JPH11261136A - Optical pulse generating element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To directly obtain laser light of 1.5 μm band by means of an optical pulse generating element which incorporates a solid-state laser medium and a saturable absorber. SOLUTION: A glass laser medium 33 containing erbium and ytterbium is used as a solid-state laser medium, and a uranium-doped calcium fluoride plate is used as a saturable absorber 37. The laser medium 33 and absorber 37 are brought into contact with each other through an intermediate film 35 which reflects part of stimulating light 50 to the glass laser medium 33 side and transmits the remainder to the absorber 37 side. An input mirror 31 is provided on the laser medium 33 side, and an output mirror 39 is provided on the absorber 37 side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pulse generating element that can be used in the fields of optical communication, optical measurement, and the like.

[0002]

2. Description of the Related Art For example, reference 1 (OPICS Letters, Sept. 15.1994, Vol. 19, No. 18, pp. 142)
7-1429) has an input mirror 11 as shown in FIG.
An optical pulse generating element 19 including a solid-state laser medium 13, a saturable absorber 15, and an output mirror 17 in this order is disclosed.

In this optical pulse generating element 19, neodymium (Nd) -doped Y is used as the solid-state laser medium 13.
An AG (Y ₃ Al ₅ O ₁₂ ) crystal is used, and a chromium (Cr) -doped YAG crystal is used as the saturable absorber 15. Input mirror 11 and output mirror 17
Constitutes a laser resonator, as is well known.

In the light pulse generating element 19, light pulses are periodically generated based on a well-known principle utilizing the excitation by the excitation light and the saturable absorption phenomenon by the saturable absorber 15. Briefly, it is as follows.

[0005] A wavelength 8 is applied to the element 19 from the input mirror 11 side.
08 nm excitation light 21 is incident. Solid laser medium 1
3 is excited by the excitation light 21, so that a laser beam having a wavelength of 1064 nm is generated in the solid-state laser medium 13. When the absorption of the laser light in the saturable absorber 15 is saturated, a part of the laser light is output to the outside of the element 19 from the output mirror 17 side. When the laser light is output outside the device,
The laser light disappears because the laser gain of the solid-state laser medium decreases, but the laser light is generated again in the solid-state laser medium 13 when the excitation by the excitation light is continued. When the absorption of the laser light in the saturable absorber 15 is saturated, a part of the laser light is output to the outside of the element from the output mirror side.
By repeating such a phenomenon, the element 19
Periodically outputs an optical pulse having a wavelength of 1064 nm.
That is, in the element 19, so-called passive Q-switch oscillation occurs.

[0006]

However, Nd ions, which are active ions in a Nd-doped YAG (Nd: YAG) crystal, are most advantageous for optical transmission using a quartz optical fiber and have an allowable irradiation density on the eyeball. Does not have an appropriate transition in the 1.5 μm band where Therefore, the above-mentioned conventional optical pulse generating element 19 has a problem that it cannot generate a laser pulse having a wavelength of 1.5 μm by itself.

Therefore, the conventional optical pulse generating element 19
In order to use the laser light, which is extremely useful in the application of laser light, to optical communication using optical fibers, and to applications requiring highly safe laser light to the eyeball, the generated laser light must be used. Further, it is necessary to further use a nonlinear crystal or an external nonlinear crystal resonator for wavelength conversion. However, in such a case, a new problem such as an increase in the number of parts and cost is caused.

[0008] The present invention has been made in view of the above points, and an object of the present invention is to provide an optical pulse generator using a solid-state laser medium and a saturable absorber,
An object of the present invention is to provide an optical pulse generating element capable of directly oscillating an optical pulse in a wavelength band of 1.5 μm.

[0009]

Therefore, according to the optical pulse generating device of the present invention, an input mirror, a solid laser medium, a saturable absorber and an output mirror are provided in this order, and excitation light is irradiated from the input mirror side. In the light pulse generating element used, the solid-state laser medium includes a glass laser medium doped with Er ions or a YLF crystal doped with Yb ions and Tm ions. An intermediate film is provided between the solid-state laser medium and the saturable absorber to reflect a part of the excitation light toward the solid-state laser medium and transmit the rest to the saturable absorber. It is characterized by.

A glass laser medium doped with Er ions generates laser light in a wavelength band of 1.5 μm. In particular, this glass laser medium is, for example, at a wavelength of 980 nm or 1
When excited by 480 nm excitation light, 1.
A laser beam of 5 μm band is generated. In addition, YLF crystals doped with Yb ions and Tm ions also have wavelengths of 1500 nm and 1569 when excited with 970 nm laser light.
It is known to emit laser light of nm (for example, Reference 2: CLEO '97, 1997, Technical Digest Series vo
l.11, Conference Edition CTuE3, p.75).

