JP3072124B2 - Optically integrated semiconductor laser device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser device, and more particularly, to an optical integrated semiconductor laser having a configuration capable of stabilizing light output, oscillation frequency or wavelength and improving spectral purity. Related to the device.

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

For this reason, distributed feedback (DFB) semiconductor lasers and distributed reflection (DBR) lasers that have achieved dynamic single mode (DSM) oscillation have conventionally used the following methods for stabilizing frequency and improving spectral purity. Such techniques have been proposed.

(1) Method for Stabilizing Frequency or Wavelength First, there is a method for precisely stabilizing the injection current to the laser and the temperature of the laser.

Next, there is a method of electrically negatively feeding the injection current to the laser using an external frequency reference. Clogging, part of the laser oscillation light is converted to Fabry-Perot etalon or molecular gas (Rb
Gas enters the cell Cs gases, etc. NH _3), by converting the fluctuation of the oscillation frequency of the laser to change the intensity of the transmitted light received by the light receiving unit, a low frequency signal of the light receiver (dc to 10kH
In z), the current returns to the laser injection current. That is, the low-frequency current is added to the injection current to obtain a laser injection current.

(2) Method of Improving Spectral Purity First, there is a method of placing a reflecting mirror outside the laser and performing optical negative feedback. As the reflecting mirror, there is a method of using a wavelength-selective mirror such as a diffraction grating, or a method of using a fiber resonator to increase the laser resonator length and increase the Q value.

Next, similarly to the above-mentioned frequency stabilization method, the injection current of the laser is electrically changed to a high frequency (10 kHz) using an external frequency reference.
There is a method of applying negative feedback at (z to 100 MHz).

Furthermore, the resonator length of a normal laser is about 300 μm, but there is a method of increasing this to about 1 mm to increase the Q value.

With these methods, the spectral line width is several tens kHz to
A few hundred kHz is currently available.

[Problems to be Solved by the Invention] However, the conventional technology has the following problems.

First, even if the injection current or temperature is precisely stabilized, there is a limit in stabilizing the oscillation wavelength.

In the case where an electric negative feedback is performed using an external frequency reference, it is difficult to reduce the size of the entire system as described above because there are a Fabry-Perot interferometer and a molecular cell that receive a part of the laser oscillation light. At the same time, when high-frequency negative feedback is performed, there is a problem that the optical path of the feedback loop is long, so that the control band is limited due to the phase delay of light. For example, if the total length of the field buck loop is 30 cm, the phase lag will be about 180 degrees in the band of about 100 MHz (wavelength is about 60 cm), and it will oscillate because it becomes positive feedback instead of negative feedback. I will. Therefore, it becomes difficult to improve the spectral purity.

When a diffraction grating, a fiber resonator, or the like is placed outside, these are vulnerable to mechanical vibration and the like, and cannot always be stably controlled. Further, this method cannot stabilize the center frequency even though the spectral purity can be improved.

Regarding the method of lengthening the laser cavity itself, the length is limited due to the balance with the loss, and the improvement of the spectral purity is limited. In addition, the center frequency cannot be stabilized with this method.

Therefore, an object of the present invention is to provide a small-sized optical integrated semiconductor laser device having a configuration capable of stabilizing an optical output and an oscillation frequency and narrowing a spectral line width in view of the above problems.

Means for Solving the Problems In order to achieve the above object, the present invention provides an optical integrated semiconductor laser device comprising: a semiconductor laser; a means for injecting a current into the semiconductor laser; A frequency reference device formed monolithically on the optical waveguide for transmitting light emitted from the semiconductor laser and a grating provided in the optical waveguide; and a semiconductor laser provided between the semiconductor laser and the frequency reference device. An integrated coupler that makes a part of the light emitted from the frequency reference device incident, and separates light reflected by the frequency reference device from light incident on the frequency reference device,
A first photodetector for detecting light transmitted through the frequency reference device, a second photodetector for detecting reflected light from the frequency reference device separated by the integrated coupler,
A feedback circuit that controls the oscillation frequency of the semiconductor laser by negatively feedbacking one of the output signals of the second and the second light receivers to a current injected into the semiconductor laser; and an output of the first and the second light receivers. Means for monitoring the wavelength of light emitted from the semiconductor laser using the other of the signals. By employing such a configuration, the wavelength monitor and the frequency control can both be performed with a simple configuration by effectively using the light emitted from the semiconductor laser.

