JPH0731334B2 - Optical pulse compressor - Google Patents

Description

The present invention relates to an optical pulse compression device for compressing an optical pulse with high efficiency and a large pulse compression ratio.

[Prior Art] The method of pulse compression has long been used for waveform processing (compression, expansion) of ultrasonic pulses using a dispersion delay line. The optical pulse compression is also the same in principle, and an optical pulse having a linear frequency change (frequency increases with time) is made incident on a medium having anomalous dispersion and is compressed. The method and apparatus are composed of three important elements as shown in FIG. The first element is a stable and narrow high-power optical pulse light source 1, which is usually a mode-locking dye laser or a CPM ring dye laser. The high power pulses from these lasers are incident on the second element, the single mode optical fiber 2. Light intensity readily 1 MW / cm chirp optical Kerr effect is a third-order nonlinear effect since ² could be more than is to be changed temporally frequency so generated along the envelope of the optical pulses in a single-mode fiber The pulses are available at the fiber output. This is called the self-phase modulation effect. This chirping pulse is compressed using the abnormal dispersion medium 3 which is the third element. That is, due to the anomalous dispersion, the propagation speed of the low frequency part located in the front part of the pulse becomes slow, and the propagation speed of the high frequency part located in the rear part of the pulse becomes fast. As a result, the light pulse is compressed.

In the optical pulse compression methods reported up to now, the high power pulse light source 1 and the single mode optical fiber 2 have the same elements, but the elements of the anomalous dispersion medium 3 are shown in FIGS.
Various methods as shown in the figure have been proposed. Figure 7 shows Na
It is composed of a vapor cell 3A and a total reflection mirror 3A ', and a method of utilizing anomalous dispersion accompanying resonance of Na vapor, FIG. 8 is a method of using two diffraction gratings 3B arranged in parallel,
FIG. 9 shows a method using a combination of two prisms 3C. The method of Fig. 7 is described by H. Nakatsuka and D. Grischkowsky in 1981 in Optics Letters Vol.6 Page.13-15 “Rec
ompression of optical pulses broadened by passage
through optical fibers ". Since this method uses the resonance of Na, the wavelength for pulse compression is 0.58μ.
There was a drawback that it was limited to around m. The method shown in FIG. 8 is the most general method and has been proposed by E. Treacy (“Optical pulse compression with diffraction gra
tings, "IEEE, J.QE, QE-5, Page 454-458,1969), 10 femtosecond (10 × 10 ^-15 seconds) pulse is possible. However, the diffraction efficiency cannot be so high. Therefore, there are drawbacks such as low output intensity, disturbance of the pulse tail, etc. The method of Fig. 9 is proposed by Zs.BOR and B.RA'CZ ("Group velocity Dispersion"). in
Prisms and Its spplication to pulse Compression a
nd Traveling-Wave Excitaion, "Opt.Commun, Volume 54, pag
e 165-170,1985) The feature is that anomalous dispersion can be created using a simple prism. However, since the wavelength dependence of the refractive index of the prism is small, it is necessary to increase the distance between the prisms 3C and 3C to several tens of meters, and there is a drawback that infrared light has a small anomalous dispersion and a limited application range.

Further, as a point common to the three methods shown in FIGS. 7 to 9, since the anomalous dispersion cannot be temporally controlled, there is a drawback that an uncompressed portion remains in the tail portion of the pulse.

[Problems to be Solved by the Invention] The present invention has the above-mentioned drawbacks of the prior art, that is, the wavelength of the pulse-compressed light is limited, the intensity of the output pulse is low, the prism interval is long, and the device is large. SUMMARY OF THE INVENTION It is an object of the present invention to provide a compact optical pulse compressing device capable of compressing an optical pulse with high efficiency over a wide frequency range, obtaining a high-output optical pulse, and solving the above problem.

[Means for Solving the Problems] In order to achieve such an object, the optical pulse compression apparatus of the present invention uses an optical pulse light source that generates a high intensity optical pulse and a positive pulse in the process of propagating the high intensity optical pulse. A single-mode optical fiber for performing chirping on the optical fiber, and an acousto-optic light deflecting device for passing and compressing a high-intensity optical pulse emitted from the single-mode optical fiber. It is characterized in that it is driven in synchronization with the pulse.