Therefore, according to the present invention, it is possible to realize an optical pulse generating element that generates an optical pulse in the 1.5 μm band in combination with the supersaturation absorption phenomenon.

On the other hand, it is known that the laser-induced emission cross section of a glass laser medium doped with Er ions is at least one order of magnitude smaller than that of Nd: YAG (for example, Reference 3: Solid State Laser Engineering). Springe-V
erlag, 4th Edition p.49 (1996) and Reference 4: SPIE vo
l.1419 Eyesafe Lasers: Components, Systems, and Appli
cations (1991)).

It is also known that a laser glass medium has only about one-eighth of a thermal shock parameter indicating a limit at which a laser medium is destroyed by heat generated at a high excitation density (as compared to YAG). For example, Reference 5: Soli
d State Laser Engineering "Springe-Verlag, 4th Editi
on p.398 (1996)).

The Mohs hardness of the YLF crystal itself doped with Yb ions and Tm ions is about 4, which is only about half that of a YAG crystal having the same hardness of 8.5.

Therefore, a glass laser medium doped with Er, a YLF doped with Yb ions and Tm ions,
Crystal (hereinafter sometimes collectively referred to as a solid-state laser medium)
In this case, if the excitation light is infinitely increased in order to efficiently generate a light pulse, the solid laser medium is destroyed by the action of heat generated in the solid laser medium.

Therefore, in order to realize an optical pulse generation element that efficiently and stably generates an optical pulse using the solid-state laser medium and the saturable absorber, the solid-state laser is efficiently used by using the pump light. It is necessary not only to improve the excitation efficiency of the medium, but also to devise such that oscillation due to saturable absorption occurs at a wide range of excitation light intensities. That is, it is important to increase the difference between the excitation density corresponding to the thermal breakdown limit of the solid-state laser medium and the excitation density corresponding to the threshold value of laser oscillation at which saturable absorption starts. In other words, the optical pulse generation element can operate between the excitation density whose upper limit is determined by the thermal breakdown limit and the threshold excitation density at which pulsed light generation starts, realizing a practical and stable optical pulse element. In order to achieve this, it is necessary to make the difference between the upper limit excitation density and the threshold excitation density as large as possible, that is, to make the operation area as large as possible.

In such a case, the optical pulse generating element of the present invention includes an intermediate film between the solid-state laser medium and the saturable absorber for branching and reflecting and transmitting the excitation light. The intermediate film reflects part of the excitation light that has not been completely absorbed by the laser beam and causes the solid-state laser medium to absorb the excitation light, so that a high-density excitation light is absorbed in the solid-state laser medium. Excitation light transmitted through the intermediate film is applied to the saturable absorber at a suitable intensity.

Here, when a calcium fluoride (U: CaF ₂ ) crystal doped with uranium is used as the saturable absorber, the calcium fluoride crystal generates excitation light of 1.5 μm generated by a solid-state laser medium. It absorbs to the same extent as light (for details, see the transmittance characteristic (FIG. 5) described in a later example). Accordingly, the saturable absorber composed of uranium-doped calcium fluoride absorbs the excitation light transmitted through the intermediate film, and the saturable absorber is promoted in saturable absorption. Therefore, the saturable absorber is easily saturated, and the minimum required excitation intensity at which laser oscillation occurs is reduced. As a result, an optical pulse generating element that efficiently and stably generates optical pulses is realized.

It should be noted that, for example, in Japanese Unexamined Patent Application Publication No. 8-316557, which was published before the filing of the present application, particularly, FIG. It is described that in a device similar to the pulse generating device, a layer serving as a fixed layer may be provided between the YAG 13 as a solid-state laser medium and the saturable absorber 15. However, this fixed layer is described as an adhesive layer for connecting the saturable absorber to the active laser medium, and is different from the intermediate layer in the present invention.

By the way, the glass laser medium has a thermal conductivity that is about one tenth smaller than that of YAG (for example, Reference 5: “Dictionary of Optical Terms”, edited by Ryuichi Hioki, published by Ohmsha, 1981, p. 56). .269, right column, first row). Therefore, when an optical pulse generation element is realized using a glass laser medium and a saturable absorption medium, the optical pulse generation element may be destroyed by heat generated due to its own laser oscillation unless some measures are taken.

In order to avoid this, when practicing the invention of the optical pulse generating element, it is preferable to provide a heat radiating portion for radiating heat generated in the optical pulse generating element at least on the input mirror side. .

