JPH0151164B2 - - Google Patents

Description

Detailed Description of the Invention "Industrial Application Field" The present invention is used in optical communication or various physical measurement fields, and relates to an optical pulse width compression method and method for generating narrow optical pulses by compressing the width of an input pulse. Related to optical pulse width compressor.

"Prior Art" Methods for compressing optical pulses, that is, narrowing the pulse width, can be roughly divided into two methods in principle. The first principle is to select a wide-width optical pulse with specific dispersion characteristics as shown in Figure 1A,
By passing this optical pulse through a dispersion medium in which the group velocity of the optical pulse propagates differs depending on the wavelength, the subsequent part of the optical pulse can catch up with the preceding part, resulting in a pulse as shown in Figure 1B. This narrows the width. The second principle uses a gate to extract only a portion of the wide pulse width optical pulse as shown in FIG. 1A as a narrow pulse width as shown in FIG. 1B.

When using the first principle, the dispersion characteristics of the optical pulse and the dispersion characteristics of the dispersion medium must be opposite, so the sign of the dispersion sign of the optical pulse is The dispersion medium must be selected depending on which of the wavelength pulse components is in the leading part and which is in the trailing part. FIG. 2 shows a schematic configuration of a conventional optical pulse width compressor using the first principle. The input optical pulse 11 is inputted to one end of the single mode fiber 13 via the lens 12, and the output optical pulse from the other end of the single mode fiber 13 is focused by the lens 14 and sequentially reflected by mirrors 15 and 16. Then, it enters a cell 17 filled with Na vapor. The light that passes through the cell 17 and is reflected by the mirror 18 passes through the cell 17 again, is sequentially reflected by the mirrors 19 and 21, and enters the optical pulse detector 22.

Since the dispersion of the single mode optical fiber 13 at a light pulse wavelength of 0.59 μm has a positive sign, a cell 17 filled with Na vapor whose dispersion has a negative sign is used as the compression medium. According to the experimental results, the optical pulse 11 with a pulse width of 3.3 psec is transmitted through the single mode fiber 1.
After passing through cell 17, the signal expands to 13 psec, and is compressed again to 3.3 psec by cell 17. In this conventional method, a medium that is transparent in the used wavelength band and has opposite dispersion characteristics is selected depending on the wavelength of the optical pulse and the dispersion characteristics of the optical pulse to be compressed, and the distance to pass through the opposite dispersion medium is adjusted. However, there are many practical disadvantages.

On the other hand, when the second principle is used, optical pulses can be compressed regardless of whether or not the optical pulse has dispersion characteristics, or regardless of its dispersion characteristics. FIG. 3 shows a conventional example that utilizes the Kerr effect of a polarization-maintaining optical fiber. Light pulse λ/2
The light is incident on one end of the polarization maintaining optical fiber 25 via the plate 23 and the lens 24 in this order. At that time, the linearly polarized light pulse incident on the polarization-maintaining fiber 25 is adjusted by the λ/2 plate 23 so that it forms an angle θ with respect to the main axis (x-axis) of the optical fiber 25, and the lens 24 The light is focused and incident on the optical fiber 25. While propagating through the polarization-maintaining optical fiber 25, the low-level, low-power portion of the optical pulse undergoes polarization rotation due to the normal linear birefringence effect.
The low power part is received by the lens 26 and emitted.
The plane of polarization is changed to the original angle θ by the λ/4 plate 27.
The light is further blocked by a polarizer 28 arranged perpendicular to the incident polarization direction. However, the high-level, high-power portion of the optical pulse undergoes nonlinear rotation of the plane of polarization due to the Kerr effect while propagating through the polarization-maintaining optical fiber 25, and is emitted from the λ/4 plate 27. Even if it passes through, it does not return to the original angle θ, and therefore passes through without being blocked by the polarizer 28. As a result, as shown in FIG. 4A, the input optical pulse is separated in its polarization direction into a low power section and a high power section at the output end of the optical fiber 25, as shown in FIG. As the light passes through, the low power portion is blocked and a compressed light pulse is obtained as shown in FIG. 4C.

Compared to the method using an inverse dispersion medium based on the first principle described above, this optical pulse compression method does not require a specific inverse dispersion medium, and can be applied even when the optical pulse does not have dispersion characteristics. It's much better. However, since optical components such as λ/2 plates, λ/4 plates, and polarizers are required for inputting and outputting optical pulses, they are not suitable for miniaturization, and have the disadvantage that their optical adjustment is complicated. are doing.