[Operation] In the present invention, anisotropic black diffraction of an ultrasonic light deflection element such as TeO _{2 is} used as a method of generating anomalous dispersion. When a linear chirping pulse with a spectral width of several 10 nm is incident on the ultrasonic deflection element, the diffraction angle varies depending on the wavelength. Ie
A diffraction grating is formed in the TeO ₂ medium, which enables highly efficient compression.

Further, in the conventional configuration, it was impossible to control the diffraction grating temporally, but according to the present invention, ultrasonic waves are temporally controlled
The diffraction grating can be temporally controlled by turning it N and OFF. Therefore, a portion of the skirt of the pulse that does not contribute to pulse compression can be removed, and a high compression rate can be obtained. The wavelength range used is wide from visible to infrared, and in particular, anomalous dispersion increases in proportion to the wavelength, which is effective for pulse compression in the optical communication wavelength range of 1 μm band.

Embodiments Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram for explaining the first embodiment of the present invention. In the figure, 1 is an optical pulse light source for generating a high-intensity optical pulse, which is composed of, for example, a mode-locking dye laser or a CPM ring dye laser, and 2 is a single-mode optical fiber. The high-intensity optical pulse generated by the optical pulse light source 1 is positively chirped in the process of propagating in the single-mode optical fiber 2, and an acousto-optic device using optical crystal TeO ₂ (hereinafter referred to as AO device)
It is incident on 4A and 4B. When the AO elements 4A and 4B vibrate with ultrasonic waves, they operate as a diffraction grating and compress an optical pulse. 5A, 5
B is an ultrasonic transducer for exciting AO elements 4A and 4B, 6A and 6B are transducers 5A and 5B, respectively.
Is a drive circuit for driving the. Reference numeral 7 denotes a pulse generator for controlling the AO elements 4A and 4B, and a pulse synchronized with the optical pulse is given from the optical pulse light source 1 to generate the pulse. 8
Is a delay circuit.

The operation of this embodiment will be described with reference to FIGS. 2 and 3.

FIG. 2 shows the case where the ON time of the control pulse is made longer than that of the compressed light pulse, and it plays the same role as a normal metal diffraction grating. Normally, the delay time of the delay circuit 8 is set to 0 and the first-stage and second-stage AO elements 4A and 4B are operated simultaneously. In this case, the non-linear chirp portion in the skirt portion of the optical pulse remains in the output waveform as it is. However, since the diffraction efficiency is 90% or higher compared to the conventional method, highly efficient compression is possible.

FIG. 3 shows the case where the ON time of the control pulse is shorter than that of the compressed light pulse. In this case, the ON / OFF of the AO element is effectively used and 1 is set by the control pulse from the pulse generator 7.
The skirt of the optical pulse is cut off by the AO element 4A in the second stage to take out a linear pulse, and the pulse compression is completed by the AO element 4B in the second stage. Extinction ratio due to ON / OFF of AO element is
1000: 1, that is, 30dB or more, so it is possible to completely cut off the hem. Normally, the delay of the delay circuit 8 is set to zero and the AO elements 4A and 4B of the first and second stages are operated simultaneously. As a result, the output pulse will be completely compressed.

Here, the distance b required for pulse compression is obtained. In FIG. 1, the group delay τ of light with respect to the first-order diffracted light of the AO element 4A in the first stage is given by the equation (1).

τ = b / c (1) where b is the length of the optical path taken by the light of the central wavelength λ of the incident light pulse, and c is the speed of light. If the distance between the two AO elements 4A and 4B is G, then b = G / cos θ. Where θ
Is the first-order diffraction angle. In order to obtain the dispersion constant δτ / δλ of the pulse compressor using this AO element, δτ / δθ is obtained to obtain the following equation.

δτ / δθ = Gsinθ / cos ² θ (2) Note that θ depends on λ, and the dispersion constant δτ / δλ of the system is Can be expressed as Where υs is the acoustic velocity of the anisotropic Bragg and fs is the ultrasonic frequency. As shown in equation (3), a larger anomalous dispersion can be generated as vs is smaller and fs is larger. This amount can be increased by using TeO ₂ anisotropic Bragg. For TeO ₂ anisotropic Bragg, υs ＝ 790m / s, fs ＝ 2
Since 00 MHz is the optimum operating condition, b = 1.3 m is obtained when the distance required for compressing an optical pulse having a wavelength λ = 580 nm, a pulse width δτ = 3 ps, and a spectrum width δλ = 20 nm is obtained.

