JPH02209784A - External resonator type semiconductor laser and wavelength multiple optical transmitter - Google Patents

Abstract

PURPOSE:To modulate at a high speed and to reduce an aberration die to an astigmatism of a laser by so forming a reflection type optical diffraction grating for composing an external resonator mirror as to have a structure in which a bent and a period are continuously varied, and representing the shapes of the gratings by part of circular or elliptical quadratic curve group. CONSTITUTION:A divergent light radiated from one end face 4 of a semiconductor laser arrives at a diffraction grating 3, only a light having specific wavelength selected according to a light dispersing effect upon optical diffraction of the grating is reflected in a direction of a semiconductor laser, condensed to one end face 4 of the laser, and optically fed back to an active layer 5 of the laser. When the shape of the grating is so set as to be radiated from the end face P of the active layer, arrived at one point G of the grating and reflected to align the phases of the light returned again P, the grating operates as an external resonator mirror. That is, when a G (X, Y) is considered as an equi-phase point of the grating, the shape of the grating is given by a formula 2PG=mlambda+(constant). When the constant at an origin is zero, the shape of the grating is represented by x<2>+(y-fsintheta)<2>=(mlambda/2+f)<2>-(fcostheta)<2>.

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to the so-called optical communication field in which information and signals are transmitted using semiconductor laser light and optical fibers.
In particular, the present invention relates to a wavelength-stabilized semiconductor laser and a wavelength-multiplexing transmission device using the same, for realizing wavelength-multiplexed optical transmission, which is a type of optical transmission that transmits more signals simultaneously.

Conventional technology In order to realize a wavelength division multiplexing optical transmission device with a large capacity, it is necessary to stably oscillate in a single longitudinal mode at a single wavelength even during current modulation and even when the ambient temperature changes, and to maintain the spectral width of the wavelength multiplexing optical transmission device. There is a demand for semiconductor lasers that are narrow and whose oscillation wavelength can be controlled. External cavity type semiconductor lasers are considered to be a promising semiconductor laser light source having the above-mentioned performance. (Asakura et al., 1985 National Conference of the Institute of Electronics, Information and Communication Engineers, 899) Figure 5 shows an outline of a conventional external cavity type semiconductor laser. 51 is a 1.3 μm band Fapley-Perot semiconductor laser, 52 is a collimating lens, and 53 is an external cavity mirror substrate arranged at an angle with respect to the optical axis of the light emitted from the semiconductor laser. A reflective diffraction grating 54 is formed on the surface. Light emitted from one end surface 54 of the semiconductor laser is converted into a parallel beam by the collimation lens 52 and reaches the diffraction grating 53. Due to the light dispersion effect accompanying the light diffraction phenomenon of this diffraction grating, only light of a selected specific wavelength is reflected in the direction of the semiconductor laser, passes through the collimation lens 52 in the opposite direction again, and is focused on one end surface 54 of the semiconductor laser. The light is emitted and optically returned to the active layer 55 of the semiconductor laser.

The specific wavelength selected here is determined by the tilt angle of the substrate, and light with wavelengths other than the specific wavelength is dispersed and diffracted in different directions and is not optically returned to the active layer. Further, an anti-reflection film 56 is formed on one end surface of the active layer, suppressing oscillation due to the Fapley-Perot resonator of the semiconductor laser itself formed between one end surface 54 and the other end surface 57.

The diffraction grating used in the external cavity mirror of this external cavity type semiconductor laser is formed by exposing a photoresist coated on a substrate using a two-beam interference method. The shape of this diffraction grating is parallel.

This external cavity type semiconductor laser exhibits stable single longitudinal mode oscillation, and the oscillation wavelength can be continuously changed by changing the tilt angle of the external cavity mirror substrate with respect to the optical axis. .

Problems to be Solved by the Invention However, in the conventional external cavity type semiconductor laser, it is necessary to arrange a collimation lens between the semiconductor laser and the external cavity mirror on which the diffraction grating is formed.
The following issues remained:

(1) Approximately 1c as the distance between the semiconductor laser and the external cavity
Since it requires m, there is a limit to miniaturization of the element size.

(2) Difficult to adjust the optical axes of the semiconductor laser, collimation lens, and external resonator (3) The travel time of light between the semiconductor laser and the external resonator (
For the reason mentioned in 1), it is long, and as a result, the high-speed modulation limit is IG.
It is about Hz, and higher speed modulation is difficult.

