JPH0493815A - Optical isolator and semiconductor laser device integrated therewith - Google Patents

Abstract

PURPOSE:To allow the execution of 100% mode conversion even if th phase matching between modes is not executed by simultaneously applying an electric field and magnetic field to a waveguide to induce the mode conversion by an electro-optical effect and magneto-optical effect within the same region, thereby compensating the degradation in the mode conversion efficiency by phase mismatching. CONSTITUTION:This optical isolator is constituted of a GaAs substrate 31, a buffer layer 32, a lower clad layer 33, a core layer 34, an upper clad layer 35, a mesa structure 36, and electrodes 37, 38. The isolator has the waveguide consisting of a magnetic semiconductor, such as CdMnTe, and induces the mode conversion between the orthogonal modes of TE light, TM light, etc., by the electro-optical effect and magneto-optical effect within the same region by simultaneously impressing the electric field E and the magnetic field H to the waveguide. The degradation in the mode conversion efficiency by the phase mismatching is compensated. The 100% mode conversion is attained in this way even if the phase matching between the modes is not executed.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention is used in semiconductor laser devices used in optical communication, optical information processing, optical measurement equipment, etc., so as not to induce noise due to return light. The present invention relates to an optical isolator and a semiconductor laser device integrated therewith.

[Background Art] In recent years, optical fiber communications and optical disk memories have been developed as the speed of information transmission increases and the capacity of information recording increases. For optical communication systems, 1.3 or 1.5μ
Trunk medium and long-distance transmission systems using wavelengths of m and
．． There are short-distance transmission systems such as LAN40 (Cal Area Network) that use the 8 μm band. In either case, the light emitted from the semiconductor laser is focused on the fiber end face and guided through the fiber. However, at this time,
Light reflected from the end face of a fiber or other optical component returns to the active layer of the semiconductor laser, resulting in unstable oscillation of the semiconductor laser and power fluctuations and wavelength fluctuations. especially,
DBR lasers (distributed reflection lasers) have major effects such as a single mode becoming multimode. In addition, coherent optical communication, which is attracting attention as a future communication method, only changes the phase of light without performing ON10 F F, so the influence of return light is particularly large. In addition to the reflected light from the components, the reflected light from the disk substrate returns to the semiconductor laser, inducing the aforementioned instability and noise.

The generation of such noise increases the error rate of transmission signals in optical communications, and causes deterioration of reproduced signals in optical disk memories.

Eliminate the returning light to the semiconductor laser as described above (the only element is an optical isolator. An optical isolator has irreversible or non-reciprocal transmission characteristics, and completely blocks the returning light to prevent the semiconductor laser from returning). This optical isolator prevents the light from entering the active layer again.This optical isolator currently has the following two problems.

One problem is the wavelength range.

As shown in FIG. 4, the device currently in practical use uses a magnetic garnet single crystal 41 such as YIG as a Faraday rotation element. The absorption edge of garnet is approximately 1μ
It is located at a wavelength of m, and is transparent to the wavelength range (1, 3 or 15 μm) used for trunk optical communications, but
Wavelength range used for short-distance optical communication or optical disk memory (
Garnet's absorption is thick for light in the 0.8 μm band. Therefore, in this 0.8 μm wavelength range, a simple optical isolator that combines a quarter-wave plate and a polarizing beam splitter is used. is only used.

The isolation ratio obtained with this simple isolator is only about 20 dB at maximum. Moreover, if the polarization plane of the returned light changes, the isolation ratio will further decrease. Naturally, the simple type cannot be used in a system that attempts to detect as a signal the rotation of the polarization plane (Kerr rotation angle or Faraday rotation angle) of light reflected or transmitted from a disk, such as a magneto-optical disk.

Another problem is that it is difficult to integrate with other optical devices.

Optoelectronic integrated circuit (OEIC) or optical integrated circuit (01C)
Development is progressing to increase the speed and efficiency of devices, and optical devices such as semiconductor lasers are made of GaAs and I
It is manufactured on a compound semiconductor substrate such as nP. Therefore,
For this reason, it is desired to integrate an optical isolator on a compound semiconductor substrate. Here, the magnetic garnet film can be grown by liquid phase epitaxy or sputtering, but since the lattice constant and coefficient of thermal expansion are different from GaAs and InP substrates, a high quality magnetic garnet film is epitaxially grown on such substrates. I can't. Therefore, as long as a magnetic garnet film is used as a Faraday material, it is considered difficult to integrate an optical isolator with other optical elements.

