JP2010054522A - Method for changing optical spectrum, and apparatus for generating light with changed spectrum - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for changing a spectrum of optical pulses capable of efficiently performing energy conversion in a C band with high efficiency by using a nonquartz-type nonlinear optical fiber. <P>SOLUTION: The method comprises introducing optical pulses into one end of a nonlinear optical fiber and generating light with changed waveforms from the other end of the nonlinear optical fiber, in which the nonlinear optical fiber is composed of glass containing Bi<SB>2</SB>O<SB>3</SB>at ≥40 mol%, and the group velocity dispersion thereof is ≤-10 ps/nm/km. There is also provided an apparatus for generating light with a changed spectrum, wherein optical pulses are introduced into one end of the nonlinear optical fiber and light with a changed spectrum is generated from the other one end of the nonlinear optical fiber, the nonlinear optical fiber is formed of glass containing Bi<SB>2</SB>O<SB>3</SB>at ≥40 mol%, and the group velocity dispersion thereof is ≤-10 ps/nm/km. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method and a spectrum change light generating apparatus for generating light having a spectrum changed from incident light (hereinafter sometimes referred to as spectrum change light) using a nonlinear optical fiber having normal dispersion, and in particular, a C band spectrum. It is suitable for generating change light in a compact and highly efficient manner.

When strong light is incident on a non-linear medium, a new frequency component is generated due to the self-phase modulation effect, and white light is generated. In particular, in an optical fiber, significant white light generation has been reported due to strong light confinement and a long action length (see Non-Patent Document 1).
However, in the generation of white light using a normal silica-based optical fiber, a plurality of nonlinear effects appear to overlap, so that the generated spectrum is intermittent and complicated, which is not practical. In order to solve this problem, a light source that controls the dispersion in the longitudinal direction of the optical fiber so as to efficiently cut out an arbitrary wavelength and generate a smooth spectrum, and gives a frequency shift in accordance with the dispersion in the incident pulse. A method to be used has been proposed (see Patent Document 1).

Japanese Patent Laid-Open No. 10-90737 Japan Journal of Applied Physics Vol. 40 (2001) pp. L365

  However, a fiber whose dispersion is controlled in the longitudinal direction of the optical fiber is not generally manufactured, and the dispersion value of the nonlinear fiber is also unstable. In addition, a technique for imparting a frequency shift in an optical pulse of about femtoseconds to picoseconds is a method used for basic research of ultrafast spectroscopy, and is not common as well. In addition, widely used silica-based nonlinear fibers have small optical nonlinearity, and it is necessary to use extremely long fibers in order to obtain a sufficient nonlinear effect. There is a problem of being vulnerable to disturbance.

On the other hand, a fiber made of a material having high nonlinearity such as Bi ₂ O ₃ or lead-containing quartz glass has a large group velocity dispersion, and it has been difficult to obtain a broadband optical nonlinearity. In recent years, with the development of optical communication technology, the optical technology in the C band of 1530 to 1565 nm has increased in maturity. However, as described above, it is difficult to obtain a broadband ultrashort pulse light source capable of efficiently converting energy within the C band using a conventional nonlinear optical fiber and general-purpose technology.
All of the white light described above is spectrum change light, but the present invention aims to provide an optical spectrum change method and a spectrum change light generator capable of solving such problems.

The present invention provides a method for generating light incident optical pulse to one end of the nonlinear optical fiber spectrum from the other end has changed, from glass nonlinear optical fiber contains Bi ₂ O ₃ more than 40 mol% And providing a method of changing an optical spectrum whose group velocity dispersion is -10 ps / nm / km or less.
Further, it is an apparatus for generating light having a spectrum changed from the other end by entering a light pulse at one end of the nonlinear optical fiber, and the optical fiber is made of glass containing 40 mol% or more of Bi ₂ O ₃ , Provided is a spectrally changing light generator having a group velocity dispersion of -10 ps / nm / km or less.

