JPH04150083A - Semiconductor raman laser - Google Patents

Abstract

PURPOSE:To enable a threshold value of an excitation light to be extremely small by including an Al-Ga-P clad layer which surrounds a GaP core layer and a multilayer dielectric film with a high transmission for the excitating light at a core part of an end face at an incidence side. CONSTITUTION:A Gap core layer 11 is surrounded by an Al-Ga-P clad layer 12. Then, a multilayer dielectric film 16 with a high transmission for the excitating light and a high transmission for a first Stokes light is formed by deposition on the GaP core layer 11, the Al-Ga-P clad layer 12 surrounding it, and an end face at an incidence side of a layer part of the core 11. Further, a selective transmission/reflection film 17 is used for an end face at output side. Then, the entered laser beam is reflected by both end faces 16 and 17, thus resulting in laser oscillation. Also, no windows are provided on the incidence end face, thus enabling loss due to the windows for the Stokes light to be excluded and obtaining a high reflectivity so that a threshold value power density can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor Raman laser that can be used for optical communication and optical measurement.

[Prior Art] The present inventors have published Japanese Patent Publication No. 51-17397 and Electronic Technology No.
We have proposed semiconductor Raman lasers in papers such as Vol. 3, No. 102-106 (published in 1965), and based on this technology we have realized a waveguide-type semiconductor Raman laser that can be miniaturized.
terJVol, 51(18), page 1457(19
(published in 1987), rLateral optica
Confinement of the het
erostructure 5eIliconduc
tor Ramanlaser J.

As shown in FIG. 3, this waveguide type semiconductor Raman laser has a GaP core layer 1 and an AlxGa1 layer on a GaP substrate 3.
-0P cladding layer 2 is formed, and GaP auxiliary layer 4 is formed.
is formed on top of these, then a multilayer dielectric reflective film 6 is formed on a part of these, and an excitation light entrance window 8 is provided using lithography technology. Oscillation of Stokes light ω can be obtained by excitation with a laser beam having an arbitrary frequency ω in the range.

Here, the relationship between ω and ω is the optical phonon frequency ω,
Then ω1. The relationship is -ωS+ω, h(1).

Incidentally, a semiconductor Raman laser is an optical frequency selective amplification oscillator that can arbitrarily select the frequency of pumping light and amplify and oscillate an arbitrary frequency ω that satisfies the above relationship. As disclosed in Japanese Patent Publication No. 62-219992, it is a means for optical heterodyne detection of broadband modulated light waves that reach the terahertz band, which cannot be detected directly by current pin photodiodes or avalanche photodiodes.

In order to obtain Stokes light oscillation, it is necessary to inject a laser beam with a frequency of ω into the core part. 6 was vapor-deposited on both end faces, and an excitation light entrance window 8 was opened in a part thereof by lithography technology.This high reflectance dielectric film 6
nfinen+entof, the heter.

5structure semiconductor R
As described in Aman 1aser J, Si
10 to 15 layers of O□ and TiO□ are deposited alternately to a thickness of λ/4 to achieve a reflectance of 90 to 99%.

[Problems to be Solved by the Invention] By the way, laser diodes are ideal as excitation light sources for Raman lasers because they can provide various arbitrary wavelengths.
The light intensity is not high.

Therefore, it is necessary to lower the threshold light intensity of the excitation light, which requires reducing the core cross-sectional area (stripe radius x stripe thickness). That is, the amplification degree within the Raman laser is proportional to the pumping light power density.

However, with lithography technology, it is difficult to make the excitation light entrance window 8 smaller than 5μ x 5μ.
Moreover, for Stokes light, opening such an excitation light entrance window 8 results in a loss;
Even if the reflectance film was deposited so as to have a high reflectance of 70%, the actual reflectance was reduced to about 70% or even lower, so the effect of reducing the core cross-sectional area could not be obtained. There was a problem in that it was not possible to achieve a low threshold of excitation light.

SUMMARY OF THE INVENTION In view of the above-mentioned problems, an object of the present invention is to provide a semiconductor Raman laser that has a simple structure, can be easily manufactured, and can have a very small threshold of excitation light.

(Means for Solving the Problems) According to the present invention, the above object is achieved by providing a GaP core layer, a Ga, -XP cladding layer surrounding the core layer, and at least the core portion of the incident side end face with respect to excitation light. This is achieved by a semiconductor Raman laser including a multilayer dielectric film having a low transmittance and a high reflectance for the first Stokes light.