From the above, one of the challenges for optical isolators is to realize an optical isolator that operates in a short wavelength range (0.8 μm band) and can be integrated with other optical devices.

In order to solve this problem, the present inventors proposed in a previous application an integrated optical isolator using a magnetic semiconductor as a Faraday material. This is shown in Figure 5. C, which is a magnetic semiconductor
dMnTe is a compound semiconductor of the II-Vl group, CdTe, in which the Cd position is replaced with Mn, and is known to be transparent in the visible range and to have a large Faraday rotation angle. Furthermore, a high quality CdTe film can be grown on a GaAs substrate 51 by MBE or MOCVD, and as shown in FIG.
1, a buffer layer 52 which is a CdTe film of the optical isolator section, a cladding layer 53, a waveguide layer 54, and a cladding layer 5
5 can be produced. In this way, the magnetic semiconductor CdMnTe
Although the integrated isolator using the above solves the above problems, it also has the following problems.

[Problems to be solved by the invention] In order to realize a waveguide type optical isolator as shown in FIG.
Phase matching between the TE wave and the TM wave is required. FIG. 6 shows how to achieve phase matching. When laser light is guided in two directions in a film as shown in FIG. 6(a), the film has a finite thickness as shown in FIG. 6(b). Then, due to shape birefringence, the refractive index nA for TE waves is larger than the refractive index nT,l for TM waves. If the propagation constant difference between TE wave and TM wave is ΔB, then
ΔB=2rt/n・(nT-nyll)+ + * +
(1), and the mode conversion efficiency R from TE wave to TM wave is R=θF'/(θF'+(Δβ/2) 2) sin"
It is given as [(θF′+(ΔB/2)”) “”ff]. Here, θ is the Faraday rotation angle per unit length, and ! is the waveguide distance. As can be seen from Section 1, the mode In order to increase the conversion efficiency (the mode is quantized by the waveguide, the rotation of the polarization plane of the propagating light is indicated by the mode conversion efficiency), △β=0, that is, n 7 F=
A condition of n 7v is required. The derivation of equation (2) is
The electromagnetic field equation of the waveguide is solved to find the electromagnetic field as a linear combination of 1'E mode and TM mode, and the ratio of optical power at a waveguide distance of 2 is found via the mode coupling equation.

For this reason, as shown in FIG. 6(C), the film thickness (t-
Efforts are being made to realize n 72 = n TM in In the case of garnet films, attempts have been made to create strain-induced birefringence using the lattice constant difference between the substrate and the film (waveguide), and to create growth-induced birefringence by controlling the temperature and composition during film growth. being done. Furthermore, in order to compensate for the difference in propagation constant, attempts have been made to attach a grating along the optical waveguide direction.

However, the above method requires strict control of film forming conditions and process conditions, and has the problem that adjustments cannot be made once the film is fabricated, making it impractical.

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide an optical isolator having a configuration capable of achieving 100% mode conversion without phase matching between modes, and a semiconductor laser integrated therewith. The goal is to provide equipment.

[Means for solving the problem] In the optical isolator according to the present invention that achieves the above object, C
It has a waveguide made of a magnetic semiconductor such as dMnTe, and allows mode conversion between orthogonal modes such as TE light and 1M light by applying an electric field and a magnetic field to the waveguide at the same time. They are caused to occur in the same region, thereby compensating for the decrease in mode conversion efficiency due to phase mismatch.

Further, the semiconductor laser device according to the present invention is configured such that the optical isolator and the semiconductor laser section as described above are integrated on the same substrate.

The present invention will be described in detail below with reference to FIG. A magnetic semiconductor (Fe, Mn such as CdMnTe) forming the waveguide 1
II-Vl group compound semiconductors containing n) are materials that have both magneto-optic and electro-optic effects. 1st
As shown in the figure, the electric field E in the sphalerite structure [1.
Consider the case where the magnetic field H is simultaneously applied in the [10] direction and the [110] direction (C, d~T n T e N 1 is G
aAs (001) substrate). A coordinate system is set as shown in FIG. 1, and the refractive indices in the x, y, and z axis directions are ntv, rlrE, and n, respectively.