  Efficient flat spectrum that can operate stably with a simple configuration using a general-purpose femtosecond laser as a light source and a highly nonlinear optical fiber that combines high nonlinearity and small group velocity dispersion compared to conventional nonlinear optical fibers. It is possible to obtain infrared white femtosecond pulsed light having

FIG. 1 is a block diagram for explaining the present invention. Note that the present invention is not limited to FIG.
In the spectrum change light generator of FIG. 1, a femtosecond pulse laser, an erbium-doped fiber amplifier (EDFA), and a nonlinear optical fiber are sequentially connected.
The femtosecond pulse laser may be of a general purpose, and the center wavelength of an optical pulse emitted therefrom is typically 1550 nm to 1560 nm, the pulse width is 100 fs to 1 ps, and the peak power is typically 10 W to 1000 W.
The optical pulse is normally transmitted after being attenuated by an attenuator (not shown) in order to suppress the generation of nonlinear effects in the transmission path.

The optical pulse is combined by pump light having a wavelength of 980 nm emitted from a pump light source (not shown) and a coupler (not shown), and enters the EDFA.
The net average power of the light pulse incident on the nonlinear optical fiber is about 10 mW to 10 W. If it exceeds 10 W, the pulse quality may be degraded and the spectrum shape may be destroyed due to nonlinear effects during transmission or stimulated scattering effects due to acoustic phonons. The peak power of the light pulse incident on the nonlinear optical fiber can be adjusted by changing the pumping light input value to the EDFA.

  The light pulse amplified by the EDFA is incident on the nonlinear optical fiber, and the spectrum width is expanded and converted into spectrum change light. The amount of expansion of the spectrum width strongly depends on the input peak power of the optical pulse. When the spectrum width is expressed by 10 dB width (Δλ), Δλ is 50 nm when the peak power is 50 W, and 140 nm is typical when it is 160 W. is there. Here, the 10 dB width is the width of the 10 dB wavelength range, that is, the wavelength range where the 10 dB intensity is weaker than the maximum value of the spectrum intensity.

  When the center wavelength of the incident light pulse is 1550 nm to 1560 nm, the pulse width is 100 fs to 1 ps, and the peak power is 10 W to 1000 W, the 10 dB wavelength range of the spectrum change light is wider than 1530 nm to 1565 nm, and 1530 nm in the spectrum change light. The power of light in the wavelength range of ˜1565 nm is typically 50% or more of the spectral change light power.

Next, a non-linear optical fiber (hereinafter sometimes simply referred to as an optical fiber) in the present invention will be described.
The optical fiber is made of glass containing 40 mol% or more of Bi ₂ O ₃ . Otherwise, the optical nonlinearity will be insufficient and the spectrum of the spectrum change light may not have a sufficient width, or the light propagation will be unstable due to modulation instability without proper normal dispersion. As a result, the spectral width of the spectrum-changing light is significantly widened, so that the wavelength conversion into the C band may be inefficient.

As such glass, Bi ₂ O ₃ 40 to 75%, B ₂ O ₃ 12 to 45%, Ga ₂ O ₃ 1 to 20%, In ₂ O ₃ 1 to 20 in terms of mol% based on the following oxide standards. %, ZnO 0-20%, BaO 0-15%, SiO ₂ + Al ₂ O ₃ + GeO ₂ 0-15%, MgO + CaO + SrO 0-15%, SnO ₂ + TeO ₂ + TiO ₂ + ZrO ₂ + Ta ₂ O ₅ + Y ₂ O ₃ + WO Examples thereof include glass consisting essentially of _{30 to} 10% and CeO ₂ 0 to 5%, and Ga ₂ O ₃ + In ₂ O ₃ + ZnO of 5% or more.

  When the group velocity dispersion (GVD) of the optical fiber exceeds -10 ps / nm / km, the spectrum width is remarkably enlarged, and the proportion of energy remaining in the C band (wavelength range: 1530 to 1565 nm) is reduced, and the smoothness of the spectrum is impaired. Or the propagation of femtosecond pulsed light may become unstable. Preferably, it is −20 ps / nm / km or less. GVD is typically -50 ps / nm / km or more.

