JPH01149032A - Method and device for optical pulse generation - Google Patents

Abstract

PURPOSE:To always efficiently generate a short optical pulse with a desired repeat frequency by forming the optical pulse from the fluctuation of the intensity of a high output continuous oscillation laser light at the time of making this laser light incident on an optical fiber and continuously varying the repeat frequency of the optical pulse. CONSTITUTION:The device is provided with a high output continuous oscillation laser 2 having an intensity fluctuation, an incident light source for incidence of a weak light 5 coinciding with the wavelength of a side band, a birefringence optical fiber 12 which induces modulation instability, a multiplexer 3 which multiplexes the laser light 2 and the incident light 5 to make the optical pulse incident, and a photodetector 6 which detects an optical pulse 7 in the exit end of the optical fiber. When the high output continuous oscillation laser light 2 is made incident on the optical fiber, the optical pulse is generated from the intensity fluctuation of the laser light by self-phase modulation due to the optical Kerr effect of the optical fiber and negative dispersion of the optical fiber and the repeat frequency of the optical pulse is continuously varied. Thus, a phase matching condition is always satisfied for the wavelength of the incident light and the optical pulse is generated with an arbitrary repeat frequency.

Description

[Detailed Description of the Invention] (Field of Industrial Application) The present invention generates short optical pulses with an extremely high repetition frequency of sub-THZ, which can be arbitrarily set in a wide frequency band, and whose pulse width is on the order of pico. The present invention relates to a method and an apparatus for doing so.

[Conventional technology]

Widely used techniques for generating so-called short optical pulses with a narrow pulse width include a method of mode-locking a laser to obtain phase synchronization, and a method of directly modulating a semiconductor laser. Short optical pulses of several tens of femtoseconds (10-'' seconds) have been obtained using this method. However, the current limit to the pulse repetition frequency is 20 GHz, which can be obtained by directly modulating a semiconductor laser.

Next, a method for generating high repetition frequency, short optical pulses using modulation instability will be explained.

When a relatively high-intensity light pulse as shown in Figure 6(a) propagates through an optical fiber, the optical Kerr effect, which is a type of nonlinear optical effect of the material of the optical fiber, causes the optical pulse to appear in Figure 6(b).
As shown in the figure, the frequency of the optical pulse becomes low at the rising edge and high at the falling edge, which is called chirping, and when this optical pulse passes through an anomalous dispersion medium with negative chromatic dispersion, it becomes a high-frequency pulse. It is known that the falling part of the support catches up with the rising part, and as a result, the pulse is compressed on the time axis as shown in FIG. Therefore, when a high-power continuous wave laser beam with fluctuations in intensity propagates through an optical fiber, the fluctuations gradually grow and become short optical pulses. In this case, the pulse repetition frequency is a sideband frequency with respect to the frequency of the incident laser beam on the optical frequency axis, as shown in FIG. It is determined as a frequency that satisfies the phase matching condition of wave mixing (induced R4 photon mixing). In addition, in this case, 3
Waves include incident laser light, Stokes light generated in sidebands,
It is anti-Stokes light, and the frequency of each wave is f
P, fs+ rAs, and the wavelength is λ2. λ3. λ,,
Then, the relationship between the repetition frequency Δf and the corresponding wavelength interval Δλ from the laser light wavelength is given by the following equation (1). Note that in the following equation, C represents the speed of light in vacuum.

Δf=fA, -f, =f, -f.

=C(1/λ□-1/λF) =C(1/λP-1/λS) Δλ=(λ,・λAs/C)Δf =(λ,・λ3/C) Δf... (1) Furthermore, in the conventional method, weak light equal to the frequency of Stokes light is injected into an optical fiber, and the formation of optical pulses is promoted by satisfying the phase matching condition of stimulated three-wave mixing.

In FIG. 8, an optical fiber with a wavelength of approximately 1- is used, and a laser beam with a wavelength of 1.
318am, injection light 1.400. This is an experimental result in the case of um. It can be seen from the figure that since the pulse time interval T is 3 ps, short optical pulses with a repetition frequency Δf of 340 GHz are obtained.

FIG. 9 shows the optical pulse generator, and the optical output continuous wave laser beam 1. lens 22 multiplexer 3. Optical fiber 4. Injected light 5. It consists of a light receiver 6, and 7 is a generated light pulse.