Since the heat generated by the light pulse generating element is radiated by the heat radiating portion, the possibility that the light pulse generating element is broken by heat can be reduced.

In practicing the invention of the optical pulse generating device, the saturable absorber can be any suitable one. Among them, it is preferable to use calcium fluoride doped with uranium. Specifically, it is preferable to use a plate obtained by polishing uranium-doped calcium fluoride to a predetermined thickness. Because, as described above, uranium-doped calcium fluoride has the property of absorbing excitation light to the same extent as laser light emitted from a laser medium, and uranium-doped calcium fluoride has a property of supersaturated absorption. This is because when used as a body, a high-output and stable light pulse can be obtained, as is clear from the experimental results described later.

On the other hand, the thickness of the saturable absorber is preferably thin from the viewpoint of radiating heat generated in the glass laser medium to the outside. Needless to say, the thickness of the saturable absorber is one of the factors that determine the pulse width of the light pulse. Therefore, from the viewpoint of obtaining a light pulse having a short pulse width (this is generally desired).
Thinner is preferred.

However, when the saturable absorber is made of, for example, a crystal exhibiting a saturable absorption phenomenon (for example, but not limited to, uranium-doped calcium fluoride), the crystal is generally polished to produce a saturable absorber. I do. for that reason,
Due to processing technology problems and handling problems, there is a limit to thinning the crystal.

In sum, when using a crystal exhibiting a supersaturation phenomenon as the supersaturated absorber, the thickness should be at most 1 mm, more preferably at most, from the results of the examples described later. The thickness is preferably 200 μm. On the other hand, the lower limit of the thickness is 30 μm, more preferably 5 μm.
The thickness is preferably set to 0 μm.

If the doping ratio of uranium in uranium-doped calcium fluoride is too high, the absorption of laser light in the saturable absorber composed of uranium-doped calcium fluoride will increase too much. Then, in order to avoid this, it becomes necessary to make the saturable absorber thinner. However, in order to make the saturable absorber thinner, there are problems in terms of the processing technique and handling described above. Therefore, the doping ratio of uranium is preferably at most 0.03 at%, more preferably at most 0.01 at%, from the results of the examples described later.

On the other hand, if the uranium doping ratio is too low, it is necessary to ensure the desired absorption of the laser beam, so that it is necessary to increase the thickness of the saturable absorber. Then, the above-mentioned problems (problems such as a decrease in heat dissipation and a reduction in pulse width) caused by increasing the thickness of the saturable absorber occur.
Therefore, it is preferable to determine the lower limit of the uranium doping ratio in consideration of these factors.

The reflectance of the intermediate film to the excitation light is
Higher values contribute to the excitation of the solid-state laser medium, but also increase heat generation in the solid-state laser medium, and, of course, do not lower the excitation light intensity at which saturation of the saturable absorber starts. If the reflectance is too low, the effect of promoting the excitation of the solid-state laser medium cannot be obtained. Therefore, the reflectance is determined in consideration of these factors.
At least, in the experimental results described later, it can be said that the reflectance should be approximately 50% at the maximum. Here, the abbreviation is 50
I think it will be around ± 5%. Further, the lower limit of the reflectance is considered to be, for example, preferably about 20%, more preferably about 30%.

[0030]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an optical pulse generating device according to the present invention will be described below with reference to the drawings. It should be noted that the drawings used in the description merely schematically show the dimensions, shapes, and arrangements of the components so that the present invention can be understood. Further, in each of the drawings, the same components are denoted by the same reference numerals, and the duplicate description thereof may be omitted.

1. First Embodiment FIG. 1A shows an optical pulse generating element 3 according to a first embodiment.
FIG. 4 is a diagram for explaining a structure of a zero. This FIG. 1 (A)
FIG. 3 is a side view of the element 30 as viewed from a direction orthogonal to the optical axis.

The optical pulse generating element 30 according to the first embodiment includes an input mirror 31, a glass laser medium 33, an intermediate film 35, a saturable absorber 37, and an output mirror 39 in this order. Furthermore, this element 30 is
1, and an antireflection film 43 provided on a surface of the heat radiating portion 41 opposite to the input mirror 31. Hereinafter, each of the components 31 to 43 will be described.

The input mirror 31 is an anti-reflection film that allows the excitation light 50 input to the element 30 to be efficiently input to the glass laser medium 33 side without being reflected to the excitation light source (not shown). And a function of a high reflection film that reflects the laser light generated in the glass laser medium 33 to the glass laser medium 33 side. The input mirror 31 can be composed of, for example, a known dielectric multilayer film.

