JPH04186690A - Optical integrated type wavelength variable semiconductor laser - Google Patents

Abstract

PURPOSE:To enable frequency control, making variable the wavelength of semiconductor oscillation light by feed-controlling a frequency fluctuation signal of laser light from a frequency reference device into a semiconductor laser. CONSTITUTION:This device is driven by a constant power source 9 which injects current into an active region 17 having a grating by way of a bias T14. A phase control region 16 having no grating drives with a constant power source 18. The control of this injection current makes it possible to change the oscillation wavelength of a laser 1. A distribution reflector 3 drives with a constant power source 10. The injection current into this distribution reflector is arranged to detect the frequency fluctuation of laser light by changing the central wavelength and oscillating with change in the oscillation wavelength of the laser 1 (This must be carried out by changing the central current into the phase control region 16). A differential signal of a light receiver 5 detects the frequency fluctuation of the laser 1. The light detected with the light receiver 4 monitors the oscillation wavelength of the laser 1, combining the drive current of the distribution reflector 3 with the detection signal of the light receiver 4. On the other hand, a signal of a light receiver 6 detects power fluctuations in the laser light.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a wavelength tunable semiconductor laser device, and in particular, to an optical integrated semiconductor laser device that can tune the oscillation wavelength while stabilizing the optical output and the oscillation frequency or wavelength. Regarding.

[Prior Art] Conventionally, a heterodyne system has been proposed as a transmission system for realizing optical frequency multiplex communication in a long-distance, large-capacity optical communication system using a single mode optical fiber. In order to perform this heterodyne detection, it is essential to stabilize the oscillation wavelength of the laser, which is the oscillation light source, and to improve the spectral purity of the oscillation light.

For this reason, distributed feedback (DFB) semiconductor lasers that achieve dynamic single mode (DSM) oscillation and distributed reflection type (DBR)
Conventionally, the following techniques have been proposed for lasers to stabilize frequencies and improve spectral purity.

(1) Method of stabilizing frequency or wavelength First, there is a method of precisely stabilizing the current injected into the laser and the temperature of the laser.

Next, there is a method of electrically applying negative feedback to the current injected into the laser using an external frequency reference. If the laser becomes clogged, part of the laser oscillation light is transferred to the Fabry-Perot etalon or molecular gas (Rb).
gas, Cs gas, NH3, etc.), and converts fluctuations in the oscillation frequency of the laser into fluctuations in the intensity of the transmitted light, which is received by a receiver, and the signal from the receiver is converted into a low-frequency (DC to
10kHz) and feeds back to the laser injection current. That is, this low frequency current is added to this injection current to obtain the laser injection current.

(2) Method of Improving Spectral Purity Firstly, there is a method of placing a reflecting mirror outside the laser to provide optical negative feedback. As this reflecting mirror, there is a method of using something with wavelength selectivity such as a diffraction grating, or of increasing the Q value by increasing the resonator length of the laser by using a fiber resonator.

Next, similar to the frequency stabilization method described above, an external frequency reference is used to electrically increase the laser injection current to a high frequency (10k
There is a method of applying negative feedback at Hz = 100 MHz).

Further, although the resonator length of a normal laser is about 300 μm, there is also a method of increasing the Q value by increasing this length to about 1 mm.

With these methods, the spectral linewidth can be reduced to several tens of kilohertz
Currently, a frequency of about 100 kHz is available.

On the other hand, a BIG-DBR type DSM laser with a Bragg wavelength controller has been proposed as a light source whose oscillation wavelength can be changed (1986 Spring 33rd Applied Physics Conference Proceedings 1st page 18).

This light emitting device has an active layer having a gain and a variable distributed reflector that can control the Bragg wavelength by changing the propagation constant of a waveguide coupled to the active layer. That is, as shown in FIG. 7, which is a vertical cross-sectional view of this light emitting device on the optical axis, there is an active J! 41. The variable distribution reflector 41 is composed of a semiconductor waveguide layer 43 on which a collagen 42 is formed, and by injecting a current into the semiconductor waveguide layer 43 to change the carrier density, the waveguide layer 43
Due to interband absorption and dispersion of plasma absorption, the refractive index of the medium changes, the propagation constant of the waveguide changes, and the Bragg wavelength can be controlled. In FIG. 7, 44 is an electrode for electrically controlling the Bragg wavelength of the variable distribution reflector 41, and 45 is an electrode.