The term TE wave or TM wave is correct only in the case of a waveguide (slab-type one that extends horizontally so far that it can be considered infinite), but here it is a plane wave approximately polarized in the X-axis direction. We will call the plane wave polarized along the y-axis the TM wave and the TE wave, and consider the mode coupling between the TE wave and the TM wave. In a zincblende cubic crystal system such as CdMnTe, the refractive index is isotropic, but when made into a thin film, shape birefringence occurs and nl and nTE differ.

The permittivity tensor ε at this time is the permittivity of vacuum ε. Then, Δn is the refractive index change due to the electric field E, and the leading angle θ of one phase is given as follows.
1 and the electro-optic coefficient T4r, Δn = n'
r 41 E/2 = E/2π・θ1... (
4) is given as Further, G is a quantity representing the magnitude of the magneto-optic effect, and has the following relationship with the Faraday rotation angle θ2 and the Weerde constant V.

G = i-n / π・θF: n/π・vH...
・(5) Using the dielectric constant tensor in equation (3), we can calculate the TM wave (complex amplitude is ATII(Z)) and the TE wave (also ATE(Z)).
) and ), where 80 is (
2S, which is the propagation constant difference between the TE wave and the TM wave given by Equation 11, is given by the following equation.

S= ((Δ73/2)2+θF'〇, 2) l/
2... (8) As shown in FIG.
When the E wave is incident (ATE (o) = i and AT,
(] = O), Equations (6) and (7) are each used here as a law, and the condition for the polarization plane to rotate by exactly 45 degrees (mode conversion motion ￥R becomes 50%) is ArE(z) =Aa, (Z
)...(11), so from now on, equation (9) and (
10) The condition that the real part and the imaginary part of the equation are equal appears as follows.

From this, if the condition 〒 in equation (12) is satisfied, even if the phase matching condition Δβ=0) is not satisfied, linearly polarized light will be converted to linearly polarized light tilted by 45 degrees. I know it will happen.

Figure 2 shows a comparison of the rotation of the plane of polarization when the phase matching condition is satisfied and when equation (12) is satisfied using the Poincaré sphere. (Δ
β: 0), the light is always linearly polarized and only the angle of the polarization plane changes (path ■). On the other hand, in the case of the condition of equation (12), it can be seen that the light is elliptically polarized at locations other than the linearly polarized light at 0 degrees and 45 degrees (the path of ■). By applying an appropriate electric field E together with H, 45 degrees, even without phase matching (Δβ=0).
The plane of polarization can be rotated, eliminating the need for waveguide growth and strict control of the process.

If this can be traced in relation to equation (9), equation (< 10), and equation IIA, then by applying the electric field E, Δr+≠0, that is, θ6≠0 (see equation (4)), and 9) The expression will have an imaginary part. Therefore, (for C for which equation 11 holds true, 4
The imaginary part of equation 1(') does not need to be zero, and ΔB≠
Even if it is 0, the condition for formula (11) to hold exists.

[Example] Figure 3 shows the husband lit! of the present invention. ! FIG. 3 is a perspective view of an example. In the figure, a buffer layer 32 of CdTe is grown on a semi-insulating GaAs (001) substrate 31 by the MBE method. By selecting appropriate growth conditions, a film 32 having the same plane orientation as the substrate 1 is grown. Thereon, a conductive path structure is fabricated using C d M n Te films 33, 34, and 35 having two types of Mn compositions. The M n wA composition of the core portion 34 is smaller than that of the upper and lower cladding portions 33.35.
y < x), the refractive index of the core portion 34 is made larger than that of the cladding portion 33.35, thereby creating a structure that confines light.

Next, the upper cladding part 35 is etched to form a mesa structure 36, and channels and skin paths are formed in an F shape.

At this time, the elongated direction of the mesa is taken as the EllOE axis, and the fi direction that is perpendicular to the mesa is taken as the BzoH axis. Subsequently, by seeing the electrodes CI rt/Au) 37, 38 on both sides of the mesa, it becomes possible to apply an electric field E in the zoi axis direction.