The phase matching condition for guided three-wave mixing is as shown in the following equation (2), where each phase term depends on the material dispersion, waveguide dispersion, birefringence, and incident laser intensity of the optical fiber. To,
It holds true when the sum of is zero. However, Kt is the total amount of phase mismatch, and Δν is a normalized frequency difference obtained by normalizing the shift amount Δf of the Stokes light and anti-Stokes light from the frequency of the incident laser light by the speed of light C.

KL (Δν) = (Δν) 10 Kw (Δν) 10 to 3 (Δ
ν) 10K11 Δν=Δf/C... (2) [Problem to be solved by the invention] However, in the past, each phase term in equation (2) was calculated for a given wavelength of laser light and injection light. Since there was no way to control Kt in some way to make it zero,
When the wavelength of the injected light mixed with the laser light does not satisfy the above phase matching condition, the optical pulse generation efficiency naturally deteriorates. FIG. 10 shows theoretical values of efficiency with respect to the amount of phase mismatch Kt, assuming that the efficiency when Kt=0 is 1. Efficiency is 0
．． 5, Kt is 0.12 cm-', and applying this to the experimental results shown in Figure 8, the wavelength of the injected light is 1.400 cm-'.
This corresponds to a deviation of 1.4 nm from μm. From this, it is theoretically clear that a slight shift in the wavelength of the injected light significantly reduces the efficiency of the generated optical pulse.

Therefore, it is actually necessary to select the wavelength that maximizes the efficiency of the generated optical pulse by sweeping the wavelength of the injected light.
There was a problem in that the repetition frequency could not be set arbitrarily.

In other words, conventionally, the repetition frequency is essentially fixed, and although the above phase matching condition is satisfied for a specific repetition frequency, the injection light The actual situation was that optical pulses could not be generated because the phase matching condition was not necessarily satisfied for the wavelength, and short optical pulses could not be obtained because the degree of modulation of the generated optical pulses was insufficient.

[Means for solving problems]

The present invention was made to solve the above problems, and
When a high-power continuous wave laser beam with intensity fluctuation due to modulation instability, which is a type of nonlinear optical phenomenon that occurs in an optical fiber, is input into an optical fiber, self-phase modulation caused by the optical Kerr effect of the optical fiber and A light pulse is formed from the intensity fluctuation of the laser beam by negative dispersion, and the repetition frequency of the light pulse is continuously varied. An injection light source that injects weak light that matches the wavelength of the waveband, a birefringent optical fiber that induces modulation instability, and a multiplexer that combines the laser beam and the injection light and inputs an optical pulse. It consists of a light receiver that detects light pulses at the output end of the optical fiber.

[For production]

The birefringence-dependent component of the amount of phase mismatch is varied.

〔Example〕

Hereinafter, specific embodiments of the present invention will be described with reference to the drawings.

First, a method of changing the value of the phase term in equation (2) in an analog manner in order to continuously vary the pulse repetition frequency will be described.

Among the phase terms in equation (2), is fixed with respect to the wavelength of the optical fiber material and the laser light source, and K8 is also a term that depends on the waveguide structure, so it remains unchanged, and 7 in the phase term is fixed with respect to the wavelength of the optical fiber material and laser light source. Although it depends on the strength, the amount of change is extremely small and the substance is considered to be semi-fixed. Therefore here the term K depends on the birefringence.

Focusing on this, we will discuss an example using a stress-applied birefringent fiber. A birefringent fiber is one in which tensile or compressive stress is applied to the core due to residual stress within the fiber, and a difference in refractive index, that is, birefringence, occurs in two orthogonal directions within the fiber cross section.

Therefore, it has been experimentally confirmed that this birefringence can be changed by applying stress from the outside.

Figure 2 shows various stress application methods; (a) is a method in which the fiber is placed on a flat plate and pressure is applied with the flat plate, (b) is a method in which the fiber is bent, and (C) is a method in which the fiber is placed in a thermostatic oven and heated. This is the way to do it.

Figure 3 shows the calculated value of δB (difference from the value when no stress is applied, which accounts for the change in birefringence by the method shown in Figures 2 (a) and (b)), and the horizontal axis is the x- and y-axis directions. The applied stress WX, W
, and the reciprocal of the bending diameter 2R. Note that the x and y axes are the long axis and short axis of the crystal principal axis of the birefringent optical fiber, respectively. Although a birefringent fiber is used here as an example, birefringence similarly occurs in ordinary fibers when stress is applied from the outside.