This is an electrode that applies current to the active layer 40.

This BIG-DBR type DSM semiconductor laser with a Bragg wavelength controller can electrically control the oscillation wavelength by injecting a current into the variable distribution reflector electrode 44, and uses a heterodyne method that requires highly accurate wavelength control. , suitable for wavelength multiplexing.

Furthermore, in response to this, a DBR type DSM laser with a Bragg wavelength controller, which is designed to reduce the size of the entire system, has been developed (see Japanese Patent Laid-Open No. 6316Q39i).

As shown in the 8th section, this element has two distributed reflectors 51 with slightly different Bragg wavelengths (that is, different pitches).
．． 52 , comprising a gain region 48 and a variable distributed reflector 46 .
After the laser light emitted from the laser emitting section consisting of 49 and phase adjustment region 47 passes through these distributed reflectors 5 and 52, the respective light intensities are detected by photodetectors 53 and 54, and a signal of the difference is detected. is calculated by the subtractor 56 and fed back to the laser emitting section, thereby realizing wavelength control of the oscillated light. In FIG. 8, 55 is an adder, 57 is an optical output control circuit, 58 is a phase control circuit, and 59 is a wavelength control circuit into which the difference signal is input.

[Problems to be Solved by the Invention] However, in order to accurately control the wavelength by detecting the oscillation wavelength of a conventional wavelength tunable semiconductor laser, it is necessary to use an external frequency reference etc. using the same method as in the conventional frequency stabilization example described above. Not only does this have the disadvantage of increasing the size of the entire system, but the conventional method also
In the case of stabilizing the lliT variable wavelength and improving spectral purity, there are the following points.

(1) When performing electrical negative feedback using an external frequency reference, there are Fabry-Perot interferometers, molecular cells, etc. that receive part of the laser oscillation light, so it is difficult to miniaturize the entire system as described above, and the high frequency When giving negative feedback,
Since the optical path of this feedback loop is long, there is a problem in that the control band is limited due to the phase delay of the light. For example, if the total length of the feedback loop is 30 cm, then 1
In a band of about 00 MHz (wavelength is about 60 cm), the phase delay becomes about 180 degrees, resulting in positive feedback instead of negative feedback, resulting in oscillation. Therefore, it becomes difficult to improve spectral purity.

(2) When a diffraction grating, fiber resonator, etc. are placed externally, these are susceptible to mechanical vibrations and cannot necessarily be controlled stably. Further, although this method can improve the spectral purity, it cannot stabilize the center frequency. Furthermore, it becomes difficult to control the external mirror when changing the oscillation wavelength in a wavelength tunable laser.

In addition, a DBR with a Bragg wavelength controller as shown in FIG.
In the case of a type DSM laser, it is difficult to control the pitch of collation, etc., and in order to improve the spectral purity, it is necessary to adopt the conventional method as described above, which results in an increase in the size of the system.

Therefore, in view of the above-mentioned ycB, it is an object of the present invention to provide a wavelength tunable semiconductor laser device which has a stable optical output and oscillation frequency, has a narrow spectral linewidth, and can be made compact.

[Means for solving the problems] In the present invention that achieves the above object, 3 is a wavelength tunable semiconductor laser such as a seed-type distributed feedback laser and an optical element to which light is coupled and which can serve as a frequency reference. A certain frequency reference device is monolithically integrated, and this frequency reference device is configured to be variable in translation wavelength in synchronization with the oscillation wavelength of the semiconductor laser, and is configured to adjust the frequency fluctuation signal of the laser light from the frequency reference device. By controlling the semiconductor laser with negative feedback, the wavelength of the oscillated light from the semiconductor laser can be varied and the frequency spread out! 20 (at least one of frequency stabilization and spectral purity improvement). This makes it possible to change the oscillation wavelength while applying electrical negative feedback over a wide band to stably stabilize the frequency of the laser beam and improve the spectral purity.

More specifically, as a frequency reference, a waveguide with a variable selection wavelength grating is provided in the direction of propagation of light.
The oscillation light emitted from one end of the wavelength tunable laser is guided through an optical waveguide formed on the substrate and is incident on this grating region.Since the grating ~ has wavelength selectivity, the change in the wavelength of the laser oscillation light is The transmitted light or reflected light from the grating is converted into a change in intensity, and this signal is detected by a photoreceiver formed on the same substrate, and amplified by an amplifier to return the injection of the wavelength tunable laser to the flow. . With such a configuration, the length of the feedback loop becomes extremely short, so that optical phase delay does not become a problem, and wideband control becomes possible. In this case, the wavelength selection region of the grating region serving as a frequency reference can be changed by current injection or the like. Therefore, by changing the selected wavelength of the grating region in synchronization with changing the oscillation wavelength of the wavelength tunable laser, it is possible to easily tune the wavelength, stabilize the frequency, and improve the spectral purity. A wavelength tunable semiconductor laser device can be realized.