The nonlinear coefficient γ for light having a wavelength of 1550 nm of the optical fiber is preferably 470 W ⁻¹ km ⁻¹ or more. If it is less than 470 W ⁻¹ km ⁻¹ , the fiber length becomes longer when attempting to increase the non-linear effect, and it becomes more susceptible to disturbances such as temperature fluctuations and vibrations. More preferably, it is 625 W ⁻¹ km ⁻¹ or more.

An optical fiber is typically a holey fiber, for example, an air-clad fiber whose cross section is conceptually shown in FIG.
The air-clad optical fiber 10 includes six holes 11, an optical transmission glass 12, a hollow glass fiber 13, and a plate glass 14.
The hollow portion of the hollow glass fiber 13 is composed of six holes 11 extending in the axial direction (perpendicular to the paper surface), and adjacent holes are partitioned by a sheet glass 14 existing between them. .

  The number of holes 11 is not limited to six, but is preferably three or more. With two, there is a risk that light confinement in the air-clad optical fiber will be insufficient. Further, the number is preferably 12 or less, more preferably 9 or less.

The light transmission glass 12 may be made of one kind of glass, or may be made of two or more kinds of glasses whose boundaries in the cross section are concentric.
In the former case, the optical transmission glass 12 is the core of the optical fiber 10 itself.
Examples of the latter case include those having a portion with a higher refractive index at the center, such as the one in which the light transmission glass 12 is composed of an internal high refractive index glass and a low refractive index glass surrounding it. By adjusting the γ and GVD by using the optical transmission glass 12 having such a structure, or when the optical fiber and the silica fiber are fused and connected, the waveguide structure disappears or the connection loss increases. This can be prevented or suppressed.

  The hollow glass fiber 13 holds the light transmission glass 12 through the plate glass 14 in the center of the hollow portion formed by the air holes 11, and light is not expected to propagate through the glass. .

  The plate-like glass 14 holds the light transmission glass 12 at the center of the hollow portion, and the thickness thereof is preferably 0.05 to 1.5 μm. If the thickness is less than 0.05 μm, when the optical fiber 10 is cut, the plate-like glass 14 may be damaged and the optical transmission glass 12 may not be held. Typically, it is 0.1 μm or more. If it exceeds 1.5 μm, light leakage from the light transmission glass 12 to the plate glass 14 becomes large, and there is a possibility that light confinement becomes insufficient. Preferably it is 0.5 micrometer or less.

  The air holes 11 are defined by the light transmission glass 12, the hollow glass fiber 13, and the plate glass 14, but at least a portion in contact with the air holes 11 in the light transmission glass 12, a portion in contact with the air holes 11 in the hollow glass fiber 13, and The plate-like glass 14 is preferably made of glass having the same composition, or made of glass having the same composition.

The plate-like glass 14 is preferably the exemplified glass, and the same applies to the light transmission glass 12.
If the optical transmission glass 12 is not such a glass, the thermal properties between the glasses are different, so that stable molding may be difficult.

The diameter (d) of the inscribed circle in the cross section of the light transmission glass 12 is usually 0.2 to 10 μm, and typically 0.5 to 4 μm.
The diameter (d ′) of the circumscribed circle in the cross section of the hollow portion of the hollow glass fiber 13 is preferably (1 + 2 ^1/2 ) d or more. If it is less than (1 + 2 ^1/2 ) d, the light is not sufficiently confined and the propagation loss may increase. More preferably, it is 3d or more, Most preferably, it is 4d or more. On the other hand, d ′ is preferably 16 d or less. If it exceeds 16 d, problems such as the strength of the optical fiber 10 being reduced, foreign matters are likely to be mixed into the holes 11, and the glass sheet 14 may be destroyed when attempting to cut the optical fiber 10 are generated. Is concerned.
The outer diameter of the hollow glass fiber 13 is the same as that of the ITU-T Recommendation G. When fused with a quartz optical fiber (SMF) standardized by 652, it is preferably 125 ± 2 μm.