In order to achieve phase matching, it is necessary to define the relationship between the crystal axis of the optical fiber and the polarization plane of the laser beam.

In the two cases described below, the polarization plane of the laser beam is incident along either the long axis (x-axis) or the short axis (y-axis) of the crystal principal axis of the birefringent fiber, and
This is a case where the light is incident on both the long axis and the short axis separately.

FIG. 4 shows a distribution curve of LpH1, which is the fundamental waveguide mode of the optical fiber. As shown in FIG. 5A, the vertical axis represents the normalized propagation constant b, and the horizontal axis represents the normalized frequency ■. For a birefringent fiber, it is shown in Fig. 2(b).

As shown in the enlarged view of (C), the degeneracy of the two fundamental modes, LPo+ and LP□ modes polarized along the x and y axes, has been resolved. (b) The figure shows the polarization directions of the laser beam, the injection light that becomes Stokes light, and the anti-Stokes light as y, x, respectively.
In this case, in equation (2),
is given by the following equation.

K1-4πB, /λ, ... (3) Note that even if the polarization directions of these three waves are set to the x, yt, and y axes, the phase matching condition can be satisfied as will be explained later. Further, the injected light may be matched to the wavelength of either Stokes light or anti-Stokes light, and if the phase matching condition is satisfied, the other light will be generated by stimulated emission and the polarization direction will be automatically determined.

FIG. 4(C) shows injection light that becomes Stokes light by dividing laser light into X-polarized light and y-polarized light.

This is a case where the polarization directions of the anti-Stokes light are made to coincide with the y- and y-axes, respectively. Note that the polarization planes of Stokes light and anti-Stokes light are respectively X.

The phase matching condition can also be satisfied when it is made to coincide with the y-axis. In this case, in equation (2) is given by the following equation.

K, -2πB, Δν ... (4) Figure 5 shows the relationship between the and -(Kw +KI) in equation (2) and the normalized frequency difference Δν between Stokes light, anti-Stokes light, and laser light wavelength. It shows. However, the term 7 that depends on the laser light intensity is ignored as it is smaller than the other terms.

Same figure (a).

(b) corresponds to the cases in FIGS. 4(b) and (C), respectively. It can be seen that depending on the stress application method described above, Δv changes as shown by the solid arrow in the figure, and Δν that satisfies the phase matching condition given as the intersection of both curves changes accordingly.

Experimentally, W=0.5 kg/am has been achieved using the method (a), and in this case, δBs#2X10-5. (
Also in the methods b) and (c), δ at R = 0°5 cm
B1'';2X10-' is achieved. If the variable range of Δν is estimated from this change using the method shown in Figure 5, it is considered to be about several 100,100 O' when the laser light wavelength is around 1.3 μm. When converted to the repetition frequency, it can be seen that the variable range is extremely wide, from several tens to several thousand Hz.

FIG. 1 shows an optical pulse generator for realizing the present optical pulse generation method, and shows an optical output continuous wave laser beam 1. lens 2
9 multiplexer 3. Injected light 5. Receiver 6° half mirror 8, mirror 9. Polarizing plate 10. λ/2 plate 11. Birefringent fiber 1
2. It consists of a stress applying device 13, and 7 is a generated optical pulse. The laser beam is split by a half mirror 8, the plane of polarization is adjusted by a polarizing plate 10, and then combined with the injection light by a multiplexer 3 and input into an optical fiber.

In this way, in order to continuously vary the pulse repetition frequency in the optical pulse generation method, the present invention changes the value of the phase term in equation (2) in an analog manner, and thereby Kt(
This makes it possible to continuously change Δν such that Δν)=0, that is, the repetition frequency Δf (=C·Δν) of the generated optical pulse. In other words, according to the present invention, the frequency f corresponding to the desired repetition frequency Δf, -(Δλ/C
)・fl (wavelength: λ8=λ, +C(Δλ/λ,') When trying to generate a short optical pulse by injecting weak light, stress is applied to the fiber to change the birefringence. As a result, phase matching can be satisfied at a certain stress value, and by always satisfying the phase matching condition for the wavelength of the injected light, it is possible to generate optical pulses at an arbitrary repetition frequency.

