JPH0477478B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor Raman laser, and particularly to a semiconductor Raman laser used for optical communication and spectroscopic measurement. In optical communications, optical heterodyne detection is especially used to generate light waves in the far infrared region.
It is used as a means of generating carrier light waves with extremely high wavelength purity and stability.

[Conventional technology]

Semiconductor Raman lasers have been developed by the inventor of the present invention.
This has been realized in GaP single crystals and epitaxial crystals, but the pumping light used has mainly been pulsed operation from YAG lasers and high-power semiconductor lasers. To use it for optical communication, it is necessary to use a low-output semiconductor laser and pump it with continuous wave (CW), and to use it for spectroscopic measurements, it is necessary to use a CW dye laser, which has a power output of 1W to 10mW.
A semiconductor Raman laser that operates with excitation laser light of extremely low power is required. The power density of optically excited laser light is already 1 to 0.7×10 ⁶
It has been confirmed that Raman oscillation occurs at W/cm ² . In principle, this means that if the incident light can be concentrated in a 1μ square area, oscillation can be achieved with a pumping light power of only 10mW. On the other hand, the inventor is an IEE Proceedings
Proceedings) Vol.134, Pt.J, No.．． ４．August
1987, pages 215 to 220, announced a Raman laser with a structure as shown in Figure 1. In Figure 1, 1 is the GaP substrate crystal, 2 is the AlxGa _1-x P first cladding layer, 3 is the GaP Raman active layer, and 4 is the GaP substrate crystal.
The AlxGa _1-x P second cladding layer, 5 and 6 are high reflectance dielectric films, and 7 is incident excitation light. The deposition film on the incident side is formed into a stripe shape using a deposition mask, and the width is about 1 mm, and the resonator length is 3 to 3 mm.
It is 4mm. The excitation light is directed diagonally forward from the edge of the deposited film.
The resonator region formed by 5 and 6 is excited by the incident light as shown in FIG. This amplifies the Stokes light and reduces the refractive index in the vertical direction.
confined by AlxGa _1-x P cladding layer,
Although there is no particular confinement effect in the lateral direction, it oscillates easily because the width of the resonator region is as wide as 1 mm.

[Problem that the invention seeks to solve]

In order for the excitation light to be incident in the method described above, the beam cross section of the excitation light must have a certain width, as shown in Figure 1, that is, a width of 100 μ or more, and it must also oscillate. Since Stokes light also spreads in the lateral direction, it usually requires a pulsed excitation light of 10 W or more, and it has been difficult to achieve oscillation with an excitation light intensity of 1 W or less. The present invention provides strength
The objective is to provide a semiconductor Raman laser that oscillates with an extremely low incident power of about 1W to 10mW. That is, an object of the present invention is to provide a semiconductor Raman laser that oscillates using a CW output injection type semiconductor laser for optical communication or a CW die laser for spectroscopic measurement as an excitation laser.

[Means for solving problems]

As shown in FIGS. 2 to 8 of the embodiment drawings of the present invention, a GaP Raman active layer is formed in a stripe shape with extremely smooth sides, and its cross section is completely surrounded by an AlxGa _1- xP cladding layer. . Furthermore, a sufficiently small hole is provided in the high-reflectance vapor-deposited surface of only a portion of the end surface compared to the end surface of the active layer, and the injection type semiconductor laser light or die laser light is focused by a lens or fiber and enters from the hole portion, that is, the entrance window. do. The active layer and cladding layer have sufficiently low carrier density,
Since there is little absorption and the end face has a high reflectance, the incident excitation light spreads within the active layer and is guided, and is reflected multiple times until it is finally lost through the entrance window. Therefore, the light intensity of the incident light at all parts of the Raman active layer is approximately equal to the light intensity at the entrance window, and Stokes light oscillation can be realized in the entire active layer having a sufficiently large end face compared to the entrance window. The size of the window is
It can be understood from the above that the limit size that can be condensed by the lens is sufficient, and it is sufficient that the threshold power density for oscillation reaches 1×10 ⁶ W/cm ² in this portion. For example, when focusing light with a lens, it is possible to focus light down to 1μφ by using a combination of lenses, and even a simple lens can narrow down the light to 5μφ, so the excitation light intensity is about 8mW for the former and about 20mW for the latter. A semiconductor Raman laser that can oscillate and oscillates when excited by a normal CW output injection type semiconductor laser can be obtained. A portion of the Stokes light to be amplified is lost through the entrance window, but the stripe width is selected with respect to the entrance window and with respect to the laser length so that the proportion of the collimated beam reaching the window due to diffraction is sufficiently small. In other words, although the entrance window is smaller than the end face of the active layer, it still causes loss of Stokes light. In order to reduce this, the first step is to make the stripe width large enough to reduce diffraction loss compared to the size of the entrance window, and to the extent that absorption loss due to multiple reflections of the excitation light inside the active layer does not become a problem. Must be designed small. This diffraction effect depends on the length of the laser. As a further improved method, the active layer stripes may be sloped as shown in FIG. 4 to reduce diffraction loss, or an excitation light introduction layer may be separately provided as shown in FIG. If the refractive index of the Raman active layer cladding layer and the excitation light introducing layer are n ₁ , n ₂ , n ₃ respectively, then if n ₁ > n ₃ > n ₂ , the excitation light will spread to the Raman active layer, but the cladding layer On the other hand, the Stokes light is confined only within the active layer, eliminating diffraction loss to the entrance window.