 In this configuration, the following is also possible.

The feedback circuit has a differentiator for differentiating the output signal of the first light receiver, and the signal differentiated by the differentiator is injected into the semiconductor laser, or the integrated coupler emits light emitted from the semiconductor laser. Of the above, the remaining portion excluding the portion to be made incident on the frequency reference device is made to be incident on the third light receiver, and the output signal of the third light receiver is negatively fed back to the current injected into the semiconductor laser. The optical output of the semiconductor laser is controlled, or an isolator is provided between the semiconductor laser and the integrated coupler to prevent light reflected by the frequency reference device from being incident on the semiconductor laser. Or a distributed feedback laser.

In a more specific example, a `` waveguide having a grating in the traveling direction of light is provided as a frequency reference, and oscillating light emitted from one end of the laser is guided through an optical waveguide formed on a substrate. Incident on the grating area,
Since the grating has wavelength selectivity, a change in the wavelength of the laser oscillation light is converted into a change in the intensity of the transmitted light or reflected light of the grating, and this signal is detected by a light receiver formed on the same substrate, and this is detected. The signal is amplified by an amplifier and returned to the injection current of the semiconductor laser. With such a configuration, the length of the field-back loop becomes very short, the phase delay of light does not matter, and wide-band control becomes possible.

With such a configuration, a semiconductor laser which is small in size, resistant to mechanical vibration, and can easily stabilize the frequency and improve the spectral purity can be realized.

Embodiment FIG. 1 is a diagram showing a configuration of an embodiment of the present invention. In FIG. 1, reference numeral 1 denotes a distributed feedback (DFB) semiconductor laser that oscillates in a dynamic single mode, and extracts laser light from the left end face.

The semiconductor laser 1 also emits laser light from the right end face, 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 incident from the laser 1 and transmitted through the optical isolator 2 is incident on the integrated coupler 7 and is divided into two, and the component that passes through and travels straight is guided in the waveguide 8 along the traveling direction of the light. Incident on a distributed reflector having a grating. Of the incident light, light of a certain wavelength (wavelength other than the Bragg wavelength) passes through the distribution reflector 3 as it is and is received by the light receiver 5, and light of a certain wavelength (Bragg wavelength) is reflected and integrated. 7, the light is reflected here and received by the light receiver 4. Therefore, the amount of light received by each of the light receivers 4 and 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. That is, FIG. 2 shows the wavelength dependence of the reflected light intensity of the distributed reflector 3 (the light received by the light receiver 4), and FIG. 3 shows the wavelength of the transmitted light intensity (the light received by the light receiver 5). Dependencies are shown.

The other split light that has entered the integrated coupler 7 from the isolator 2 and has been reflected there is received by the light receiver 6. Therefore, the light quantity corresponding to the output of the laser oscillation light enters this light receiver 6.

 Next, the driving method of the present embodiment will be described.

The distributed feedback semiconductor laser 1 having the above configuration has a bias T1
4 through a constant current source 9. In addition, distributed reflector 3
Are driven by the constant current source 10. The center wavelength λ ₁ shown in FIGS. 2 and 3 can be changed by the injection current to the distributed reflector 3.

In the embodiment shown in FIG. 1, light transmitted through the distributed reflector 3 (received by the light receiver 4; see FIG. 2) is used as a frequency reference.
Since the wavelength dependence of the light received by the light receiver 4 is as shown in FIG. 2, it is necessary to fix the oscillation wavelength of the laser light at the wavelength λ ₀ which is the center of the negative slope in FIG. I do. The wavelength λ ₀ can be adjusted by changing the current injected into the distributed reflector 3. As a result, the frequency or wavelength fluctuation of the laser 1 can be detected by the light receiver 4.
That is, the center of the oscillation wavelength of the laser 1 is the center wavelength λ in FIG.
The change in the amount of light received by the light receiver 4 becomes maximum at the position where it coincides with ₀ , and the negative feedback is performed using this signal.