The input mirror 31 may be formed directly on the surface of the glass laser medium 33. Alternatively, the input mirror 31 is formed on another suitable member, and the other member is attached to the glass laser medium 33 such that the surface on which the input mirror 31 is formed is on the glass laser medium 33 side, for example, using a transparent adhesive ( An adhesive that is transparent to the light used.
Same. ). In the first embodiment, the latter example, that is, the heat radiating portion 4 as another member is used.
1 (described later in detail), the input mirror 31 is formed by, for example, a vacuum film forming technique.
Side, and is adhered to the glass laser medium 33.

As the transparent adhesive, for example, LEN
S BOND (Product name. United States SUMMERS LABORATORIES DIV
ISION ENS AQUISITION CORPORATION).

The glass laser medium 33 is typically a known medium in which glass is doped with a rare earth element. Here, since we want to obtain light in the 1.5μ band,
A glass laser medium doped with Er (erbium) ions is used. In particular, in this embodiment, a glass laser medium doped with Er ions and Yb (ytterbium) ions is used. The glass laser medium 33 can have an arbitrary shape such as a square plate or a disk.

The intermediate film 35 is a film that reflects a part of the excitation light 50 toward the glass laser medium 33 and transmits the rest toward the saturable absorber 37. This intermediate film 35 can be composed of, for example, a known dielectric multilayer film.

The intermediate film 35 is preferably formed on the glass laser medium 33 by a known film forming technique. This is because the saturable absorber 37 is often made thin, and when the saturable absorber is made of, for example, calcium fluoride, the saturable absorber 37 itself is relatively brittle. Therefore, it is considered that forming the intermediate film 35 on the glass laser medium 33 is easier than forming the intermediate film 35 on the saturable absorber 37.

The saturable absorber 37 can be made of any suitable material exhibiting a saturable absorption phenomenon.
For example, calcium fluoride doped with uranium, or
Organic dyes such as phthalocyanines can be mentioned as preferred examples of the constituent material of the supersaturated absorber.

When the saturable absorber 37 is made of, for example, uranium-doped calcium fluoride, the uranium-doped calcium fluoride crystal is processed into a predetermined shape and thickness, and the processed sample is used as the glass laser medium 33. Is bonded to the surface provided with the intermediate film 35 by using a transparent adhesive.
When the saturable absorber 37 is made of, for example, an organic dye, the organic dye may be formed on the intermediate film 35 by an arbitrary suitable film forming method such as a vapor deposition method, and may be used as the saturable absorber 37. I can do it.

The output mirror 39 is a mirror that can output a part of the laser light generated in the glass laser medium 33 to the outside and reflects the rest toward the glass laser medium 33 side. The output mirror 39 can be formed of, for example, a known dielectric multilayer film.

The output mirror 39 may be formed on the surface of the saturable absorber 37. Alternatively, the output mirror 39 may be formed on another suitable member, and this member may be bonded to the saturable absorber 37 such that the surface on which the output mirror 39 is formed contacts the saturable absorber 37.

However, when calcium fluoride doped with uranium is used as the saturable absorber 37, for example, the output mirror 39 is not formed on the saturable absorber 37 itself because it is fragile and thinned. Is preferred.

In such a case, the output mirror 39 is formed on another member, and the other member is saturated with a transparent adhesive or the like so that the surface on which the output mirror 39 is formed is in contact with the saturable absorber 37. It is good to adhere to the body 37.

In the case of the embodiment described later with reference to FIG. 1B, a second heat radiating section 45 is prepared as another member, and the output mirror 39 is formed on this member. Then, the second heat radiating section 45 is set so that the surface on which the output mirror 39 is formed is in contact with the saturable absorber 37 so that the saturable absorber 37
2 shows an example of bonding with a transparent adhesive.

The heat radiating section 41 radiates heat generated in the light pulse generating element 30. The heat radiating portion 41 is made of any suitable material that is transparent to the light used and has a high thermal conductivity. For example, a sapphire substrate is cited as an example of a constituent material of the heat radiating section 41.

The anti-reflection film 43 prevents the excitation light 50 from being reflected on the surface of the heat radiating section 41 and
0 is efficiently input to the element 30. The anti-reflection film 43 can be composed of, for example, a known dielectric multilayer film.

The operating principle of the optical pulse generator 30 of the first embodiment is the same as that of the conventional optical pulse generator. However, since a predetermined glass laser medium or a YLF crystal was used as the solid laser medium, and a predetermined intermediate film was provided between the solid laser medium and the saturable absorber,
The optical pulse generating element 30 is an element that directly generates an optical pulse in the 1.5 μm band with a high intensity that has not been achieved in the past (for details, refer to a later embodiment).