[Effect]

As mentioned above, the excitation light focused on the entrance window,
Both the Stokes light is confined within the stripe, and the excitation light is subjected to multiple reflections, so that loss due to absorption does not become a problem, and the entire active layer should reach a power density comparable to the power density of the excitation light at the entrance window, and oscillate. The loss of Stokes light from the entrance window is not a substantial problem, and oscillation with extremely low CW excitation light power is possible.

The semiconductor Raman laser thus formed is extremely suitable for optical heterodyne detection and far-infrared light generation, which have already been proposed. Further, since the semiconductor Raman laser of the present invention is entirely composed of semiconductors, including the excitation laser, it has a long life, high reliability, and is compact. Furthermore, the semiconductor Raman laser of the present invention has
Since it is composed of an optical resonator in which the reflection, absorption, and diffraction losses of the resonator are extremely low, that is, the Q is extremely high, the stability of the oscillation frequency is extremely high compared to an injection type semiconductor laser. In other words, the Q value is about 70 times that of a semiconductor injection laser compared to the end face reflectance, and the frequency purity is correspondingly higher. Also, the half-width of Raman scattering is the gain band, but in the case of GaP
When the frequency is about 30GHz and the incident laser is about 1μ,
The corresponding spectral width is on the order of 0.5 Å. In other words, it does not matter if the excitation light has this degree of fluctuation. On the other hand, since the longitudinal mode spacing determined from the cavity length of the semiconductor Raman laser is about 15 GHz (when the length is 4 mm), the longitudinal mode is a single mode.

In addition, since Raman scattering is not an absorption phenomenon, internal heat generation can be ignored, and there is no frequency instability due to heat as in injection lasers. Therefore, it has extremely high frequency stability as a carrier light wave for optical communications, making it ideal for ultra-multiplex communications.

〔Example〕

The present invention will be described in detail below based on embodiments shown in the drawings.

(Example 1) FIG. 2 shows a semiconductor Raman laser showing an example of the present invention. In this figure, 1 is a GaP crystal substrate, 2 is an AlxGa _1-x P first cladding layer, and 3 is a GaP Raman active layer formed in a stripe shape.
4 is AlxGa _1-x covering the top and side surfaces of the Raman active layer
The P second cladding layer and 9 are the GaP buffer layer.
5 is a dielectric high reflection film on the incident side with a reflectance of 99% or more. Reference numeral 8 denotes an excitation light entrance window, which is an extremely small hole made using photoresist technology in a part of the end face of the Raman active layer. 6 is a high reflectance film on the output side, and the reflectance is also a high value of 99%. Reference numeral 7 denotes excitation laser light that is focused by a lens or optical fiber and enters the stripe region from 8.