According to the present invention, a multilayer dielectric film having a high transmittance for the second Stokes light and a high reflectance for the first Stokes light is further formed in the core portion of the output end face. I can do it.

The GaP core layer has a cross section of a size equal to or smaller than the diffraction limit of a beam of excitation laser light incident through a lens or an optical fiber.

[Function] According to the present invention, a selective wavelength shape having high transmittance for the excitation light ω and conversely a high reflectance for the Stokes light ω is provided at least at the incident end face of the excitation light. Since the adhesive film of the lens is formed as a weed, the incident light only needs to irradiate the entire core cross-sectional area, so the core cross-sectional area can be made very small compared to conventional lenses. It also becomes very easy to narrow down the incident beam using a fiber. That is, in the conventional method, the core cross-sectional area is 40
The practical limit was μm×10 μm, but according to the present invention, the core cross-sectional area can be reduced to 1 um×1 pm.

Unlike conventional Raman lasers, there is no need to form a window opening on the excitation light incident end face, so not only does lithography not be required, but the laser is extremely easy to manufacture.

Further, according to the present invention, since there is no loss of Stokes light due to the excitation light entrance window, a high reflectance can be obtained and the threshold light power density can be lowered. According to the conventional Raman laser, the threshold power density is large because an excitation light entrance window is formed, and an incident light intensity of at least ]W or more is required, but according to the present invention, an output of less than IW is required. The optical power density is 400% for the same incident intensity compared to the conventional type mentioned above.
It can even double.

In addition, in the conventional method, there is a multiple internal reflection effect of the incident light, so the incident light actually makes two to three round trips inside the system, whereas in the present invention, it only makes one round trip, so it is actually about 200 times more efficient. However, it is clear that the effect is significant.

〔Example〕

Hereinafter, the present invention will be explained in more detail based on examples.

FIG. 1 shows the structure of the incident end face of the Raman laser 10 of the present invention, and FIG. 2 is a schematic diagram showing the structure of its axial cross section.

In FIGS. 1 and 2, 11 is a GaP core layer; 12 is a GaP core layer;
1 is an Alx Ga1-x P cladding layer, 13 is a GaP substrate, and 14 is a GaP auxiliary layer. 16 is a multilayer dielectric film having high transmittance for excitation light and high reflectance for first Stokes light;
is the GaP core layer 11 and the AlxGa surrounding this core layer 11.
1-A bonding layer is formed on the incident side end face of the 1-A P cladding layer 12 and core 11 portion.

Such a selective transmission reflection film 16 may be constituted by a conventionally known multilayer dielectric film. As described in the "Laser Handbook" (edited by the Laser Society of Japan, published by Ohmsha), for example, two sets of Ti0z and 5ioz multilayer films with a thickness of λ/4 and a center wavelength of λL are formed at a distance of λ from each other. By laminating 15 to 20 layers of laminated structures, it is possible to obtain a transmittance T of 90 to 95% at λ and a reflectance R of 98 to 99% at λ.

Here, in the GaP crystal, the optical phonon frequency is ωph = 12 TH2 in the case of vertical optical phonons, so for example ωL = 2827 + (in the case of z, ω.

=2707Hz, and converting this into a wavelength, λ, =1.
064 μm, λs = 1.112 μm, that is, the difference in wavelength is 48 nm. Therefore, it is easy to obtain a transmittance of 90% or more at λ and a reflectance of 99% at λ.

As a means for inputting light into the Raman laser, condensing light using a lens or direct connection with an optical fiber may be employed.

In the case of focusing by a lens, the beam radius r□7 is
It is stated in general optics books that at the diffraction limit, r□, , >(λ/D)·f.

Here, D is the diameter of the incident parallel beam and f is the focal length of the lens. For example f! : 1lOss + D 2 -, since r□ and 1z are 4 μm, the core cross-sectional area of the Raman laser can be about 84 m×8 μm or less. In addition, if the line point distance of the input lens is made too small,
The spread of the incident angle of the incident beam becomes too large, making it impossible to confine the light within the core.

The relationship between the critical incident angle and the A1 composition required for the gland layer is described by K, 5uto, S, Ogasawa.
ra, T.