2. Second Embodiment In the above-described first embodiment, an example in which the heat radiating portion 41 is provided only on the input mirror 31 side of the optical pulse generating element 30 has been described. However, in order to obtain a higher heat radiation effect, heat radiation parts may be provided on both sides of the optical pulse generating element. The second embodiment is an example.

The light pulse generating element 30 a of the second embodiment has a second heat radiating portion 45 in contact with the output mirror 39. The second heat radiating portion 45 is made of a material that is transparent to light to be used and has high thermal conductivity. For example, a sapphire substrate is an example of a material forming the second heat radiating portion 45.

Similarly to the optical pulse generating element 30 of the first embodiment, the optical pulse generating element 30a of the second embodiment directly outputs an optical pulse in the 1.5 μm band and has an unprecedented high pulse. The element is generated at a high intensity (for details, see a later example).

3. Third Embodiment As a third embodiment, an application example of the optical pulse generation device according to the present invention will be described.

As is apparent from the results of the embodiments described later, the optical pulse generating element of the present invention generates an optical pulse having a wavelength of 1.5 μm and a high output and a small time jitter. That is, even if the optical pulse is input to the quartz optical fiber, an optical pulse having a small loss and a high output and a small time jitter is generated.

Therefore, the optical pulse generating element of the present invention
For example, it can be used as a light source for performing so-called OTDR (optical time domain reflectometry) for inspecting whether there is a defect in the optical fiber. The brief description is as follows.

An optical pulse generated from the optical pulse generating element of the present invention is input to an optical fiber to be inspected. If there is a defect in the middle of the optical fiber, the inputted optical pulse is reflected by the defect and returns. It is monitored whether or not the optical pulse returns, and when the optical pulse returns, it can be determined that a defect exists in the optical fiber to be inspected. Of course, when the light pulse returns, the position where the defect exists can be estimated from the time from when the light pulse is input to when it returns.

Of course, the application of the optical pulse generating element of the present invention is not limited to the above example, but can be used in various fields such as an optical communication field and an optical measurement field.

[0057]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Some embodiments and comparative examples will be described below for better understanding of the optical pulse generating device of the present invention.

First, the system used in the experiment (experimental system) will be described. FIG. 2 is a diagram showing the configuration of this experimental system.

This experimental system includes an excitation light source 61, a condensing optical system 63, and an optical pulse generating element 30 according to the second embodiment.
a and the optical pulse measuring device 65 are provided in this order along the optical axis of the excitation light.

However, an antireflection film 67 is provided on the surface on the outside of the heat radiating portion 45 of the light pulse generating element 30a in order to efficiently output light having a wavelength of 1.54 μm to the outside.

Further, in this embodiment, the excitation light has a wavelength of 98 nm.
The light has a wavelength of 0 nm. Further, a glass laser medium containing erbium and ytterbium is used as the glass laser medium 33.

An excitation light source 61 for obtaining excitation light
OPC-A001-980 manufactured by Optopower Corporation
-Use FC / 100. This light source 61 has a wavelength of 980.
It emits light with a beam diameter of about 100 μm in nm.

The condensing optical system 63 includes an excitation light source 61
Is provided for selectively inputting the excitation light generated from a part of the glass laser medium 33 of the optical pulse generation element 30a. The configuration of the light collecting optical system 63 is not particularly limited.

The light pulse generating element 30a used has the following configuration.

The heat radiating portion 41 is formed of a sapphire substrate.
Then, the input mirror 31 is formed on one surface of the sapphire substrate 41, and the antireflection film 43 is formed on the other surface.

However, the input mirror 31 has a wavelength of 980 n
function as an anti-reflection film for m light, wavelength 1.54μm
A dielectric multilayer film functioning as a highly reflective film for light is used.

Further, as the glass laser medium 33, Kaiger Inc. (USA, containing erbium and ytterbium)
A glass laser medium called QX / ER (trade name) manufactured by KIGRE Inc. is used. However, the thickness is 1mm
And Note that QX / ER is a glass laser medium containing 0.48% by weight of erbium in terms of oxide.

The intermediate film 35 is a dielectric multilayer film that functions as an anti-reflection film for light having a wavelength of 1.54 μm and functions as a partial reflection film for light having a wavelength of 980 nm. Here, a reflective film having a reflectance of 50% with respect to light having a wavelength of 1.54 μm is used. This intermediate film 35 is formed on the glass laser medium 33.