〔Effect of the invention〕

As explained above, the present invention provides an optical Kerr effect in an optical fiber when a high-power continuous wave laser beam with intensity fluctuations due to modulation instability, which is a type of nonlinear optical phenomenon that occurs in an optical fiber, is incident on the optical fiber. An optical pulse is formed from the intensity fluctuation of the laser beam by the self-phase modulation caused by the self-phase modulation and the negative dispersion of the optical fiber, and the repetition frequency of the optical pulse is continuously varied. A high-output continuous wave laser, an injection light source that injects weak light that matches the wavelength of the sideband, a birefringent optical fiber that induces modulation instability, and a light pulse that combines the laser light and the injection light. The structure consists of a multiplexer that inputs the optical pulse, and a receiver that detects the optical pulse at the output end of the optical fiber, thereby eliminating the semi-fixed state that was considered to be a drawback of high repetition frequency and short optical pulse generation methods that use modulation instability. The repetition frequency can be varied continuously over a wide frequency band because the birefringence-dependent component of the amount of phase mismatch is varied, and short optical pulses can always be efficiently generated at the desired repetition frequency. It has this effect.

Therefore, applications are conceivable in fields such as light sources for ultra-high-speed optical communications that are about 1000 times faster than conventional ones, and light sources for optical sampling used in ultra-high-speed waveform analysis.

[Brief explanation of the drawing]

FIG. 1 is an optical pulse generator showing the configuration of an embodiment of the present invention, FIGS. 2(a), (b), and (c) are perspective views for explaining the stress applying method, and FIG. A graph showing the relationship between the change in refraction and the applied stress 2 bending radius, Figure 4 (
a), (b), and (c) are the dispersion curve of the fundamental mode of the optical fiber and the laser beam. Graphs showing the mode relationship between Stokes light and anti-Stokes light, Figures 5(a) and 5(b) are the phase amount - (Ki
+ K%1), graphs showing the relationship between Klm and normalized frequency difference, Figures 6(a), (b), and (e) explain pulse compression due to optical pulse chirping and anomalous dispersion. Graph for Figure 7(a). (b) is a timing diagram showing the frequency/wavelength relationship between stimulated three-wave mixing laser light, Stokes light, and anti-Stokes light. 9A timing diagram showing the generated pulses. Figure 8 is a timing diagram showing the pulses generated. FIG. 9 is a waveform diagram showing experimental results of pulse generation, FIG. 9 is a block diagram showing a conventional optical pulse generator using phase instability, and FIG. 10 is a graph showing the relationship between generation efficiency and phase mismatch amount. ■... Optical output continuous wave laser beam, 2... Lens, 3
... Multiplexer, 5... Injected light, 6... Light receiver, 7.
...Generated light pulse, 8... Half mirror 19...
・Mirror, 10...Polarizing plate, 11...λ/2 plate, 1
2... Birefringent fiber, 13... Stress applying device. Patent applicant: Nippon Telegraph and Telephone Corporation Agent: Kazuka Yamakawa (one idiot) Figure 1 to Figure 3 1/2R (cmll) Figure 2 Figure 4 (a) (b) (c) ■■ 5th Figure (a) nFk4ctfl=IfLET, ΔV (cm-1) (b
) Figure 6 Satsuma (psec) Time lv! (psec) Time (psec) Figure 7 Figure 8

Claims (3)

[Claims] (1) When a high-power continuous wave laser beam with intensity fluctuations due to modulation instability, which is a type of nonlinear optical phenomenon that occurs in an optical fiber, is input into an optical fiber, self-phase modulation occurs due to the optical Kerr effect of the optical fiber. An optical pulse generation method that forms optical pulses from intensity fluctuations of a laser beam using negative dispersion of an optical fiber, the method comprising continuously varying the repetition frequency of the generated optical pulses. (2) The repetition frequency of the optical pulse is changed by applying stress from the outside to the birefringence of the optical fiber, and the Stokes' sideband wavelength corresponds to the laser wavelength corresponding to the repetition frequency of the optical pulse to be generated. 2. The optical pulse generation method according to claim 1, wherein the optical pulse generation method is continuously varied by always satisfying a phase matching condition for stimulated three-wave mixing between light, anti-Stokes light, and the laser light. (3) a high-output continuous wave laser with intensity fluctuation; an injection light source that injects weak light that matches the sideband wavelength corresponding to the repetition frequency of the optical pulse to promote pulse generation; and the modulation. Optical pulse generation consists of a birefringent optical fiber that induces instability, a multiplexer that combines laser light and injected light to input optical pulses, and a light receiver that detects optical pulses at the output end of the optical fiber. Device.