"Objective of the Invention" The object of the present invention is that it does not require a reverse dispersion medium, can be applied to optical pulses without dispersion characteristics, does not require many optical parts, and can be easily adjusted by optical adjustment.
An object of the present invention is to provide an optical pulse width compression method and a compressor that can be configured in a compact size.

"Structure of the Invention" According to the method of the present invention, the signal light pulse is first
A control optical pulse is branched and supplied to the second optical waveguide, and is superimposed with the signal pulse after a certain time delay than the signal optical pulse, and propagated to the first optical waveguide, and the signal optical pulse is phase-modulated by the Kerr effect. give. An optical phase difference is provided between the signal light pulses propagating through the first and second optical waveguides, and the phase difference is set to π in a state where the phase modulation is not performed. The lights from the first and second optical waveguides are combined, and the control optical pulse component is removed from the combined light to obtain a compressed optical pulse.

According to the compressor of the present invention, the output end of the input optical waveguide to which the signal light pulse is supplied is branch-connected to one end of the first and second optical waveguides. The first and second optical waveguides are made of a material having a Kerr effect, and their other ends are connected to the input end of the output optical waveguide. The output end of the control optical waveguide to which the control optical pulse is supplied is connected to the first optical waveguide, and the control optical waveguide gives the control optical pulse a delay smaller than the pulse width of the signal optical pulse. Delay means are provided. In addition, in a state where the signal light pulse of the first optical waveguide is not subjected to Kerr effect phase modulation by the control light pulse, a phase difference of π is created between the output signal light pulses of the first and second optical waveguides at their optical wavelengths. A phase difference imparting means is provided. Furthermore, a control light pulse removing means is provided for removing the control light pulse from the light propagating in the output optical waveguide and outputting a compressed signal light pulse.

Embodiment FIG. 5 shows an example of an optical pulse width compressor according to the present invention. The input optical waveguide 31 receives a signal light pulse 32
is supplied and propagated, and its output end is branch-connected to one end of each of the first and second optical waveguides 33 and 34. The other ends of the first and second optical waveguides 33 and 34 are multiplex-connected to the input end of an output optical waveguide 35. First,
The second optical waveguides 33 and 34 are made of the same material having the Kerr effect. In this example, the first and second optical waveguides 33 and 34 have the same optical path length, and the signal light pulse from the input optical waveguide 31 is , are branched into the second optical waveguides 33 and 34 at the same ratio, and when both of the branched lights are combined in the output optical waveguide 35, the second optical waveguide 34 is used as a phase difference imparting means for giving a phase difference of π. A phase modulation section 6 is provided, and the phase modulation section 3
A DC electric field is applied to 6 from a power source 37 to impart phase modulation to the light passing through the second optical waveguide 34 by an electro-optic effect. These optical waveguides 31, 33, 34,
35 and phase modulation means 36 and 37 constitute a so-called Matsuha-Zehnder interferometer.

The output end of the control optical waveguide 38 is connected to the first optical waveguide 33, and the control optical pulse from the control optical waveguide 38 can be supplied to the first optical waveguide 33 and propagated. Delay circuit 39 in control optical waveguide 38
is provided, and can give the control pulse a delay δt shorter than the pulse width of the signal light pulse 32. It is preferable that the delay circuit 39 is capable of changing its delay amount δt. Control optical waveguide 38
In synchronization with the signal light pulse 32, or the signal light pulse is branched and supplied as a control light pulse 41, delayed by a delay circuit 39, and the light pulse 42
The light is input to the first optical waveguide 33 as a signal.

FIG. 6 schematically explains the operating principle of optical pulse width compression in this embodiment using a step-type pulse. This invention is based on the second principle, and (a) shows the control light pulse 41.
(b) is the case where the control pulse 41 is input, where the horizontal axis is time and the vertical axis is the amplitude of the optical pulse.