Example 1
The composition expressed in terms of mol% is Bi ₂ O ₃ 53.23%, B ₂ O ₃ 27.61%, Ga ₂ O ₃ 8.96%, In ₂ O ₃ 1%, ZnO 4.48%, BaO 4. The raw materials were prepared and mixed so that a glass of 23% and CeO ₂ 0.5% was obtained, and 250 g of the prepared raw material was produced. This mixed raw material is put into a platinum crucible and melted by holding at 1000 ° C. for 2 hours in an air atmosphere. The obtained molten glass is poured into a plate shape, and then kept at 370 ° C. for 4 hours and then cooled to room temperature. went.

  A glass plate having a thickness of 1 mm and a size of 20 mm × 20 mm was prepared from the glass thus obtained, and the refractive index for light having a wavelength of 1550 nm was measured for a sample plate obtained by mirror-polishing both surfaces of the model plate. It was 2.111 when measured using a 2010 prism coupler.

Also, a sample block in which a right-angled triangular prism having a thickness of 10 mm with an oblique side of 40 mm, a short side of 20 mm, and an angle between the oblique side and the short side of 60 ° is manufactured from the glass, and the oblique side and the long side are mirror-polished. The glass material dispersion D _m (unit: ps / nm / km) was calculated as follows. That is, the refractive index n _λ at the wavelength (λ) of 492 to 1710 nm of the sample block was determined by the minimum deviation method using a precision refractive index measuring device GMR-1 manufactured by Kalnew Optical Industry Co., Ltd. This n _λ was fitted to the Selmeier polynomial in the equation (1), and the fitting parameters p ₁ , p ₂ , p ₃ and p ₄ were determined.
_nλ ² = p ₁ + p ₂ · λ ² / (λ ² −p ₃ ) + p ₄ · λ ² (1)
When D _m at a wavelength of 1550 nm was calculated from the equation (2) using n _λ represented by the equation (1), it was −170 ps / nm / km.
D _m = −10 ¹⁵ (λ / c) · d ² n _λ / dλ ² (2).

The molten glass obtained in the same manner as described above is poured into a SUS310S tea tube mold (cylindrical mold having a bottom surface) having an inner diameter of 28 mm and a height of 120 mm, and slowly cooled to obtain a glass rod. It was.
Six through holes with an inner diameter of 4 mm were formed on this glass rod using an ultrasonic machine USM-3CNC manufactured by Prosonic Corporation. The central axis of these six holes was 5 mm away from the central axis of the glass rod, and the interval between adjacent holes was 1 mm.

Using this glass rod, the air-clad nonlinear optical fiber was produced as follows. First, a rod glass having six holes formed, a diameter of 28 mm, and a length of 130 mm was redrawn at 418 ° C. to obtain a glass rod having a diameter of 3.5 mm. Next, one end of the glass rod was sealed, put in a glass tube having an outer diameter of 15 mm and an inner diameter of 6 mm with the sealing portion down, and then the lower end of the glass tube was sealed.
The space between the glass rod and the glass tube is depressurized at −60 kPa, and the glass rod and the glass tube are simultaneously heated by heating to 425 ° C. while the six holes of the glass rod are pressurized and expanded at 30 to 40 kPa. Redraw was performed to obtain a preform having a diameter of 5 mm.

The preform was drawn under the conditions of a drawing temperature of 425 ° C. and a drawing speed of 6 mm / min while pressurizing the hole at 5 kPa, d was 2.8 μm, d ′ was 17.3 μm, fiber diameter was 125 μm, the plate A nonlinear optical fiber having a glass-like thickness of 0.25 μm was obtained (hereinafter referred to as optical fiber A).
A scanning electron microscope (SEM) photograph of a cross section of the optical fiber A is shown in FIG. The inserted photograph is an enlarged photograph of the hollow portion.

When the GVD of the optical fiber A was measured using the homodyne interferometry, the absolute value was 20 ± 10 ps / nm / km. By having the hollow portion, the GVD could be made significantly smaller than the material dispersion Dm calculated by the above formula (1).
The nonlinear constant γ calculated for the optical fiber A by four-wave mixing was 700 ± 70 W ⁻¹ km ⁻¹ .