(4) There is a limit to stabilizing the oscillation mode due to lens aberrations and aberrations due to astigmatism of the semiconductor laser.

The present invention overcomes the problems of the conventional external cavity type semiconductor laser as shown above, and enables high-speed modulation with a small size.
Moreover, the aberration caused by astigmatism of the semiconductor laser is reduced, and an extremely high-performance external cavity type semiconductor laser is provided.

Means for Solving the Problems The present invention provides the following features: (1) Only light of a specific wavelength of the diverging radiation emitted from one end face of a semiconductor laser and a semiconductor laser active layer is directed to one end face of the active layer. It is an external cavity type semiconductor laser that has the function of directly concentrating light and performing optical feedback, and is formed by including an external cavity mirror composed of a reflective optical diffraction element formed on a flat substrate. The reflective optical diffraction element constituting the external resonator mirror is a diffraction grating having a structure in which the curve and period change continuously, and the shape of each grating is a group of quadratic curves of a circle or an ellipse. It is expressed by a part of .

Further, the present invention provides: (2) an external cavity semiconductor laser in which the shape of each grating of the diffraction grating is a part of a group of circles expressed by the following formula.

x2+(y-fsinθ}2 = (mλ/2+f) 2- (fcosθ}2 where The set pressure HE1 θ is the angle between the axis connecting the end face of the active layer and the origin and the y-axis of the flat substrate on which the diffraction grating is formed,
λ is the set oscillation wavelength of the semiconductor laser, and m is an integer.

Further, the present invention is characterized in that (3) in (1), the shape of each grating of the diffraction grating is a part of an ellipse group expressed by the following formula.

x2/((mλ/2+ f ) 2- (f @co゛sθ}2)+ (y-(Δf + f) sinθ}2/
[(mλ/2+(Δ f +r)) 2− {(Δ
f + f) cos θ} 2 co = 1 where X and y are orthogonal coordinates on the flat substrate on which the optical diffraction element is formed, f is the set distance between the active layer end face and the origin of the coordinate system, and θ is the active layer The angle formed by the axis connecting the layer end face and the origin and the y-axis of the flat substrate on which the diffraction grating is formed, λ is the set oscillation wavelength of the semiconductor laser, and m is an integer. Also, Δf is the astigmatism difference,
It shows the distance between the divergence center point of laser light spreading from the active layer end face in the X direction of the coordinate system and the divergence center point of the laser light spread from the active layer end face in the X direction of the coordinate system.

In addition, in (4) (1), the present invention provides that the light of a specific wavelength of the diverging radiation emitted from one end surface of the semiconductor laser active layer is
When the external resonator focuses the light on one end surface of the active layer and returns the light, light of an adjacent oscillation wavelength that can be oscillated by the resonator of the semiconductor laser itself with respect to the specific wavelength is transmitted to the outside of the active layer. The light is focused on.

(5) In (1), the diffraction grating constituting the external resonator mirror is formed by a computer-controlled electron beam exposure method.

(6) In (1), in the semiconductor laser, a non-reflective film is formed on one end surface from which light is emitted and where the light is focused by the external resonator mirror.

Furthermore, the present invention provides: (7) a plurality of semiconductor lasers, and a plurality of external resonators with convergence and wavelength dispersion properties installed on one end surface of each semiconductor laser to directly return light of different specific wavelengths. In addition, in (8) and (7), a plurality of semiconductor lasers are formed in an array on the same semiconductor substrate, and each A diffraction grating forming an external resonator for directly optically feeding back light of different wavelengths to the semiconductor laser is formed on the same substrate.

(9) In (8), the set distances between the active layer end faces of the plurality of semiconductor lasers and the plurality of external cavity mirrors and the angle formed by the semiconductor laser substrate and the external cavity mirrors are constant, and This provides a wavelength multiplexing optical transmission device in which the specific wavelengths to be optically returned are different.

Function The present invention uses a reflection type diffraction grating having a structure in which the bending and period change continuously to redirect only light of a specific wavelength out of the diverging radiation emitted from the end face of a semiconductor laser active layer. By continuously changing the reflection angle of the diffracted light, it is possible to convert the diffracted light into a condensing light beam, to directly focus the reflected light on the end face of the active layer, and to perform optical feedback to the semiconductor laser. It is an application of this. In addition, since no lens is interposed between the semiconductor laser and the external cavity mirror, the device can be made smaller and aberrations can be prevented from occurring in the optical path of optical feedback, resulting in the realization of a high-performance external cavity type semiconductor laser. It is also possible.