The DC electric field generated by the output of the power source 37 is set so that the optical phase difference φ between the signal light pulses passing through the first and second optical waveguides 33 and 34 becomes π. In FIG. 6a, the signal light pulse 3 supplied to the input optical waveguide 31
2 is branched into first and second optical waveguides 33 and 34 into signal light pulses 32a and 32b of equal amplitude as shown in FIGS. 6A and 6B, respectively. As shown in FIG. 6C, the signal light pulse 32b is not input to the control light pulse 41, and the signal light pulse 32b is not input to the phase modulation unit 36.
are subjected to phase modulation, and the phase difference φ between the two signal light pulses 32a and 32b that reach the output optical waveguide 35 is π and has equal amplitude, so they cancel each other out, and as shown in FIG.
As shown in FIG. 3, no optical pulse is output from the output optical waveguide 35.

On the other hand, in FIG. 6B where the control light pulse 41 is input, the control light pulse 42 is delayed by δt with respect to the signal light pulse 32 and is supplied only to the first optical waveguide 33, as shown in FIG. 6C. Ru. Therefore, the control light pulse 4 of the branched signal light pulses 32a and 32b
The light in the portion before 2 is mutually canceled out by the output optical waveguide 35 with a phase difference φ of π. However, the signal light pulse 3 of the first optical waveguide 33 is caused by the control light pulse 42.
2a undergoes phase modulation Δφ due to the Kerr effect, so after the leading edge of the control optical pulse 42, the signal light pulses 32a and 32b have a phase difference φ of π+Δφ in the output optical waveguide 35, and are canceled out by each other. It is output without any error. The signal light pulse 32 and the control light pulse 42 are guided as TE mode and TM mode in which the polarization planes are orthogonal to each other, respectively, and the light output from the output light waveguide 35 is passed through the analyzer 43. By making the polarization plane orthogonal to the polarization plane of the control optical pulse 42, the control optical pulse 42
can be removed, and only the compressed signal light pulse 44 can be extracted as shown in FIG. 6D.

By changing the delay time δt of the control optical pulse, the width of the output signal optical pulse 44 can be changed. FIG. 7 shows an example of the optical delay circuit 39, and FIG. 7A shows a combination of a total reflection mirror 45, a half mirror 46, and a position-variable total reflection mirror 47, and reflects light 48 to these as shown by the arrow. By moving the total reflection mirror 47 back and forth as shown by the arrow 49, the optical path length is changed and the optical delay amount δt is changed. Figure 7B
The optical pulse is passed through the optical fiber 51 and an arbitrary delay time δt is given to the control optical pulse by changing the length of the fiber.

The maximum phase change amount Δφm that the peak value of the control optical pulse 42 gives to the signal optical pulse is expressed as Δφm=2πn ₂ LI ₀ /λ (1). However, n ₂ is a nonlinear coefficient of refractive index representing the Kerr effect of the material of the first optical waveguide 33, and λ
is the wavelength in vacuum, L is the length of the optical waveguide where the control light pulse 42 and the signal light pulse interfere, and I ₀ is the peak power density of the control light pulse. Taking LiNbO ₃ , which is widely used as an optical waveguide material, as an example, n ₂
is 4.2×10 ^-10 (mw/cm ² ) ^-1 , L=2cm, λ
=0.84μm The peak power value of the control optical pulse 42 is
If the dimensions of the optical waveguide 33 are 4μm×0.04μm, then I ₀ =6.3× ¹⁰⁴ mw/ ^μm2 , so △φm/π=
0.63 is obtained. The peak power value of 0.1 μm of the control light pulse 42 is an extremely small value. For example, when the control light pulse 41 is extracted separately from the signal light pulse 32, it is only 1/107 of the signal power of 1 W.
The loss imparted to the signal light pulse in order to extract the control light pulse 41 can be ignored.

FIG. 8 shows the ratio of the rms pulse width σ ₀ of the compressed signal light pulse 44 to the width σ _i of the input signal pulse 32 as the compression ratio of the signal light pulse 32, and the loss caused by the compression of the signal light pulse 32. and maximum phase difference △
The relationship between φm/π is shown by curves 52 and 53, respectively. From this figure, the width of the signal light pulse 32 is approximately 3/5
It can be seen that the loss at that time can be reduced to 3 dB with △φm = 1.3π. In the previous numerical example, to obtain △φm/π=1.3, the control light pulse 4
The peak power value of 2 is required to be 0.2 μw. Although FIG. 8 shows the case where the delay time δT of the control light pulse is 1, it was confirmed that almost the same pulse compression ratio can be obtained even when δT is changed. Here, ΔT is the delay time Δt normalized by the pulse width of the input signal optical pulse, and the pulse in this example has a Gaussian shape, and the half width is used as the pulse width.