[Embodiment] FIG. 1 is a diagram showing the configuration of an embodiment of the present invention. In the figure, 1 oscillates in a dynamic single mode, and the phase adjustment region 16
This is a 344-pole type branch feedback (DFB) semiconductor laser having active regions L7 on both sides thereof, and the first output is taken out from the left end facet. This semiconductor laser 1
is equipped with a grating only in the active region L7 of both . . . 11, and the phase adjustment region 16 does not have a grating. This semiconductor laser 1 also emits laser light from the right end surface, and this laser light is coupled to an optical waveguide 8 formed on a substrate (not shown) and enters the optical isolator 2. The optical isolator 2 guides light only from left to right in FIG. 1, and does not guide light from right to left.

The light from the laser 1 that passes through the optical isolator 2 enters the integrated coupler 7 and is split into two parts, of which the component that passes through and travels straight is guided through the waveguide s and is split in the direction of light travel. It is incident on a distributed reflector 3 having a grating along it. Of this incident light, the light with the wavelength (wavelength other than the Bragg wavelength) passes through the distributed reflector 3 as it is, and the light with the wavelength (Bragg wavelength) is reflected and returns to the integrated coupler 7, where it is reflected. Then, the light is received by the light receiver 4.

Therefore, the amount of light received by each light receiver 4.5 is determined by the relationship between the wavelength of the laser oscillation light and the Bragg wavelength.

The wavelength characteristics of the distributed reflector 3 at this time are shown in FIGS. 2 and 3. That is, the second section shows the wavelength dependence of the reflected light intensity of the distributed reflector 3 (received by the light receiver 4), and FIG. 3 shows the wavelength dependence of the transmitted light intensity (received by the light receiver 5). Indicates dependence.

The other split light that enters the integrated coupler 7 from the isolator 2 and is reflected there is received by the light receiver 6. Therefore, a light amount corresponding to the output of the laser oscillation light enters the light receiver 6.Next, a driving method of this embodiment will be explained.

The wavelength tunable distributed feedback semiconductor laser 1 having the above configuration is driven by a constant current source 9 that injects current into the active region 17 where the grating is provided through the bias T'14. The phase adjustment region 16 without a grating is driven by a constant current #18, and the oscillation wavelength of the laser 1 can be changed by controlling this injection current. In FIG. 5, an injection current 1 into the gain region 17 is shown.
．． The relationship between the ratio of the current I5 injected into the phase adjustment region 16 (to be precise, /x, +I) and the oscillation wavelength of the laser I is shown, and when necessary, the oscillation wavelength can be changed. Just change it.

Further, the distributed reflector 3 is driven by a constant current source 1o. This current injected into the distributed reflector 3 changes the center wavelength shown in FIGS. In the embodiment shown in Fig. 1, the transmitted light of the distributed reflector 3 (received by the light receiver 5, see Fig. 3) is used as the frequency reference. used as The differential waveform (obtained by the differentiator 19) of the wavelength-dependent curve of the light received by the light receiver 5 shown in FIG.
As shown in the figure, the signal detected by the light receiver 5 is transferred to the differentiator 1.
If negative feedback is applied to the active region 17 of the laser 1 so as to fix the center of the oscillation wavelength of the laser beam at the wavelength shown in FIG. 4, the frequency can be stabilized and the spectral purity can be improved. . That is, the more the center of the wavelength of the light species of the laser 1 matches the center wavelength in the second section and FIG. The above negative feedback is performed using (any one is fine).

To synchronize the selected wavelength of the distributed reflector 3 with the laser oscillation wavelength, the differential waveform (1
The DC component (obtained by the 1 divider 19) may be fed back to the constant current source 10, and the distributed reflector 3 may be controlled so that the signal of this DC component becomes O.

In other words, if the selected wavelength 11, E, matches the laser oscillation wavelength, the above differential waveform will be point symmetrical with respect to E in Figure 4, and the DC component will be 0, so feedback can be applied as described above. It is.