The saturable absorber 37 has a thickness of 200
A calcium fluoride plate having a thickness of μm and containing 0.01 atomic% of uranium is used. The saturable absorber 37 is prepared by polishing a uranium-doped calcium fluoride crystal obtained from Spectragen Inc. (SPECTRAGEN Inc., USA).

The output mirror 39 has a wavelength of 1.54 μm.
The dielectric multilayer film reflects 97% of the m light. However, the output mirror 39 is formed on one surface of the heat radiating portion 45. The other surface of the heat radiating portion 45 has a wavelength of 1.54 μm
An antireflection film 67 for efficiently outputting light to the outside is formed.

The heat radiating section 41 having the input mirror 31 and the antireflection film 43, the glass laser medium 33,
The saturable absorber 37 and the heat radiating portion 45 provided with the output mirror 39 and the anti-reflection film 67 are bonded with a transparent adhesive as shown in FIG.

An excitation light having a wavelength of 980 nm is input to such an optical pulse generating element 30a. However, the excitation light irradiation conditions and the like are as follows.

The center wavelength of the excitation light is 980 nm. The excitation light wavelength width is about 2 nm. Excitation light irradiation diameter is about 120
μm. Excitation light irradiation numerical aperture (irradiation NA) is about 0.1
To 0.12. Excitation light irradiation total power is about 35
0 mW.

As a result, the light pulse generating element 30a
Produced an optical pulse having a repetition frequency of about 10 KHz, a pulse width (FWHM) of 20 nsec or less, and a peak power exceeding 100 W. FIG. 3 is a diagram in which the light pulse output to the display unit of the light pulse measuring device is copied. FIG. 3 shows the light intensity on the vertical axis and the time on the horizontal axis.

The repetition frequency of the generated light pulse was stable at 10 KHz ± 50 Hz or less.
Therefore, it can be seen that an optical pulse generating element having a time jitter as small as 0.5% or less can be realized.

Further, since a single light is emitted in both the horizontal mode and the vertical mode with a peak power of 100 W or more,
It can be seen that a high-output optical pulse generating element, which has never existed before, can be realized in terms of brightness.

Apart from the above experiments, the doping ratio of uranium was 0.01 atomic%, 0.03 atomic% and 0.05 atomic%.
Atomic% calcium fluoride crystal (manufactured by Spectragen)
Prepare each. And each of these crystals,
100 μm, 200 μm, 400 μm and 100 thickness
Each is polished to 0 μm (= 1 mm). That is, a total of 12 types of saturable absorbers having three types of doping rates and four types of thickness are prepared.

Next, using these 12 types of saturable absorbers, optical pulse generating elements are manufactured in the same manner as described with reference to FIG. Then, an optical pulse generation experiment is performed in the same manner as in the above-described embodiment.

As a result, the light pulse generating element having a thickness of 100 μm and a saturable absorber composed of calcium fluoride having a uranium doping rate of 0.01 atomic% is the element (thickness) of the above embodiment. The device emits a light pulse similar to that of the device having a thickness of 200 μm and a uranium doping rate of 0.01 atomic%, and does not emit a light pulse otherwise.

From these results, when the saturable absorber is composed of uranium-doped calcium fluoride, its thickness is preferably 30 μm to 1 mm, more preferably 50 μm to 2 mm.
It is considered that 00 μm is good. The maximum uranium doping ratio is preferably 0.03 atomic%, more preferably 0.1%.
It is considered that 01 atomic% is good.

Next, the device of the embodiment which generated the light pulse,
That is, an optical pulse generating element (hereinafter referred to as an example) in which the reflectance of the intermediate film to excitation light is 50%, and the saturable absorber is a calcium fluoride crystal having a thickness of 100 μm and a uranium doping rate of 0.01 atomic%. Example 1 except that the thickness of the saturable absorber was set to 200 μm.
The relationship between the input power of the excitation light and the peak power of the optical pulse output from the device is examined for an optical pulse generating device having the same configuration as the device (hereinafter, device of Example 2). That is, in the measurement system of FIG. 2, the excitation light power is set to various intensities, and the output pulse peak power at that time is measured.

Further, this measurement was performed in Comparative Examples 1 to 5 as described below.
This is also performed for each element of Comparative Example 3.

(Element of Comparative Example 1) The same as the element of Example 1 except that a film having a state in which the reflectance with respect to the excitation light is not effectively (here, the reflectance is 5% or less) is provided as the intermediate film 35. An optical pulse generating element (element of Comparative Example 1) having such a configuration is prepared.