Embodiment A schematic diagram of an external cavity type semiconductor laser according to the present invention is shown in FIG. FIG. 1 schematically shows an external cavity type semiconductor laser according to the present invention. 1 is a Fapley-Perot semiconductor laser in the 1.3 μm band; 2 is an external cavity mirror substrate arranged obliquely with respect to the optical axis of the semiconductor laser; a reflective type is provided on a part of the surface of the substrate; A diffraction grating 3 is formed. The diverging light emitted from one end surface 4 of the semiconductor laser reaches the diffraction grating 3, and due to the light dispersion effect accompanying the optical diffraction phenomenon of this diffraction grating, only the light of a specific wavelength selected is directed toward the semiconductor laser. The light is reflected by the semiconductor laser, is focused on one end surface 4 of the semiconductor laser, and is optically returned to the active layer 5 of the semiconductor laser. The specific wavelength selected here is determined by the shape of the diffraction grating and the tilt angle of the substrate, and light with wavelengths other than the specific wavelength is dispersed and focused in different directions and is not returned to the active layer. do not have. Further, an anti-reflection film 6 is formed on one end surface of the active layer, suppressing oscillation due to the Fapley-Perot resonator of the semiconductor laser itself formed between one end surface 4 and the other end surface 7.

Further, it is designed so that light having a wavelength of an adjacent sub-line mode that can be oscillated by the Fabry-Perot mode of the semiconductor laser 1 itself is focused at a position 2 μm above or below the active layer 5.

The design principle of the external resonator mirror used in the present invention is illustrated using FIG. x on the flat substrate on which the diffraction element is formed;
Assuming an orthogonal coordinate system of y, the end face P of the active layer, which is the divergence point and convergence point of light, is y in the perpendicular direction from the origin of the coordinates.
Exists on a line that makes an angle of 0 in the axial direction, and is located at a distance f from the origin.
Assume that the distance is set to F. When the shape of the diffraction grating is set so that the phases of the light emitted from P, reaching one point G of the diffraction grating, being reflected and returning to P again are aligned, this diffraction grating will work as an external cavity mirror. Become. That is, if point G(x, Y) is considered as an equiphase point of the diffraction grating, the shape of the diffraction grating is given by the following equation.

2PG=mλ+(constant) (Equation -) Here, PG
is the distance between points P and G, λ is the set oscillation wavelength of the semiconductor laser, and m is an integer.

When the constant at the origin is set to zero, the shape of the diffraction grating is expressed by the following equation in terms of x and y coordinates.

x2+ (y −fslrl) 2=(mλ/2+f
) ” -(fcosθ}2 (2nd equation) Therefore, the shape of the diffraction grating is: The radius centered at the point (0, f 5lnO) is v'((mλ/2+f) 2- (fcosθ}2)
It is a daily total that is a function of m.

Furthermore, when an astigmatism difference Δf exists in the radiation beam of the semiconductor laser, a divergence point P in the X direction is similarly located on a line forming an angle θ in the y-axis direction from the origin of the coordinates.
1 and the divergence point P2 in the y direction, and assuming that they are set at distances f and f+Δf from the origin, respectively, the shape of the diffraction grating is similarly given by the following equation.

x21 ((mλ/2+f)2- (f 5cosθ}
2) 10y−(Δf +f)sinθ}2/[(mλ/2
+(Δ f +f)) 2- ((Δ f+f)c
osθ} 2co = 1 (3rd equation) FIG. 3 schematically shows the external resonator mirror used in the embodiment of the present invention. FIG. 3(A) shows a schematic cross-section of the external resonator mirror. By irradiating the electron beam resist 32 of about 0.6 μm formed on the 51 substrate 31 used as the substrate 2 with an electron beam in a curved shape expressed by the above-mentioned second equation, and then immersing it in a developer. A concavo-convex structure is formed by removing the portions irradiated by the electron beam. A thin Au film 33 is formed on the surface of the electron beam resist having the uneven structure, thereby forming a diffraction grating with a high reflectance. FIG. 3(B) schematically shows the shape of the diffraction grating 34 formed by the electron beam. The size of the area where the diffraction grating 34 is formed is 0.1x O
, Ic m2. The shape of the concave portion of the diffraction grating is expressed by the second equation, and the setting parameters are f = 2 mm1 θ =
48.2' λ=1.3 μm.

In the diffraction grating 34, the curves composing each grating are part of the daily scale expressed by the second equation, and the radial intervals of the curves gradually change as shown in FIG. As already mentioned, this grating has a bending and chirping structure.