The length of the Raman active layer is 3 mm to 4 mm, the thickness of each layer is 1 to 5 μm for the AlxGa _1-x P cladding layer, 1 to 10 μm for the GaP Raman active layer, or the stripe width of the GaP Raman active layer is about 100 μm to 10 μm. The window for incident light is formed by removing the dielectric film deposited on the entire end face in stripes as shown in FIG. If the thickness of the GaP Raman active layer is 5 μm and the width of the entrance window is 5 μm, the area of the entrance window is 25 μm ² , and unless the light entering through it is lost due to absorption or residual transmittance of the dielectric, Multiple reflections are repeated until almost the same intensity is reached throughout the active layer. Then, approximately 1×10 ⁶ W/cm ² or 10 mW per 1 μ square
Since this is the threshold power density in the case of GaP, the required pumping light power is 250 mW, which can be pumped with a CW die laser with an output of 1 W or less, or a high-output injection type CW semiconductor laser (wavelength 0.8 to 0.85 μm) with a wavelength of 0.6 to 1.1 μm. can. The stripe width of the active layer at this time varies depending on the carrier density of the active layer. In other words, a semiconductor Raman laser can only oscillate in an extremely transparent medium such as GaP, but even then there is residual absorption that is approximately proportional to the carrier density.
In the case of GaP, in the 1μ band, the absorption coefficient α is approximately α/N0.05cm ⁻¹ /10 ¹⁶ cm ⁻³ where N is the carrier density. Therefore, when the carrier density is 1×10 ¹⁶ cm ^-3 and the resonator length is 4 mm, after about 17 reciprocations, the intensity decreases to about 1/2 due to absorption.
Therefore, the stripe width is approximately 5 μm of the hole width.
If it is set to about 17 times, that is, about 85μ, the intensity of the excitation light inside the active layer will be about the same as the intensity of the excitation light focused at the entrance window. If the stripe width is made larger than this, the light intensity inside the active layer decreases and the incident power must be increased, which is not preferable. Furthermore, if the stripe width is made smaller than this, the conversion efficiency from excitation light to Stokes light will decrease accordingly. Furthermore, the intensity distribution of the Stokes light is as shown in FIG. 3, and decreases on the entrance window side, but since the Stokes light is lost from the entrance window due to diffraction, the narrower the stripe width, the greater the diffraction loss. That is, assuming the stripe width S, wavelength λ, refractive index n, and cavity length l, the spread width δ of the Stokes light towards the entrance window due to diffraction can be approximately estimated by δλ/2nS・l...(1) is possible, S~85μ, λ~1μ,
If n~3.1 and l~4 mm, then δ7.6 μm. That is, this portion reaches the entrance window through diffraction and exits from there, resulting in a loss.

Therefore, this diffraction loss is given by L _D δ/S and is about 9%, which is the loss due to the reflective film L _R and the absorption loss L〓
The sum of about 6% cannot be ignored. The above diffraction loss is an approximation based on Fraunhofer diffraction, and since it is actually Fresnel diffraction, the diffraction loss is reduced to 2 to 3%. however,
As the stripe width becomes narrower, the diffraction loss obviously increases, and it can no longer be ignored and the incident power must be increased.

When the carrier density of the active layer is 2×10 ¹⁶ cm ^-3 , the stripe width of the active layer is 1/2 of the above, that is,
It must be around 40μm or less.

Therefore, diffraction loss increases, and the intensity required as excitation light must be greater than 250 mW.

The difference in refractive index between AlP and GaP is not precisely known, but AlP has a smaller refractive index, approximately Δn
of the order of 0.5 and in the above example AlxGa _1-x P
The appropriate value of x for the layer is between 0.3 and 0.07, and x
0.1 is optimal. Methods for forming stripes, especially stripes with smooth sides, are summarized at the end of the examples.

(Example 2) In Example 1, the Stokes light is a nearly parallel beam, but the intensity decreases at the entrance window and has an intensity distribution as shown in Figure 3. Since the light reaches the window, the loss from the entrance window becomes large. Therefore, the shape of the Raman active layer stripe viewed from above is made to have an inclination toward the entrance window as shown in FIG. 4, thereby reducing the effect of Stokes light reaching the entrance window by diffraction and being lost to the outside. In other words, Stokes light travels along the dotted line in the figure. As in Example 1, when the laser length is 4 mm, the active layer stripe width S is 85 μm, and the entrance window width W is 5 μm,
If the length l' of the sloped part is 100μ and the stripe width on the incident side is 90μ, the loss due to diffraction at the part l' will be reduced to less than 1/40 by replacing l in equation (1) with l'. It can be completely ignored. The smaller l′ reduces the diffraction loss, but since the angle of incidence of the incident light increases, the loss due to reflection on the stripe side increases, and in extreme cases, the confinement effect due to the difference in refractive index for the excitation light is lost. be exposed. However, since the incident light condensed by a lens or the like spreads inside the crystal, the inclination angle θ≈W/l' may be approximately the same as the spread angle. Typically, the spread angle of the excitation light inside the crystal is about 0.01, and the slope l' is allowed to be up to about 50μ.

