accurate measurement of the nonlinear refractive index constant can be realized with a simple configuration using a low-power laser light source.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a configuration diagram of an apparatus for measuring a third-order nonlinear optical constant according to an embodiment (Example 1) of the present invention. In this figure, 1 is a semiconductor-doped glass of a sample to be measured optically polished to a thickness of 300 μm, and 2 and 3 are wavelengths λpro
An optical reflector 4 having a reflectivity of 90% at be = 0.83 μm and a reflectivity of 12% at a wavelength of gate light of λgate = 0.675 μm, 4 is a piezo element whose thickness is minutely changed by voltage application. Reference numeral 5 denotes a dye pulse laser that generates gate light having a wavelength of 0.675 μm and a pulse time width of 9 ns, and reference numeral 6 denotes a steady oscillation semiconductor laser that generates probe light having a wavelength of 0.83 μm. 7,
Reference numeral 8 denotes a lens, 9 denotes an optical filter that cuts gate light and transmits only probe light, 10 denotes a photomultiplier, and 11 denotes an oscilloscope. The sample to be measured 1 is an optical reflecting mirror 2,
Put between three.

The distance between the mirrors 2 and 3 constituting the optical resonator is about 800 μm.
m, and thus, the optical length (resonator length) of the resonator l _cav, including the sample _{1 l cav = nol s + l} a = 1.56 × 300μm + 5
00 μm = 970 μm (where n ₀ is the refractive index of the sample, l
_s and l _a the thickness of the sample and the air layer).

The rear reflecting mirror 3 is attached to and fixed to the piezo element 4, and by changing the voltage applied to the piezo element 4, the value of the resonator length l _cav can be changed with the accuracy of the order of wavelength.

FIG. 2A shows the transmittance characteristics of the resonator when the light (probe light) of the semiconductor laser 6 is condensed by the lens 7 and is incident on the mirrors 2 and 3 of the resonator. . The probe light is reflected multiple times in the resonator, and as a result, exhibits a sharp resonance peak at an interval of λprobe / 2. L _{cav on the} horizontal axis corresponds to the voltage applied to the piezo element 4.

The procedure for measuring the nonlinear refractive index of Sample 1 in this example is as follows. First, the cavity length l _cav in FIG. 2A is fixed to a certain length, and in this state, the pulse laser beam of the gate light generated from the pulse laser 5 is irradiated through the lens 7 to the same position as the spot of the probe light. I do. Due to this irradiation, the refractive index of the sample 1 changes according to the above equation (2), so that the cavity length is l _cav = (n ₀ + n ₂ I) l _s + l _a
And the optical length changes by Δl _cav = n ₂ I ₁₅ (4) as compared to the OFF state where no gate light is applied. Here, I is the power density of the gate light. Since the transmittance of the resonator with respect to the probe light changes in a pulse shape according to FIG. 2A, the value of n ₂ can be calculated by analyzing the change waveform of the probe light.

FIG. 2 (B) shows that the peak value of the gate light intensity is I = 6.6 MW /
The time waveform of the transmitted light (output light) of the gate light and the probe light observed by the oscilloscope 11 through the filter 9, the lens 8, and the photomultiplier tube 10 at cm ² is shown. Since the values of the output light intensity at the time of OFF and at the time of ON should correspond to the vertical axis of FIG. 2 (A), the state of the resonator at the time of OFF and ON is indicated by the arrow in FIG. 2 (A). It is determined as follows.
That is, from the waveform of FIG. 2 (A), the change in the resonator length during ON-OFF can be read as Δl _cav = 0.18 × (λprobe / 2) = 0.0747 μm. From this value and the above equation (4), the nonlinear refractive index of Sample 1 is determined to be n ₂ = 3.8 × 10 ⁻¹¹ cm ² / W. Further, X ⁽³⁾ = 1.76 × 10 ⁻⁹ esu is calculated from equation ⁽³⁾ .

In the embodiment of the present invention, since the reflectance of the resonator mirrors 2 and 3 with respect to the gate light is as small as 12%, the fact that the gate light does not cause multiple reflection in the resonator is fundamentally different from the conventional example shown in FIG. One of the different features. Therefore, in estimating the value of I in the equation (4), the analysis of the multiple reflection in the resonator, which has been a problem in the past, is completely unnecessary, and as a result, high geodetic accuracy can be realized. Furthermore, in the embodiment of the present invention, multiple reflection is used for the probe light, and as a result, high-precision measurement is one of the major features as compared with the conventional example of FIG. . Therefore, in the embodiment of the present invention, a low-power pulse laser can be used as an input light source,
The device can be reduced in size.

1. Other Embodiments In the embodiment of the present invention shown in FIG. 1, the gate light is irradiated on the sample 1 to be measured through an optical path different from that of the probe light, but this is changed to use a semi-transmissive mirror (not shown). It is, of course, also possible to combine these two lights and make the gate light incident on the sample 1 to be measured on the exactly same optical path as the probe light. Further, a semiconductor detector or a high-speed streak camera can be used instead of the photomultiplier tube 10 described above. Although the example of the semiconductor-doped camera has been described as the sample 1 to be measured in FIG. 1, exactly the same measurement is possible for an organic material having the same third-order nonlinear optical constant.

[Effects of the Invention] As described above, according to the present invention, probe light having a wavelength showing sharp resonance characteristics of a resonator and gate light having a wavelength showing almost no resonance characteristics are used as its basic elements. Therefore, the measurement accuracy is high and the data analysis is easy. As a result, a highly accurate nonlinear optical constant measuring apparatus with a simple configuration can be realized. Therefore, the measuring apparatus of the present invention exerts its power on various nonlinear optical materials, especially high-speed responsive materials such as semiconductor-doped glass and organic materials, which have been difficult to measure in the past. It is expected to be an indispensable measuring device for the development of devices such as gate optical switches and optical bistable elements.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an apparatus for measuring a third-order nonlinear optical constant showing an embodiment of the present invention. FIG. 2 is a diagram showing the transmittance of the optical resonator in the embodiment of FIG. FIG. 3 is a waveform diagram showing pulse waveforms of the gate light and the output light (FIG. 3B). FIG. 3 is a principle diagram showing an example of a conventional measuring device for a third-order nonlinear optical constant. FIG. FIG. 5 is a principle diagram showing an example of an apparatus for measuring a nonlinear optical constant, and FIG. 5 is a characteristic diagram showing the transmittance of the optical resonator shown in FIG. 1 ... Sample to be measured, 2,3 ... Reflector with 90% reflectivity,
4 ... Piezo element, 5 ... Pulse laser, 6 ... Stable oscillation laser, 7,8 ... Lens, 9 ... Optical filter, 10 ...
… Photomultiplier tube, 11… Oscilloscope, 12… Semi-transmissive mirror, 13,14 …… Reflective mirror.

Claims (1)

(57) [Claims] 1. An optical resonator comprising two mirrors having a high reflectance of about 90% at a wavelength λprobe and a sufficiently low reflectance at a wavelength λgate different from the wavelength λprobe; After the sample to be measured is set inside the optical resonator, a probe light having the wavelength λprobe and the pulsed gate light having the wavelength λgate are incident on the optical resonator. Measuring means for determining a non-linear refractive index of the sample to be measured from a temporal change in the amount of light transmitted from the optical resonator.