FIG. 4 shows a second embodiment of the present invention. This example is AO
Only one element is used, and an optical pulse is reciprocally passed through the AO element 4A using the retro prism 9A to perform compression. In this embodiment, the number of AO elements is one, and the device can be simplified.

FIG. 5 shows a third embodiment of the present invention. In this embodiment, a vertical retro prism is added to the embodiment shown in FIG.
9B is added, and the optical pulse makes two round trips to the AO element 4A, that is, 4
Pass twice. In this device, there are two compression processes from the input to the output of the beam, and the output beam returns along the same optical path (except for the vertical change of the retro prism 9B), so that it has the same diameter as the input beam. Since the beam is separated into upper and lower parts by the retro prism 9B, it is easy to extract the beam.

In the embodiment shown in FIGS. 4 and 5, the ON / OFF time of the control pulse for the AO element is the same as that described in the first embodiment.

In the conventional prism combination shown in FIG. 9,
In the wavelength range longer than 0.7 μm, the anomalous dispersion is extremely reduced and the compression function is lost. However, according to the present invention, the dispersion constant is increased in proportion to the wavelength as shown in the equation (3), so that the wavelength range is longer. Also in, pulse compression is possible. The present invention is also effective for compressing a relatively wide Q-switch optical pulse.

[Effects of the Invention] As described above, by using anisotropic Bragg diffraction of the TeO ₂ AO element, a diffraction efficiency of 90% or more is possible, so there is an advantage that highly efficient pulse compression can be realized. See also A.
O. By turning the element on and off temporally, the tail portion of the pulse that does not contribute to pulse compression can be removed, so there is an advantage that a high compression rate can be obtained. The wavelength range used is wide from visible to infrared, and in particular, anomalous dispersion increases in proportion to the wavelength, which is effective for pulse compression in the optical communication wavelength range of 1 μm band.

[Brief description of drawings]

FIG. 1 is a block diagram of the first embodiment of the present invention, FIGS. 2 and 3 are diagrams for explaining the operation of the first embodiment, and FIGS. 4 and 5 are other drawings of the present invention. FIG. 6 is a diagram illustrating an embodiment, FIG. 6 is a diagram showing a configuration of a conventional pulse compression device and its waveform change, and FIGS. 7 to 9 are diagrams showing a configuration of an anomalous dispersion medium in the conventional device, respectively. 1 ... Optical pulse light source, 2 ... Single-mode fiber for self-phase modulation effect generation, 3 ... Anomalous dispersion medium, 4A, 4B ... Acousto-optic optical deflector (AO element), 5A, 5B ... Ultrasonic wave Transducer, 6A, 6B ... AO element drive circuit, 7 ... A.
O. Element control pulse generator, 8 ... Delay circuit, 9A, 9B ...
… A total reflection retro prism.

Front page continued (72) Inventor Yasuro Kimura Tokai-mura, Naka-gun, Ibaraki Prefecture 162 Shiraku, Shirahane, Nippon Telegraph and Telephone Corporation, Ibaraki Telecommunications Research Institute (72) Inoue Yoshiyuki Tokai-mura, Naka-gun, Ibaraki Prefecture 162 Shirakata, Shirahoji, Nippon Telegraph and Telephone Corporation, Ibaraki Research Institute of Electrical Communication (56) References Applied Physics Letters vol. 44, No. 8, pages 729-731

Claims (2)

[Claims] 1. An optical pulse light source for generating a high-intensity optical pulse, a single-mode optical fiber for positively chirping in the process of propagating the high-intensity optical pulse, and the light emitted from the single-mode optical fiber. An acousto-optic light deflector for passing and compressing a high-intensity light pulse, wherein the acousto-optic light deflector is driven in synchronization with the high-intensity light pulse. 2. The optical pulse compression device according to claim 1, wherein the acousto-optical light deflection device comprises one ultrasonic light deflection element and a retro prism.