This signal is amplified by the amplifier 12, and a negative feedback is applied to the injection current of the laser 1 through the bias T14 at an appropriate feedback rate. The band of the amplifier 12 is set to about DC to 10 kHz if the center wavelength of the oscillation light is stabilized, and is set to about 10 kHz to 100 MHz to improve the spectral purity. It is also possible to use multiple amplifiers and control both at the same time.

Since the light detected by the light receiver 5 has a wavelength dependency as shown in FIG. 3, it can be used for wavelength monitoring for monitoring the current laser wavelength.

On the other hand, since the signal of the photodetector 6 is a detection of the power fluctuation of the laser beam, similarly, if the negative feedback is applied to the electrode of the laser 1 through the amplifier 11 and the bias T14, the light output can be stabilized at the same time.

By the above-mentioned method, it is small (because the light receiving unit, the distributed reflector, etc. are integrated with the laser light emitting unit), the light output is stabilized (since the signal of the light receiving unit 6 is negatively fed back), and the frequency is stabilized. (Because the signal of the light receiver 4 is negatively fed back), a distributed feedback laser as described above can be realized, and a spectral line width of several hundred kHz is obtained.

Next, the manufacturing process of this embodiment will be described with reference to FIG. In this embodiment, a GaAs ridge laser structure is employed. In FIG. 5 (a), on a GaAs substrate 20, Al
GaAs lower clad layer 21, active layer 22, light guide layer 23, upper clad layer 24, cap layer 25 are epitaxially grown in this order, and photoresist 1 is patterned and etched to form laser 1, light receiving sections 4, 5, and 6. The epitaxial layer is removed leaving only the epitaxial layer.

Next, in FIG. 5 (b), on the removed portion, a lower clad layer, an optical guide layer, an upper clad layer, and a cap layer are epitaxially grown in this order to form a regrown region 26. Only the portion corresponding to the distributed reflector 3 is etched to the light guide layer, and then a grating 27 having a predetermined pitch is formed. At this time, the grating pitch in the two regions is the same.

In FIG. 5C, an upper clad layer and a cap layer are regrown on the grating 27 to form a region 28. Subsequently, in FIG. 5D, only the position with respect to the isolator 2 is etched up to the substrate 20, and the CdTe clad layer 2 is etched.
9. CdTe / CdMnTe multiple quantum well (MQW) waveguide 30 and CdTe cladding layer 31 are grown in this order.

Next, in FIG. 5 (e), mesa etching is performed on the ridge structure while slightly leaving the upper cladding layer 24, and FI
The integrated coupler 32 (portion where a 45-degree mirror is formed) is formed by etching with B (focused ion beam), and an SiO ₂ film is formed. Thereafter, the SiO ₂ above the ridge is removed, electrodes are deposited, and the electrodes are further separated to complete the structure shown in FIG.

The circuit shown in FIG. 1 may be connected to the above structure, but a magnet is required to exhibit the function of an optical isolator utilizing the magneto-optical effect in use.

Next, a second embodiment according to the present invention will be described. FIG. 6 shows the structure and driving method of the device.

In the second embodiment, a signal for frequency control of the laser 1 is detected by the light receiver 5. At this time, the wavelength dependence of the transmitted light of the distributed reflector 3 is as shown in FIG. The differential waveform of this curve is as shown in FIG. Thus, through the differentiator 19 the signal detected by the light receiver 5 can perform the frequency stabilization secure the laser wavelength at a fourth view of a wavelength lambda _1, is more of the second embodiment, the As compared with the first embodiment, there is an advantage that a wavelength range in which a wavelength change can be detected can be widened. As a result, similarly to the first embodiment, it is possible to perform electrical negative feedback over a wide band for stabilizing the frequency of the laser beam and stably improving the spectral purity.

 Other points are the same as the first embodiment.

In the embodiment shown in FIGS. 1 and 6, the feedback control to the laser 1 is performed by an external electronic circuit.
An amplifier may be integrated after 5 or 6. As this amplifier, a GaAs FET or the like is suitable.

By integrating the amplifier in this way, the length of the feedback loop is further shortened, and the feedback control to the laser 1 can be further broadened. Further, the whole system can be further downsized.

In the above embodiment, the GaAs / AlGaAs ridge type tunable semiconductor laser has been described. However, the present invention is applicable to other waveguide structures (buried type, rib type, etc.) and semiconductor lasers using InP type semiconductors. The invention is applicable.