Frequency spread of laser 1! ll (wavelength stability, spectral purity improvement) will be explained in more detail.

Since the differential signal from the photodetector 5 detects the frequency fluctuation of the laser 1, it is amplified by the amplifier 12 and negatively fed back to the active region 17 of the laser 1 through the bias TL4 at an appropriate feedback rate. The band of the amplifier 12 should be about DC ~ 10 kHz to stabilize the center wavelength of the oscillation light, and about 10K) (z ~ 100 kHz to improve the spectral purity).
The frequency should be approximately MHz. It is also possible to control both at the same time by using multiple stages of amplifiers.

Since the light detected by the light receiver 4 has wavelength dependence as shown in Figure 2, the drive current of the distributed reflector 3 (this determines the angle in Figure 2) and the detection signal of the light receiver 4 must be matched. The oscillation wavelength of the laser 1 can be monitored.

On the other hand, since the signal from the photoreceiver 6 is obtained by detecting the power fluctuation of the laser beam, the optical output can be stabilized at the same time by providing negative feedback to the active region 17 of the laser 1 through the amplifier 1 and bias T14. By the above method, it is possible to achieve compactness (because the light receiver, distributed reflector, etc. are integrated with the laser emitting part), stabilize optical output (because the signal from the light receiver 6 is fed back negatively), and stabilize the frequency. A wavelength tunable distributed feedback laser was realized that achieved this (because the signal of the photoreceiver 5 was negatively fed back), and a wavelength tunable width of about 20 m and a spectral linewidth of several 100 kHz were obtained.

Next, the manufacturing process of the main part embodiment will be explained with reference to FIG. This embodiment employs a GaAs-based ridge laser structure. In FIG. 6(a), a lower cladding 121 of AlGaAs and an active layer 22 are placed on a GaAs substrate 20.
, light guide layer 23, upper cladding layer 24, cap layer 2
By patterning and etching the photoresist, laser 1 and light receiving part 4.5 are formed by epitaxial growth in the order of 5.
．． The epitaxial layer is removed leaving only the portion numbered 6.

Next, in FIG. 6Cb), the lower cladding layer, the optical guide layer, the upper cladding layer, and the cap layer are epitaxially grown in this order on the removed portion to form a regrowth region 26 and activate the distributed feedback laser l. Only the region 17 and the portion corresponding to the distributed reflector 3 are etched down to the light guide layer, and then a grating 27 with a predetermined pitch is formed. At this time, the grating pitches of the two regions are made the same.

In FIG. 6C, the upper cladding layer and the cap layer are regrown on the grating 27 to form the region 28. Subsequently, in FIG. 6(d), only the position relative to the isolator 2 is etched down to the substrate 2o to form a CdTe cladding layer 29, a CdTe/CdMnTe multiple quantum well (CMQW) waveguide 30, and a CdTe cladding M.
It grows in the order of 31.

Next, in FIG. 6(e), mesa etching is performed on the ridge structure, leaving a small portion of the upper cladding layer 24,
Etching is performed using FIB (focused ion beam) to form an integrated coupler 32 (portion in which a 45-degree mirror is formed), and a Si 02 film is formed. Thereafter, the SiOt on the upper part of the ridge is removed, electrodes are deposited, and the electrodes are further separated to complete the structure shown in FIG.

The circuit of the first section may be connected to the above structure, but a magnet is required to perform the function of an optical isolator that utilizes the magneto-optic effect during use.

In the embodiment shown in FIG. 1, feedback control to the laser 1 is performed by an external electronic circuit, but an amplifier may be integrated after the photoreceiver 5.6. This amplifier is suitable for GaAs-based FETs, etc. By integrating the amplifier in this way, the length of the feedback loop can be further shortened.
Feedback control to the laser 1 can be made more broadband. Moreover, it will be possible to further downsize the entire system.

By the way, although the above embodiments have been explained for GaAs/AlGaAs-based ridge type wavelength tunable semiconductor lasers, the present invention also applies to semiconductor lasers using other waveguide structures (buried type, rib type, etc.) and InP-based semiconductors. The invention is applicable.

, In addition, although the frequency-based distributed reflector 3 changes the selected wavelength by injecting a current and using a plasma effect, other methods (methods that utilize the quantum confined Stark effect (QCSE) by applying a reverse bias, etc.) But that's fine.