"Effects of the Invention" As explained above, the present invention can be implemented as a waveguide type element, and can be configured compactly with a small number of optical components, and optical adjustment is easy. By using currently available materials and extremely weak control optical pulses, it is possible to compress the optical pulse width and reduce the loss to 3 dB, and the time response of the Kerr effect of the material is usually less than 1 ps. It is extremely fast. Further, in the above example, the pulse width compressor can be easily made using an existing Matsuha-Zehnder interferometer.

From the above, the optical pulse width compression method and compressor of the present invention can be used not only as a circuit for optical pulse waveforms used in the field of high-speed optical communication, but also for various physical measurements to investigate transient response phenomena of materials using ultrashort pulses. In the field, it is suitable for obtaining ultrashort pulses by combining with a light source.

Furthermore, the present invention can be practiced by similarly utilizing the Kerr effect in conventional optical fibers. In this case, there is an advantage that the compression element of the Matsusha-Zehnder interferometer can be easily constructed using an optical fiber. Further, in order to provide a phase difference of π between the signal light pulses of the first and second optical waveguides, other means may be used, such as by providing an optical path length difference between the first and second optical waveguides. Furthermore, the control light pulse component can be removed from the combined light of the output optical waveguide, not only by making the polarization planes of the signal light pulse and the control light pulse orthogonal and removing it by the analyzer 43, but also by other means. You can read it.

[Brief explanation of drawings]

Fig. 1 is a diagram showing an input optical pulse waveform and a compressed pulse waveform, Figs. 2 and 3 are schematic diagrams showing a conventional optical pulse width compressor, respectively, and Fig. 4 is an explanation of the operation of Fig. 3. FIG. 5 is a schematic configuration diagram showing one embodiment of the present invention, and FIG.
A diagram showing the waveforms of each part in the figure, Figure 7 is the optical delay circuit 3
FIG. 8 is a diagram showing the relationship between the input/output rms pulse width ratio, loss, and Δφm/π. 31... Signal light pulse input waveguide, 32... Signal light pulse, 33... First optical waveguide, 34... Second optical waveguide, 35... Output optical waveguide, 36... Gives a phase difference of π to the branched signal light pulse. Phase modulation unit as part of means, 37...DC electric field≠power source for application, 3
8... Control optical waveguide, 39... Delay circuit, 41, 4
2... Control light pulse, 43... Analyzer as control pulse removal means.

Claims (1)

[Claims] 1. A signal light pulse is branched and supplied to first and second optical waveguides using materials having the Kerr effect, and the signal light pulse is present after a certain time delay than the signal light pulse. During this time, a control optical pulse is propagated through the first optical waveguide, and phase modulation is applied to the signal optical pulse propagating through the first optical waveguide by the Kerr effect, thereby reducing the Kerr effect in the signal optical pulse. A phase difference is imparted to the light propagating through the first and second optical waveguides so that the optical phase difference between the portion not subjected to phase modulation by the signal light pulse and the signal light pulse of the second optical waveguide is π. , an optical pulse width compression method for obtaining a signal optical pulse with a compressed width by multiplexing output light from a second optical waveguide and removing the control optical pulse from the multiplexed light. 2. an input optical waveguide to which a signal light pulse is supplied;
The output end of the input optical waveguide is branch-connected to each end, and the first and second optical waveguides are made of a material having the Kerr effect, and the other ends of the first and second optical waveguides are combined with the input end. The connected output optical waveguide has an optical phase difference of π when the signal light pulse light branched and supplied from the input optical waveguide to the first and second optical waveguides is combined in the output optical waveguide. A phase difference imparting means for providing a phase difference between the lights in both the first and second optical waveguides, and a control optical pulse is supplied and connected to the first optical waveguide to transmit the control optical pulse a control optical waveguide for supplying the optical waveguide to the optical waveguide; a delay means provided in the control optical waveguide for delaying the control optical pulse by an amount smaller than the pulse width of the signal optical pulse; and the output optical waveguide. An optical pulse width compressor comprising a control optical pulse removing means for removing a component of the control optical pulse from light propagating in a wave path and outputting a signal optical pulse whose width is compressed.