In addition, spectrum changing light was generated as follows using the optical fiber A having a length of 46 cm.
The output femtosecond optical pulse is output to an IMRA fiber laser (trade name: femtolite) that outputs a femtosecond optical pulse having a center wavelength of 1560 nm, a pulse width of 500 fs, an average output of 4 mW, and a repetition frequency of 48 MHz. An attenuator for attenuating 15 dB was connected.

A separately prepared pump light source (light output: 200 mW) for generating pump light of 980 nm and the attenuator were connected to a coupler, and the femtosecond optical pulse and the pump light were combined by the coupler.
The coupler was connected to an EDFA and femtosecond light pulses were amplified up to 250W.

  The amplified femtosecond optical pulse was incident on the optical fiber A, but the optical power was partially lost during transmission due to connection loss or the like, so that the maximum of the incident femtosecond optical pulse was 120 W. .

  The spectrum of the femtosecond optical pulse output from the optical fiber A was measured with an optical spectrum analyzer AQ6317 manufactured by Yokogawa Electric Corporation. FIG. 4 shows the spectrum measurement results. Further, the time waveform was measured with an autocorrelator HAC150 manufactured by Arnea.

The 10 dB wavelength range of the femtosecond optical pulse output from the optical fiber A was 1495 to 1615 nm, and the 10 dB width was 120 nm. The spectral spread of the femtosecond light pulse incident on the optical fiber A was 13 nm with a 10 dB width.
Further, no change was observed in the time waveform of the femtosecond light pulse incident on the optical fiber A and the femtosecond light pulse output from the optical fiber A, and a high-quality infrared white femtosecond pulse could be obtained.
In addition, 45% of the total energy in the femtosecond optical pulse output from the optical fiber A was present in the C band (hereinafter, the ratio of the energy of light in the C band in the output femtosecond optical pulse is expressed as follows. Called wavelength conversion efficiency).

When the output of the excitation light source is lowered, the spectrum of the femtosecond pulse output from the optical fiber A becomes narrower, but the wavelength conversion efficiency increases.
When searching for a condition where the wavelength conversion efficiency is maximized under a condition where the 10 dB wavelength range is wider than the C band, the wavelength conversion efficiency is obtained when the peak power of the femtosecond light pulse incident on the optical fiber A is 50 W. The maximum was 61%. FIG. 4 shows a spectrum under this condition.
Table 1 shows the relationship between the peak power (unit: W) of the femtosecond optical pulse incident on the optical fiber A and the wavelength conversion efficiency (unit:%). When the peak power is 83 to 50 W, the 10 dB wavelength range is wider than the C band, and at 25 to 42 W, the 10 dB wavelength range stays within the C band. In this example, the peak power capable of generating a highly efficient infrared white femtosecond pulse (spectrum changing light) in which the optical power in the C band is 50% or more of the total optical power was 50 to 67 W.

(Example 2)
The numerical calculation according to the present invention was performed as described below for a case where femtosecond pulsed light having a wavelength of 1550 nm, which is near the center of the C band, was incident.
That is, a numerical calculation of the nonlinear Schrodinger equation describing the time evolution of pulsed light propagating in the nonlinear optical fiber was performed using the split step Fourier method. Here, the non-linear Schrodinger equation means G. P. This is the formula (2.3.35) described by Agrawar Nonlinear Fiber Optics Original Edition 2nd edition published by Yoshioka Shoten.
FIG. 5 shows a flowchart of the calculation program.

The measurement result of Example 1 was calculated under the conditions of GVD = −25 ps / nm / km, γ = 800 W ⁻¹ km ⁻¹ , propagation loss 2 dB / m, incident pulse center wavelength 1560 nm, and peak power 120 W. As shown in FIG. 6 together with the results, the calculation results reproduce the actual measurement results well.