Kimura J, rs by NiN15hiza
etaiconductorRaman 1aser
as a tool for wideb
and optical coeu + unicatio
A paper entitled ns4 (IEEE PI?0CEE[1
INGS, Vol. 137. PtJ, No. 1
．． (Page 43, February 1990), in order not to deteriorate the crystallinity, it is necessary to reduce the A1 group composition within this range. In addition, by keeping the cross-sectional area of the core at or below the diffraction limit of the incident beam, the core of the Raman laser is irradiated in a nearly uniform manner, so the excitation light guided within the core is of the lowest order in the lateral direction. This is the most desirable mode for exciting the fundamental mode of Stokes light.

The same applies to the case where the excitation light is input by directly connecting an optical fiber. In the case of nonglumot core ino, the fiber core diameter is several micrometers, so the core cross section of the Raman laser is made to have the same size or smaller, and the matching liquid is connected directly to the end face of the Raman laser and the fiber. By entering the gap between the end faces, the excitation light can be incident with almost no loss. Koaino＼
In the case where a hemispherical lens is attached to the end face of the excitation light, the excitation light is incident in the same way as in the case of incidence through the lens.

Furthermore, by using the selective transmission/reflection film 16 on the incident side end face and at the same time use the selective transmission/reflection film 17 on the output side end face, a new semiconductor Raman laser can be obtained that has effects that cannot be obtained with conventional semiconductor Raman lasers. .

That is, regarding the output side end face, it has a high reflectance for the Stokes light (first Stokes light) ω, 1=ω1−ω, and has a high reflectance for the second Stokes light ω5□−ωL 2ω.
For p6, a dielectric film having high transmittance is used. As a result, the first Stokes light ω5I results in laser oscillation because both end faces have high reflectance. Here, when the reflectance is 98 to 99% on both end faces, even if the photoelectric intensity of the first Stokes light reaches the same level as the intensity of the incident light, the output through both end faces is the same as the number of incident light inputs. %, so there is almost no reduction in the incident power.

This means that the internal intensity of the first Stokes light easily exceeds the intensity of the incident light. As a result, the first Stokes light has the same mechanism as the incident light but with a higher amplification factor, and is ωs2−ωsI−ωph−ω.
Excite the second Stokes light given by −2ω2h.

That is, when the first Stokes light is oscillating, a higher degree of amplification is obtained for the second Stokes light than for the first Stokes light.

Furthermore, if the output side end face is made of a vapor-deposited film that provides high transmittance to the second Stokes beam, the output side end face (meaning the side opposite to the incident side) will be more ω.

2ω2. When signal light in the second Stokes wavelength band is input, the second Stokes light is amplified by the first Stokes oscillation light and is reflected at the input side end face (the side where the excitation light is incident) and returns to the output side end face. This amplified light can be separated and extracted using a Faraday rotator. In other words, it functions as an optical amplifier. With this method, when a semiconductor Raman laser is used as an optical heterogain demodulator, it is possible to obtain the effect that the amplification degree is higher than when using the first Stokes beam, and the isolation against light outside the band is also high.

Further, by adopting such a structure, since the second Stokes oscillation does not occur, there is also the advantage that saturation of the power of the first Stokes oscillation light does not occur.

The present invention will be further explained below using specific examples.

[Specific Example 1] As shown in FIG. 1, the core layer 11 made of GaP has a substantially rectangular cross section with a stripe width of 10 μm and a stripe thickness of 10 μm on the incident side end face of the semiconductor Raman laser body, and A1. The crat layer 12 made of Ga1-0P is XY02.

FIG. 2 shows an axial cross section of the Raman laser body and the incidence of excitation light through the lens 15. The axial length is 4 mm, and the core portion of the incident end face of the Raman laser body is located at the position of the minimum beam diameter by the lens.

The multilayer vapor deposited film 16 on the incident side is composed of about 15 layers, and has a transmittance T=90% for the wavelength band of 1.064 μ and a reflectance R≧98% for the first Stokes light wavelength band of 1.112 μ. This is a narrow band transmission type multilayer vapor deposited film. The deposited film is a λ/4 thick film (L) of SiO□ with a low refractive index and a T film with a high refractive index.
It has a multilayer structure of a film (H) of iO□ with a thickness of λ/4. On the other hand, the vapor deposited film 17 on the output side has the same structure as the conventional one, and has a reflectance of 98 to 99% in the 1.064 to 1.11 μ band.

For the incident laser beam, a CW with a diameter of 2cm.