(Example 3) In FIG. 5, a is a top view, and b is an end view as seen from the incident side. The Raman active layer 3 has a stripe shape with a width S, and an excitation light introducing layer 10 having a width S' equal to or less than the active layer width is formed thereon. The latter is a layer of AlyGa _1- yP and y<x.

If the refractive indices of the active layer, cladding layer, and excitation light introduction layer are n ₁ , n ₂ , and n _{3 ,} respectively, then n ₁ > n ₃ > n ₂ ,
The excitation light spreads throughout the active layer, but the Stokes light is confined only to the active layer. The entrance window is the end face 10 of the excitation light introduction layer shown in Figure b. This is formed by removing a portion 11 of the deposited film 5 by photolithography.

For example, x=0.15, y=0.1, the width of the AlyGa _1-y P layer is 2μ, the thickness is 2μ, and the width of the active layer is 20μ to 50μ. The length of the excitation light introduction layer does not need to span the entire length of the resonator. That is, since the incident light that enters through the entrance window spreads over the entire layer including the active layer, a structure may be adopted in which the light narrows in the middle and disappears as shown in FIG. Since the area of the entrance window is 4 μm ² , it can be excited with an injection type CW semiconductor laser of approximately 40 mW.

(Example 4) The stripe of the active layer is narrowed through a tapered part in most of the middle part of the total length of 4 mm, and only the vicinity of the input end and output end has the dimensions and structure shown in Example 2. 7th
In the top view of the stripe shown in the figure, if the stripe width in the wide part is S ₁ , the average stripe width in the narrow part is S ₂ , and the length is l ₂ , then the excitation light intensity in the narrow part is S ₁ / Since S is _doubled , the excitation light power can be reduced by a factor given by f=[1+l ₂ /l(S ₁ /S ₂ -1)] ^-1 . . . (2) compared to Example 3. l=
4 mm, l ₂ = 3 mm, S ₁ = 85 μm, and S ₂ = 10 μm, this factor becomes 0.15, and the required excitation light power is only 38 mW compared to 250 mW in the second embodiment.

The shape of the taper may be arbitrary as long as it is gentle.
Typically, the length of each tapered part is 250 μm, and the length of each wide part is 250 μm.
It is 250μm.

(Example 5) It goes without saying that stripes having a completely similar structure are also possible in the structure of Example 3. For example, in Example 3, if the width S ₁ of the active layer is 20 μm and the narrow part is the same width as the excitation light introduction layer, that is, S ₂ = 2 μm, the factor f becomes 0.13, and the excitation light power increases from 40 mW to 5 mW. It can be lowered to By narrowing the stripes by the tapered portions in this way, not only can the power of the excitation light be lowered, but also the intensity of the light near the end face can be lowered, thereby making it possible to avoid the surface breakdown phenomenon caused by the strong electric field of the light.

(Example 6) The above method narrows the width of the stripe, but
Of course, as shown in FIG. 8, the thickness may be gradually narrowed except for the vicinity of the end face. FIG. 8 is a side view and is schematically shown with its length shortened. Also, both may be performed at the same time. For example, in Example 4, the thickness d ₁ near the end face is 5 μ, the average thickness d ₂ near the center is 1 μ, the length of that part is 3 mm, and the total length is 4 mm.
Then, since the factor f is given by an equation similar to equation (2), it becomes 0.25, and the required excitation power decreases from 38 mW in Example 4 to 9.5 mW.

The manufacturing method of Examples 1, 2, and 4 is as follows.

An impurity-free AlxGa _1- xP first cladding layer and GaP are grown on a GaP substrate by temperature difference liquid phase growth.
Grow the active layer. Next, a striped resist film is formed on the GaP active layer by lithography technology, and using this as a mask, reactive ion etching (RIE) is performed using PCl ₃ gas.

_PCl3 gas is particularly effective against GaP. This creates a nearly vertical and smooth side surface.

Since the gain of a Raman laser is small, even if there is a scattering loss of several percent on the side surfaces, the threshold value will decisively increase, but if etched with PCl ₃ gas, the side surfaces will be sufficiently smooth and no scattering loss will occur substantially.