Wavelength conversion efficiency (ratio of energy contained in C band) when a femtosecond optical pulse having a center wavelength of 1550 nm and a pulse width of 500 fs is incident on an optical fiber having a length of 50 cm and γ of 800 W ⁻¹ km ⁻¹ The calculation results are shown in FIG.
When the peak power is 40 W or less, the wavelength conversion efficiency is 50% or more. However, at 20 W or less, the 10 dB wavelength range does not reach the entire C band. Therefore, in this embodiment, the peak power of the incident femtosecond pulse light that generates a highly efficient white femtosecond light pulse is 20 W to 40 W.

(Comparative example)
A bismuth oxide glass having a GVD of −280 ps / nm / km and a γ of 1100 W ⁻¹ km ⁻¹ is prepared. When generated, the 10 dB wavelength range was 1531 to 1582 nm and did not cover the C band. Further, although the pulse width of the spectrum change light has increased to 3 ps or more, such an increase in the pulse width is not preferable for high-speed optical communication.

  It can be used for broadband ultrashort pulse light sources such as wavelength multiplexing optical communication, medical observation equipment (such as coherent tomography), ultrafast spectroscopy, and infrared spectroscopy.

It is a block diagram for demonstrating this invention. It is an example of the conceptual diagram of the cross section of the holey fiber used by this invention. It is an example of the scanning electron microscope (SEM) photograph of the cross section of the holey fiber used by this invention. It is a figure which shows the spectrum of the femtosecond optical pulse output from the nonlinear optical fiber. It is a figure which shows the flowchart of a calculation program. It is a figure which shows the calculation result and measurement result of the spectrum of the femtosecond optical pulse output from the nonlinear optical fiber. It is a figure which shows the calculation result of the ratio of the energy contained in C band in the femtosecond optical pulse output from the nonlinear optical fiber.

Explanation of symbols

10: Air-clad optical fiber (Holy fiber)
11: Hole 12: Optical transmission glass 13: Hollow glass fiber 14: Sheet glass

Claims (8)

A method in which an optical pulse is incident on one end of a nonlinear optical fiber to generate light having a spectrum changed from the other end, wherein the nonlinear optical fiber is made of glass containing 40 mol% or more of Bi ₂ O ₃ , and the group An optical spectrum changing method in which velocity dispersion is −10 ps / nm / km or less.   The method of changing an optical spectrum according to claim 1, wherein the group velocity dispersion is -50 ps / nm / km or more. 3. The optical spectrum changing method according to claim 1, wherein a third-order nonlinear coefficient for light having a wavelength of 1550 nm of the nonlinear optical fiber is 470 W ⁻¹ km ⁻¹ or more.   4. The method of changing an optical spectrum according to claim 1, wherein the nonlinear optical fiber is a holey fiber. Glass containing the _Bi 2 _{O 3} more than 40 mol%, in mol% based on the following _{oxides, Bi 2 O 3 40~75%,} B 2 O 3 12~45%, Ga 2 O 3 1~20 %, In ₂ O ₃ 1-20%, ZnO 0-20%, BaO 0-15%, SiO ₂ + Al ₂ O ₃ + GeO ₂ 0-15%, MgO + CaO + SrO 0-15%, SnO ₂ + TeO ₂ + TiO ₂ + ZrO ₂ Any one of claims 1 to 4, which consists essentially of + Ta ₂ O ₅ + Y ₂ O ₃ + WO ₃ 0 to 10% and CeO ₂ 0 to 5%, and Ga ₂ O ₃ + In ₂ O ₃ + ZnO is 5% or more. Of optical spectrum change. A device for generating light having a spectrum changed from one end of a nonlinear optical fiber by entering a light pulse, wherein the optical fiber is made of glass containing 40 mol% or more of Bi ₂ O _{3 and} has a group velocity. Spectral light generator with a dispersion of -10 ps / nm / km or less.   7. The spectrum change light according to claim 6, wherein the optical pulse has a wavelength of 1550 nm to 1560 nm, a pulse width of 100 fs to 1 ps, a peak power of 10 W to 1000 W, and a femtosecond pulse laser capable of generating such an optical pulse. Generator.   8. The spectrum change light generator according to claim 7, further comprising an erbium-doped fiber amplifier connected between the femtosecond pulse laser and the optical fiber.

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