(Element of Comparative Example 2) The same as the element of Example 2 except that a film having a state in which the reflectance with respect to the excitation light is not practically effective (here, the reflectance is 5% or less) is provided as the intermediate film 35. An optical pulse generating element (element of Comparative Example 2) having such a configuration is prepared.

(Element of Comparative Example 3) A device having the same configuration as that of the element of Example 1 except that a film having a reflectance to excitation light as high as possible (here, the reflectance is 95%) is provided as the intermediate film 35. An optical pulse generating element (element of Comparative Example 3) is prepared.

(Element of Comparative Example 4) A device having the same configuration as that of the element of Example 2 except that a film having a reflectance to excitation light as high as possible (here, the reflectance is 95%) is provided as the intermediate film 35. An optical pulse generating element (element of Comparative Example 4) is prepared.

FIG. 4 shows the relationship between the pump light input power and the output pulse peak power of each of the devices of Examples 1 to 4 of the present invention. However, the characteristics of Comparative Example 4 are not shown in FIG. 4 because no Q-switch oscillation occurred. In FIG. 4, the horizontal axis indicates the input power (m
W), and the vertical axis represents the output pulse peak power (W).

The intensity of the pumping light (oscillation threshold) for starting the oscillation of each element of the first and second embodiments is 23
It is about 0 mW. Further, in each of the elements of the first and second embodiments, the output light intensity sharply increases with the increase of the excitation light intensity, and the oscillation stops after passing the peak value. The reason why the oscillation stops is due to a lens effect that occurs in the element as a precursor to thermal destruction. The peak value of the output light is about 84 W for the device of the first embodiment and about 104 W for the device of the second embodiment. The intensity of the excitation light at which oscillation stops is about 380 mW in the device of Example 1, and 370 mW in the device of Example 2.
It is about. In addition, the range of the excitation light intensity capable of oscillating is about 230 to 380 mW in the device of the first embodiment, and
Is about 240 to 370 mW.

On the other hand, the device of Comparative Example 1 has an oscillation threshold of about 260 mW, which is slightly higher than that of the Example. Also, in the device of Comparative Example 1, even if the intensity of the excitation light is increased, the increase in the intensity of the output light is only about １ ／ of that of the embodiment. Further, the peak value of the output light intensity is also about 18 W, which is about 1/5 as compared with the embodiment. The range in which oscillation is possible is about 260 to 390 mW.

The device of Comparative Example 2 has an oscillation threshold of about 300 mW, which is considerably higher than that of the Example. Further, in the device of Comparative Example 2, even if the intensity of the excitation light is increased, the increase in the intensity of the output light is only about 1/5 of that of the embodiment. Also, the peak value of the output light intensity is about 19 W, which is about 1/5 as compared with the embodiment. The range in which oscillation is possible is about 300 to 380 mW.

The device of Comparative Example 3 has an oscillation threshold of about 250 mW, which is slightly higher than that of the device of Example. In the device of Comparative Example 3, even if the intensity of the excitation light was increased, the increase in the intensity of the output light was only about 40% of that of the embodiment. Although the peak value of the output light intensity is about 130 W, which is higher than that of the embodiment, the range in which oscillation is possible is about 250 to 290 m.
W, which is only about 1/3 of the embodiment.

In the device of Comparative Example 4, no Q-switch oscillation occurred. This is because the saturable absorber cannot be saturated.

As is clear from the descriptions of Examples 1 to 4, when the reflectivity of the intermediate film is very low and when the reflectivity of the intermediate film is very high, the optical pulse which can perform Q-switch oscillation with stable and high peak power is used. It can be seen that the generation element cannot be realized. In other words, it can be said that the preferable value of the reflectance R of the intermediate film with respect to the excitation light is in the range of 5% <R <95%. By optimizing in this way, the peak power is high,
It can be seen that an optical pulse generating element having excellent stability can be obtained. According to the inventor's consideration, the excitation light is 2
It is considered that a film that reflects about 0 to 70%, more preferably about 30 to 60%, and still more preferably about 45 to 55%, and transmits the rest should be used.

Next, the transmittance characteristics of the uranium-doped calcium fluoride crystal will be described.

FIG. 5 shows that the uranium concentration is 0.01 atomic%,
It is the figure which showed the measurement result of the spectral transmittance characteristic of each of the calcium fluoride crystal | crystallization of 0.03 atomic% and 0.05 atomic%. The horizontal axis represents wavelength (nm) and the vertical axis represents transmittance (%). In addition, the thing made from a spectragen company was used for all the calcium fluoride.