The active layer stripes are relatively deep, ranging from 2μ to 10μ.
In addition, it is necessary to perform near-vertical etching to prevent light scattering. Etching can be performed at a high speed of 1 μm/min by introducing PCl ₃ gas into a vacuum chamber with opposing electrodes and applying high frequency power of 3.2 MHz and 1200 W at a pressure of approximately 0.05 Torr. An ordinary positive photoresist may be used as the etching mask. After the stripe formation process, an AlxGa _1-xP second cladding layer and a GaP buffer layer are grown using the same growth method.

In Examples 3 and 5, the first cladding layer, active layer, and excitation light introduction layer are grown first. Next, using a mask for the excitation light introduction layer, only this layer can be subjected to RIE as described above.
The process forms stripes. Next, using the mask for the active layer, apply a resist film to the width of the active layer, and repeat the same RIE process. After that, a second cladding layer and a buffer layer are grown.

In Example 6, in which the thickness is gradually changed, after the active layer is grown, a resist film having an opening in a 3 mm long part near the center of the stripe is used, and this part is etched to a depth of 1 .mu.m by RIE process.
Next, a new resist film having openings about 100 .mu. wide on both sides, that is, opening width 3.2 mm, is formed and etched by 1 .mu.m again. If the RIE process is performed four times by sequentially widening the openings in the same way, the thickness of the narrow portion will be 1μ, and the length of each tapered portion will be 300μ. 2
The step caused by the first etching will be smoothed out by the second RIE, but if you want to make it even smoother, you can add regular wet etching at the end.

〔Effect of the invention〕

The semiconductor Raman laser of the present invention has the above configuration, and operates with a CW excitation laser having a much lower power than conventional ones, that is, 250 mW to 10 mW or less. . In particular, since a CW operation injection type semiconductor laser is operated as an excitation laser, a CW operation semiconductor Raman laser including the excitation source is made of a semiconductor. Since it does not require a discharge tube or a large power source, it has an extremely long life, high reliability, and is compact. The present inventor has already proposed that semiconductor Raman lasers can be used for optical heterodyne detection, far-infrared light wave generation, and carrier light wave generation with extremely high wavelength purity stability. Since it operates with a power injection type semiconductor laser, current injection type semiconductor lasers for optical communication can be used as is, and it provides a means for ultra-wideband optical communication, that is, ultra-multiplexing, and is of extremely high industrial value. be.

[Brief explanation of drawings]

Figure 1 shows a conventional semiconductor Raman laser;
FIGS. 2 to 8 are diagrams showing the structure of an embodiment of a semiconductor Raman laser according to the present invention. 1...GaP crystal substrate, 2...AlxGa _1-xP first cladding layer, 3...GaP Raman active layer, 4...
AlxGa _1-x P second cladding layer, 5... high reflectivity dielectric film on the input side, 6... high reflectance dielectric film on the output side,
7... Focused incident excitation light, 8... Incident window, 9
...GaP... buffer layer, 10... excitation light waveguide layer,
11...Dielectric film removed portion.

Claims (1)

[Claims] 1. A first clad layer of AlxGa _1-x P formed by epitaxial growth on a GaP substrate crystal, and formed in a stripe shape on the first clad layer.
a Raman active layer of GaP, formed on the Raman active layer and surrounding the top and sides of the Raman active layer;
A dielectric film with a high reflectance is formed on two opposing end faces of the second cladding layer of AlxGa _1-x P, and a part of the dielectric film directs the focused excitation light to the Raman active layer. An entrance window is opened, and the excitation light incident through the entrance window is confined inside the cladding layer and undergoes multiple reflections, so that its intensity almost reaches the intensity at the entrance window.
A semiconductor Raman laser characterized in that the area of the entrance window is designed to be small so that the loss of Stokes light from the entrance window is sufficiently small. 2. The semiconductor Raman laser according to claim 1, wherein the stripes of the Raman active layer have an inclination near the entrance window to reduce loss of Stokes light from the entrance window. 3. An excitation light introducing layer AlyGa _1-y P layer is provided inside the cladding layer in contact with the Raman active layer,
Claim 1, characterized in that n ₁ > n ₃ > n ₂ when the curvature indexes of the Raman active layer and the cladding layer excitation light introduction layer are n ₁ , n ₂ , and n ₃ , respectively. semiconductor Raman laser. 4. The semiconductor Raman laser according to any one of claims 1 to 3, wherein the stripe of the Raman active layer is at least partially narrowed through a tapered portion. 5. The semiconductor Raman laser according to any one of claims 1 to 4, wherein the stripes of the Raman active layer are formed by etching with PCl ₃ gas.