By using a YAG laser beam and a lens with a corner distance f=10 mm, the effective minimum beam diameter is about 8 μm, which is close to the core cross-sectional area.

In this example, the incident light intensity oscillates at 8001.

The oscillation output is several tens of 11−, and the wavelength is 1.112·μ.

(Specific Example 2) A Raman laser with a core cross section of 3 μm x 3 μm and a length of 4 μm is used, and the dielectric film has a transmission wavelength of 830 n- and a transmittance of 90.
%, and has a reflectance R~98% at 860 nm of the first Stokes light.

As excitation light, GaAlAs laser diode light is introduced into one end of a single-mode fiber with a core diameter of several μm by conventional means, and the other end of the fiber is directly connected to the core of the Raman laser by the method described above.

This results in an incident intensity of approximately 100 mW (wavelength 830 nm).
) oscillation occurs. The output is several Ta- and the wavelength is 860r.
It is +-.

(Specific Example 3) The dielectric film and excitation laser were the same as those in Specific Example 2, except that the core cross section was 2μ×2μ.

A hemispherical lens is connected to the output end face of the single mode fiber, and the core end face of the Raman laser is placed at a spot position with a diffraction limit radius of about 1 μm, and the light is incident thereon.

In this case, oscillation occurs with an incident light intensity of approximately 5()+W. The output is the number 1 and the wavelength is 860n-.

[Specific Example 4] The incident side end face and the incidence method are the same as those in Specific Example 1, and the output side end face is of the same type II structure as the incident side end face, but the center transmission wavelength is changed to the second Stokes light wavelength 1. 164 μm, and has a transmittance T-=90%. in this case,
First Stokes light 1.112 μ and incident light 1.064
For μ, the reflectance is 98-99%.

When a signal light having a wavelength of 1° 164 μm is input from the output side end face, the degree of amplification becomes 10 times. YA as signal light
The second Stokes oscillation light of another Raman laser excited by the G laser is used.

[Specific Example 5] Same as Specific Example 4, but with a wavelength of 1.35 as the excitation light.
A laser diode with μ and an output of 100 mW is used. The first Stokes light wavelength is 1.44μ, and the second Stokes light wavelength is 1.51μ. The signal light is a laser diode light with a wavelength of 1.51μ, and the amplification degree is 10 times as in the case of the fourth example.

[Effects of the Invention] As described above, according to the present invention, a selected wavelength having a high transmittance for excitation light and a high reflectance for Stokes light can be obtained without providing a window on the incident end face. By providing a shaped vapor-deposited film, the loss due to the window for Stokes light is eliminated, and therefore a high reflectance is obtained, making it possible to lower the threshold power density.
This makes it possible to further reduce the core cross-sectional area and to obtain higher optical power density for a given incident intensity.

In addition, if a selective transmission/reflection film is also used on the output side end face, if the first Stokes light has a high transmittance with respect to the second Stokes light, the second Stokes light can be extracted. , which acts as an optical amplifier and can provide high isolation for out-of-band light.
In addition, power saturation of the first Stokes light can also be eliminated, etc.
Various effects are achieved.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view showing the structure of the incident end face of the semiconductor Raman laser of the present invention, and FIG. 2 is a schematic view showing the structure of the axial cross-section. 3V is a schematic cross-sectional view showing the structure of a conventional semiconductor Raman laser. 10... Semiconductor Raman laser; 11... Core layer;
12...Clad layer: 13...GaP substrate; 1
4 auxiliary layers: 15... Lens; 16, 17... Multilayer dielectric film (selective transmission/reflection film). : Disconnection: Nishi: Suto: Patent attorney: Patent attorney: Patent attorney

Claims (3)

[Claims] (1) GaP core layer and Al_xGa_ surrounding the core layer
1_-_xP cladding layer, a multilayer dielectric film having high transmittance for excitation light and high reflectance for first Stokes light at least in the core portion of the incident side end surface;
A semiconductor Raman laser characterized by comprising: (2) A claim characterized in that the core portion of the output side end face has a multilayer dielectric film having a high transmittance for the second Stokes light and a high reflectance for the first Stokes light. The semiconductor Raman laser according to item 1. (3) The semiconductor according to claim 1 or 2, wherein the GaP core layer has a cross section of a size equal to or smaller than the diffraction limit of a beam of excitation laser light incident through a lens or an optical fiber. Raman laser.