The diffraction grating formed as described above is used as an external cavity mirror in the first
As a result of fabricating the external cavity type semiconductor laser shown in the figure in 11i, single longitudinal mode oscillation was realized at a wavelength of about 1.3 μm. Furthermore, the oscillation wavelength could be continuously controlled within a wavelength range of approximately 0.02 μm centered around 1.3 μm by changing the angle formed between the semiconductor laser substrate and the external cavity mirror.

Further, the sub-mode suppression ratio was 30 dB or more. Furthermore, when modulation was performed using a sinusoidal alternating current, it was confirmed that the semiconductor laser light output could sufficiently follow modulation frequencies of IQGHz or higher.

In the above embodiments, the shape of the diffraction grating forming the external cavity mirror is a part of the diode, but the semiconductor laser used is, for example, a gain guide type semiconductor laser, If there is a gap difference, by configuring the shape of the diffraction grating that forms the external resonator mirror to be a part of the diary scale expressed by the third elliptic equation, external resonance that similarly oscillates in a stable single longitudinal mode can be achieved. It is possible to construct a container-shaped semiconductor laser. This problem can only be solved by inserting an lindrical lens in addition to the collimation lens, or by using a high-f, 17-degree aspherical lens that performs both functions. This is a serious problem, and it can be easily achieved with the present invention.

Above, we have shown an external cavity type semiconductor laser that uses a reflective diffraction grating, which has the function of converting divergent light into convergent light, as an external cavity mirror. Figure 4 shows this external cavity type semiconductor laser. A schematic configuration diagram of a light source of the wavelength division multiplexing optical transmission device used is shown.

41a-h are a plurality of semiconductor lasers in the 1 and 3 μm band formed in an array on the same InP semiconductor substrate;
Reference numeral 2 denotes an external resonator mirror substrate, on which a plurality of diffraction gratings 43a-h are formed for directly optically feeding back light of different wavelengths to each semiconductor laser.

The shape of each diffraction grating is determined by the set distance between the active layer end faces 44a-h of the plurality of semiconductor lasers 41a-h and the plurality of external cavity mirrors 43a-h, and the distance between the semiconductor laser substrate and the external cavity mirror. They were designed so that the angle θ formed was constant, and the specific wavelengths to be optically fed back differed by 10 A, and were manufactured using an electron beam exposure apparatus in the same manner as in the previous example. Other setting parameters are similarly f = 2
mm1 θ=48.2'. A straight line connecting each of the active layer end faces 44a-h of the semiconductor laser and a straight line connecting the coordinate origin of each of the corresponding external resonator mirrors 43a-h are arranged parallel to each other and separated by 2 mm, and the external resonator mirror substrate By tilting the wavelength at an angle θ with respect to the optical axis of the semiconductor laser, it was possible to realize a stable light source for a 10-wavelength wavelength multiplexing optical transmission device.

The oscillation wavelength interval between adjacent semiconductor lasers is approximately 1 OA, and the oscillation wavelength can be shifted and controlled as a whole by changing the tilt angle of the external cavity mirror substrate with respect to the optical axis of the semiconductor laser. there were.

In the above embodiment, the semiconductor laser is 1.3 μm.
Although the m-band was used, it is not necessarily limited to this, but the 0, 8 μm band using a gallium arsenide substrate, or the 0. 6μ
It is obvious that exactly the same effect can be obtained by using a mμm semiconductor.

Advantages of the Invention As described above, the present invention addresses the drawbacks of conventional external cavity type semiconductor lasers, namely (1) the distance between the semiconductor laser and the external cavity is long, which limits miniaturization of the device size.

(2) It is difficult to adjust the optical axes of the semiconductor laser, collimation lens, and external resonator. (3) The travel time of light between the semiconductor laser and the external resonator is long, making it difficult to reach the high-speed modulation limit.

(4) The oscillation mode is unstable due to aberrations caused by astigmatism of the lens and semiconductor laser.

The present invention overcomes these problems and realizes a compact wavelength-stabilized semiconductor laser capable of ultra-fast modulation and a wavelength-multiplexed optical transmission system, which is of great value.