Furthermore, the wavelength tunable laser method is also fi! ! For example, a two-electrode distributed feedback laser having only an active region, or a distributed reflection laser without a grating in the active region (the propagation constant is changed by grating opening) may be used.

[Effects of the Invention] As explained above, the present invention provides a compact optical integrated wavelength device that can stabilize the frequency, have a narrow spectral linewidth, and stabilize the optical output simply by driving with a simple electronic circuit. Since the entire system that implements the tunable laser device has been miniaturized, the control band of feedback control is no longer limited due to the phase delay of the light, and the frequency of the laser oscillation light cannot be sufficiently controlled. A wavelength tunable semiconductor laser that can be used for communication, etc. can be provided.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the structure and drive system of an optically integrated wavelength tunable semiconductor laser device according to the present invention, FIG. 2 is a diagram showing the wavelength dependence of reflected light from a distributed reflector, and FIG. 3 is a diagram showing the wavelength dependence of reflected light from a distributed reflector. Figure 4 shows the wavelength dependence of transmitted light. Figure 4 shows the differential waveform of Figure 3. Figure 5 shows the relationship between injection current and oscillation wavelength of a wavelength tunable laser. Figure 6 (a) - (b) is a diagram showing the manufacturing process of the wavelength tunable semiconductor laser device according to the present invention,
FIGS. 7 and 8 are diagrams showing conventional examples. 1... Distributed feedback laser, 2... Optical isolator, 3... Distributed reflector, 4.5.6... Light receiver, 7... Integrated coupler, 8...・Leading wave path, 9・
. . . Constant current source for laser driving, 10. . . Constant current source for driving distributed reflector, 11. . . Amplifier for stabilizing optical output, 1
2... Frequency control amplifier, 14... Bias T, 16... Phase adjustment region of distributed feedback laser, 1
7... Active region of distributed feedback laser, 18...
Constant current source for driving phase adjustment region, 19...Differentiator, 2
0...GaAs substrate, 21...AlGaAs cladding layer, 22...AlGaAs active layer, 23...
・-AIGaAs light guide layer, 24...A I G
a As cladding layer, 25... GaAs cap layer, 26... cladding layer, optical guide layer, region regrown in this order, 27... grating, 28... Region where cladding layer and cap layer are regrown on grating, 29...CdTe cladding layer, 30...CdMnTe/CdTeMQW, 3
1---- ・CdTe cladding layer, 32...45 degree mirror formed part

Claims (1)

[Claims] 1. A wavelength tunable semiconductor laser and a frequency reference device to which light is coupled to the semiconductor laser are monolithically integrated,
The frequency reference device is configured to be selectively variable in wavelength in synchronization with the oscillation wavelength of the semiconductor laser, and the frequency fluctuation signal of the laser light from the frequency reference device is controlled by negative feedback to the semiconductor laser. An optically integrated wavelength tunable semiconductor laser device characterized in that it is configured to be able to control the frequency of laser oscillation light while changing its wavelength. 2. The laser device according to claim 1, wherein the negative feedback control stabilizes the frequency of the oscillation wavelength of the semiconductor laser by electrically applying negative feedback at a low frequency. 3. The laser device according to claim 1, wherein the negative feedback control improves the spectral purity of the semiconductor laser by applying negative feedback electrically at high frequency. 4. The integrated coupler is used to split the laser light, and the frequency reference device that receives one of the split lights detects the frequency fluctuation of the laser light, and the light receiver that receives the other of the split lights changes the optical output of the laser light. 2. The laser device according to claim 1, wherein the laser device is configured to simultaneously perform frequency control and optical output stabilization. 5. The laser device according to claim 4, wherein the light receiver for receiving the other of the branched lights is also monolithically integrated. 6. The laser device according to claim 1, wherein the laser device is configured so that the oscillation wavelength of the semiconductor laser can be monitored by a signal from the frequency reference device. 7. The laser device according to claim 1, wherein the wavelength tunable semiconductor laser is a three-electrode distributed feedback laser. 8. The laser device according to claim 1, wherein the frequency reference device is a distributed reflector, and its transmitted light or reflected light is used as a frequency fluctuation signal of the laser beam. 9. The laser device according to claim 1, further comprising an integrated amplifier for negative feedback of the frequency fluctuation signal to the semiconductor laser. 10. The laser device according to claim 1, further comprising an integrated optical isolator to prevent the return light from the frequency reference device from returning to the semiconductor laser.