In the frequency-based distributed reflector 3, the selected wavelength is changed by the plasma effect by injecting a current, but another method (a method using a quantum confined Stark effect (QCSE) by applying a reverse bias) or the like may be used. .

[Effects of the Invention] As described above, according to the present invention, a small-sized optical integrated semiconductor that can stabilize the frequency, narrow the spectral line width, and stabilize the light output only by driving with a simple electronic circuit. A laser device is realized.

Since the whole system is miniaturized, the control band of the feedback control is not restricted due to the phase delay of light, so that the frequency control of the laser oscillation light cannot be sufficiently performed.
A semiconductor laser applicable to coherent optical communication or the like can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing a structure and a driving system of an optical integrated semiconductor laser device according to a first embodiment of the present invention, FIG. 2 is a diagram showing the wavelength dependence of reflected light of a distributed reflector, and FIG. FIG. 4 shows the wavelength dependence of the transmitted light of the reflector, FIG. 4 shows the differential waveform of FIG. 3, and FIGS. 5 (a) to 5 (e) show the manufacturing steps of the semiconductor laser device according to the present invention. FIG. 6 shows the structure of the second embodiment. 1 ... distributed feedback laser, 2 ... optical isolator, 3 ...
... distributed reflectors, 4, 5, 6 ... light receivers, 7 ... integrated couplers, 8 ... optical waveguides, 9 ... constant current sources for driving lasers,
10: constant current source for driving a distributed reflector, 11: amplifier for stabilizing optical output, 12: amplifier for frequency control, 14: bias T, 19: differentiator, 20: GaAs substrate, 21 AlGaAs cladding layer, 22 AlGaAs active layer, 23 AlGaAs light guide layer, 24 AlGaAs cladding layer, 25 GaAs cap layer,
26: a region in which the cladding layer, the light guide layer, and the cladding layer are regrown in this order; 27: a grating; 28: a region in which the cladding layer and the cap layer are regrown on the grating; 29
…… CdTe cladding layer, 30… CdMnTe / CdTeMQW, 31… Cd
Te clad layer, 32 …… 45 degree mirror formed part

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-262366 (JP, A) JP-A-62-158927 (JP, A) JP-A-63-64380 (JP, A) JP-A-2- 97914 (JP, A) JP-A-60-195985 (JP, A) JP-A-61-55985 (JP, A) JP-A-63-58982 (JP, A) (58) Fields investigated (Int. ⁷ , DB name) H01S 5/00-5/50

Claims (5)

(57) [Claims] 1. A semiconductor laser, means for injecting a current into the semiconductor laser, an optical waveguide monolithically formed on the same substrate as the semiconductor laser and transmitting light emitted from the semiconductor laser, and the optical waveguide A frequency reference device comprising a grating provided in the semiconductor laser and a frequency reference device provided between the semiconductor laser and a part of the light emitted from the semiconductor laser and incident on the frequency reference device, reflected by the frequency reference device An integrated coupler that separates light from light incident on the frequency reference device, a first light receiver that detects light transmitted through the frequency reference device,
A second photodetector for detecting reflected light from the frequency reference device separated by the integrated coupler; and one of output signals of the first and second photodetectors being negatively fed back to a current injected into the semiconductor laser. A feedback circuit for controlling the oscillation frequency of the semiconductor laser,
Means for monitoring the wavelength of light emitted from the semiconductor laser using the other of the output signals of the photodetector. 2. The feedback circuit according to claim 1, further comprising a differentiator for differentiating an output signal of the first light receiver, and negatively feeding back the signal differentiated by the differentiator to a current injected into the semiconductor laser. 2. The semiconductor laser device according to 1. 3. The integrated coupler causes the remaining portion of the light emitted from the semiconductor laser excluding the portion to be incident on the frequency reference device to be incident on a third light receiver, and the output signal of the third light receiver. 3. The semiconductor laser device according to claim 1, wherein the optical output of the semiconductor laser is controlled by negatively feeding the current to a current injected into the semiconductor laser. 4. 4. An isolator for preventing light reflected by a frequency reference device from being incident on a semiconductor laser between said semiconductor laser and an integrated coupler. Semiconductor laser device. 5. The semiconductor laser device according to claim 1, wherein said semiconductor laser is a distributed feedback laser.