In addition, the magnetic field F (<
If the wave direction (snao) is also applied in the EIIOT axis direction, the external t
The relationship between AE, H and the crystal axis of the 2# wavepath is the same as in FIG.

Therefore, the electric field C that satisfies the above mE<', Equation 2)
E), magnetic field 1), propagation distance (I2) (corresponding to 2 in equation (12)), and propagation constant difference (Δβ) can be found, as shown in Figure 3, in the y-axis direction. Since polarized TE light enters and exits as linearly polarized light tilted at 45 degrees, an integrated isolator can be realized.

The manner of accumulation is the same as in the building shown in Figure 5.

Propagation constant difference △β is 1 orad/cm, propagation distance β is Lm
m, then solve Equation (12) to find the Faraday rotation angle (θ
1) and the phase advance angle (OF) due to the electro-optic effect as follows.

θc = 5 rad/cm = 286'/cm θF: 4
-7 rad/cm2269°/cm From the formula (15),
Welde constant 3 of CdMnTe is 0, 1 degree/C
m Oe, the required magnitude of the field H is approximately 27 kOe obtained from the torque r-θF/v, which is a value that can be sufficiently applied with a permanent magnet. Also, from equation (4), the wavelength (e) of the light is 0.8 μm, and the refractive index (n) is 2
．． 84, electro-optic coefficient rr4+) 4.5x 10-1
Assuming 0 cm/V, the required electric field strength E is E: θ6/
Obtained from πn″r4, 1,2 x 10' V/
cm.

If the distance between the two electrodes 37 and 38 in Fig. 3 is 3 μm, the required voltage is 3.6 V.
(transistor-transistor 1
It can be seen that a logic level circuit is sufficient.

From the above estimation, it was shown that an integrated isolator with a waveguide distance of 1 mm can be realized by simply applying a magnetic field H = 3 kOe and a pressure of about 4 mm. , According to the present invention, electro-optic ? Mo by fruit of A and magnetic light ♀ effect
Therefore, conventional optical isolators should not be used for two-phase matching (by appropriately selecting the combination of electric and magnetic fields (propagation constants i and ΔB can be determined from the structure). An optical isolator that can rotate the plane of polarization by 45 degrees without causing a 1 phase delay (predetermined) is realized.

Further, the magnetic semiconductor waveguide described above can be manufactured without requiring close control of film formation conditions or process conditions, and furthermore, it can be easily integrated with a semiconductor laser.

[Brief explanation of drawings]

Fig. 1 is a diagram for explaining the present invention in detail, Fig. 2 is a diagram showing a Poincaré sphere for explaining mode conversion by comparing the present invention and a conventional example, and Fig. 3 is a diagram of the present invention. Figure 4 is a diagram showing an example of the invention, Figure 4 is a diagram showing a conventional example, Section 5 is a diagram showing an example of a previous proposal by the present inventor, and Figure 6 is a diagram explaining a conventional method for eliminating the difference in propagation constants. This is a diagram for 1--Cd M n T e "A wave path, 3l-Ga
As(001) substrate, 32...777 layers, 33-
- Lower cladding layer, 34...-core layer, 35...-upper cladding layer. 36...Mesa structure, 37.38...-Electrode supplier: Canon Co., Ltd.

Claims (1)

[Claims] 1. It has a waveguide made of a magnetic semiconductor, and an electric field and a magnetic field are simultaneously applied to the waveguide to cause mode conversion by electro-optic effect and magneto-optic effect in the same region, thereby creating a phase difference. An optical isolator characterized in that it compensates for a decrease in mode conversion efficiency due to matching. 2. The optical isolator according to claim 1, wherein the electric field is applied in the [110] direction in the zincblende structure. 3. The optical isolator according to claim 1, wherein the magnetic semiconductor is CdMnTe. 4. The optical isolator according to claim 1, wherein the mode conversion is performed between TE light and TM light. 5. The optical isolator according to claim 1, wherein the electric field and the magnetic field are orthogonal to each other. 6. The optical isolator according to claim 1, wherein the waveguide is fabricated on a GaAs (001) substrate. 7. A semiconductor laser device, wherein the optical isolator according to claim 1, 2, 3, 4, 5, or 6 and the semiconductor laser section are integrated on the same substrate.