[Brief explanation of the drawing]

Figure 1 is a +a schematic diagram of the external cavity type semiconductor laser of the present invention, Figure 2 is a diagram explaining the principle and shape of the diffraction element constituting the external cavity mirror used in the present invention, and Figure 3 (A). ,
(B) is a schematic cross-sectional view and a plan view of an external resonator mirror used in an embodiment of the present invention, and FIG. FIG. 5 is a diagram schematically showing a conventional external cavity type semiconductor laser. No. 1 No. 1 semiconductor laser, 2 (31) ・O・Substrate, 3
(34)...sha diffraction grating. Name of agent: Patent attorney Shigetaka Awano, 1 person (Figure 1, Figure 6, Figure cA), Figure 5, Figure 5, Figure 56, Anti-IF Jr.

Claims (9)

[Claims] (1) Wavelength dispersion in which only light of a specific wavelength of the diverging radiation emitted from one end face of a semiconductor laser and a semiconductor laser active layer is focused directly onto one end face of the active layer for optical feedback. The external resonator mirror is formed of an external resonator mirror that has the functions of light-gathering and light-converging functions and is formed of a reflective optical diffraction element formed on a flat substrate. The reflection type optical diffraction element is formed by a diffraction grating having a structure in which the curve and period change continuously, and the shape of each grating is represented by a part of a group of quadratic curves of a circle or an ellipse. External cavity type semiconductor laser. (2) The external cavity type semiconductor laser according to claim 1, wherein the shape of each grating of the diffraction grating is a part of a circle group expressed by the following formula. x^2+(y-fsinθ)^2= (mλ/2+f)^2-(fcosθ)^2where x,
y is the orthogonal coordinate on the plane substrate on which the optical diffraction element is formed, f is the set distance between the end surface of the active layer and the origin of the coordinate system,
θ is the angle formed between the axis connecting the end face of the active layer and the origin and the y-axis of the flat substrate on which the diffraction grating is formed, λ is the set oscillation wavelength of the semiconductor laser, and m is an integer. (3) The external cavity type semiconductor laser according to claim 1, wherein the shape of each grating of the diffraction grating is a part of an ellipse group expressed by the following formula. x^2/{(mλ/2+f)^2-(f・cosθ)^
2}+{y-(Δf+f)sinθ}^2/[{mλ/
2+(Δf+f)}＾2−{(Δf+f)cosθ}＾
2] = 1 where x and y are orthogonal coordinates on the plane substrate on which the optical diffraction element is formed, f is the set distance between the end face of the active layer and the origin of the coordinate system, and θ is the distance between the end face of the active layer and the origin of the coordinate system. The angle formed by the axis connecting the origin and the y-axis of the flat substrate on which the diffraction grating is formed, λ is the set oscillation wavelength of the semiconductor laser, and m is an integer. Further, Δf is an astigmatism difference between the divergence center point of the laser beam spreading from the active layer end face in the X direction of the coordinate system and the divergence center point of the laser light spreading from the active layer end face in the Y direction of the coordinate system. Show distance. (4) When light of a specific wavelength of the diverging radiation emitted from one end surface of the semiconductor laser active layer is focused on one end surface of the active layer by the external resonator and optically returned, Claim 1, wherein light having an oscillation wavelength in an adjacent longitudinal mode that can be oscillated by a resonator of the semiconductor laser itself is focused outside the active layer.
External cavity type semiconductor laser described in . (5) The external cavity type semiconductor laser according to claim 1, wherein the diffraction grating constituting the external cavity mirror is formed by a computer-controlled electron beam exposure method. (6) External resonance according to claim 1, wherein an antireflection film is formed on one end surface of the semiconductor laser from which light is emitted and where the light is focused by an external resonator mirror. Vessel-shaped semiconductor laser. (7) A plurality of semiconductor lasers and wavelength dispersion properties that directly return light of different specific wavelengths from among the light emitted from one end surface of each semiconductor laser to one end surface of each of the semiconductor lasers. A wavelength multiplexing optical transmission device using multiple external cavity lasers equipped with external cavities. (8) A plurality of semiconductor lasers are formed in an array on the same semiconductor substrate, and a diffraction grating forming an external resonator mirror for direct optical feedback of light of different wavelengths to each semiconductor laser is on the same substrate. 8. The wavelength division multiplexing optical transmission device according to claim 7, wherein the wavelength division multiplexing optical transmission device is formed on the top. (9) Specify that the set distances between the active layer end faces of multiple semiconductor lasers and multiple external cavity mirrors, and the angles formed between the optical axes of the semiconductor lasers and the external cavity mirrors are constant and optical feedback is provided. 9. The wavelength division multiplexing optical transmission apparatus according to claim 8, wherein the wavelengths of the wavelength division multiplexing optical transmission apparatus are different from each other.