A characteristic to be noted in the spectral transmittance characteristic is the excitation light wavelength, regardless of the uranium concentration.
The transmittance near 0 nm and the oscillation wavelength of 1540 nm
The point is that the transmittance in the vicinity is almost the same. this is,
In a uranium-doped calcium fluoride crystal, this means that supersaturation absorption is promoted even by excitation light. Accordingly, since the saturable absorber is easily saturated, the minimum required excitation intensity at which laser oscillation occurs is reduced. As a result, it is considered that an optical pulse generation element that efficiently and stably generates an optical pulse is realized.

In the above, the embodiments and examples of the optical pulse generating element of the present invention have been described. However, the present invention is not limited to the above embodiments and examples, and many modifications are possible. Or, changes can be made.

For example, in the above description, an example was described in which a laser medium containing erbium and ytterbium was used as the solid-state laser medium. However, even when a YLF (YLiF ₄ ) crystal doped with Yb (ytterbium) ions and Tm (thulium) ions is used as the solid-state laser medium, the operation and effect of the intermediate film can be obtained.

[0099]

As is apparent from the above description, according to the optical pulse generator of the present invention, the input mirror, the solid-state laser medium, the saturable absorber, and the output mirror are provided in this order, and the pump light is emitted from the input mirror side. The solid-state laser medium is provided with a predetermined glass laser medium or a predetermined YLF crystal as the solid-state laser medium, and the pump is provided between the solid-state laser medium and the saturable absorber. An intermediate film that reflects a part of the light toward the solid-state laser medium and transmits the remaining light toward the saturable absorber.

Therefore, an optical pulse generator using a solid-state laser medium and a saturable absorber, having a wavelength of 1.5 μm
An optical pulse generating element that can directly oscillate an m-band optical pulse can be realized.

[Brief description of the drawings]

FIG. 1A is a side view illustrating an optical pulse generator according to a first embodiment, and FIG. 1B is a side view illustrating an optical pulse generator according to a second embodiment.

FIG. 2 is an explanatory diagram of an experimental system used in Examples.

FIG. 3 is a diagram simulating an example of a light pulse observed in an example.

FIG. 4 is a diagram showing the relationship between the excitation light intensity and the output pulse peak power for each element when the reflectance of the intermediate film is different.

FIG. 5 is a diagram showing a spectral transmittance characteristic of a uranium-doped calcium fluoride crystal.

FIG. 6 is a diagram illustrating a conventional optical pulse generation element.

[Explanation of symbols]

 30: Light pulse generating element of the first embodiment 30a: Light pulse generating element of the second embodiment 31: Input mirror 33: Glass laser medium 35: Intermediate film 37: Supersaturated absorber 39: Output mirror 41: Heat radiating part 43: Anti-reflection film 45: Heat radiating part (second heat radiating part) 50: Excitation light 61: Excitation light source 63: Condensing optical system 65: Optical pulse measuring device 67: Anti-reflection film

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yuichi Tanaka 1-13-23 Wada, Suginami-ku, Tokyo Co., Ltd. Applied Photonics Laboratory, Inc. (72) Inventor Takao Kobayashi 7-13-13 Bunkyo, Fukui City, Fukui Prefecture No.608

Claims (7)

[Claims] 1. An optical pulse generating element which is provided with an input mirror, a solid laser medium, a saturable absorber, and an output mirror in this order, and is used by irradiating excitation light from the input mirror side, wherein Er is used as the solid laser medium. A glass laser medium doped with (erbium) ions, or a YLF (YLiF ₄ ) crystal doped with Yb (ytterbium) ions and Tm (thulium) ions, wherein: An optical pulse generating element comprising: an intermediate film that reflects a part of the excitation light toward the solid-state laser medium and transmits the remainder toward the saturable absorber. 2. The optical pulse generator according to claim 1, wherein the saturable absorber is a calcium fluoride crystal doped with uranium. 3. The optical pulse generation device according to claim 1, wherein the reflectance R of the intermediate film with respect to the excitation light is 5% <R.
<95%. An optical pulse generating element, characterized by being in the range of <95%. 4. The optical pulse generation device according to claim 1, wherein a reflectance of the intermediate film to the excitation light is approximately 50%. 5. The optical pulse generation device according to claim 1, wherein the thickness of the saturable absorber in the optical axis direction is 30 μm to 1 m.
m, an optical pulse generating element. 6. The optical pulse generating device according to claim 2, wherein the uranium doping ratio is at most 0.03 atomic%. 7. The optical pulse generating device according to claim 1, further comprising a heat radiating unit for radiating heat generated in the optical pulse generating device at least on the input mirror side. .