JP3220259B2 - Laser device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-power single-mode oscillation semiconductor laser having a wavelength tunable function which is useful as a light source for coherent optical transmission and optical measurement.

[0002]

2. Description of the Related Art In recent years, practical use of a coherent optical transmission system has been awaited for expansion, subdivision, and high integration of an optical communication network. Semiconductor lasers as coherent light sources, which will be the key device of this new technology for controlling light at the frequency level, have very high expectations, but there are still many issues to be solved in the future. is the current situation. In the optical heterodyne system, which is regarded as one of the promising coherent optical transmission systems, the signal light on the transmitting side interferes with the local light on the receiving side to extract the signal from the interference component. It is possible to transfer a plurality of different signals simultaneously.
Therefore, the laser used as a light source must be capable of high-power single longitudinal mode oscillation, be capable of frequency tuning sufficiently fast, and have a spectral line so that signals transmitted in one channel are more accurate and dense. There is a demand that the width is as narrow as possible and that the variable width of the wavelength is as wide as possible so that many channels that can be transmitted by one light source can be secured. These requirements remain the same even when the semiconductor laser is used as a light source for optical measurement, merely by substituting the names such as resolution, measurement band, and measurement speed.

(Prior Art 1) A conventionally known semiconductor laser will be described below. A well-known wavelength-tunable single longitudinal mode oscillation laser is a distributed Bragg reflection laser (hereinafter referred to as a DBR laser) in which a diffraction grating having a uniform pitch Λ is provided in a wavelength control region coupled to a light emitting region. ). Since the diffraction grating having the pitch Λ reflects only the wavelength of λ = 2nΛ (n is the equivalent refractive index of the waveguide), a single longitudinal mode oscillation can be obtained by using this, and the carrier can be added to the wavelength control region. Implantation can change the wavelength by the plasma effect. However, in the reflection caused by a diffraction grating having a uniform pitch, the diffraction wavelength is limited to only one value, and the amount of change in the equivalent refractive index due to the plasma effect is at most several percent. However, it is about 10 nm.

(Prior Art 2) On the other hand, as a method of performing single mode selection without using a diffraction grating, there is a so-called composite resonator laser having a Y-shaped waveguide whose top view is shown in FIG. This laser guides light emitted from the waveguide of one light emitting region 3 to two waveguides of the wavelength control region 21 via the coupler 16 to form two resonance means. Two types of Fabry-Perot modes are obtained by two resonance means, and only a mode having a wavelength overlapping among these modes oscillates. The wavelength can be changed by passing a current through the waveguide in the wavelength control region 21 and changing the effective resonator length of each to change the wavelength at which the mode overlaps. According to this laser, a wide range of modes can be selected and a sufficient output can be obtained, but the interval between adjacent modes in the Fabry-Perot mode is extremely small (usually about several angstroms), and the laser oscillates in a plurality of modes simultaneously. It often happens.

(Prior Art 3) Therefore, as a method for obtaining a mode selection in a wide range while providing a single wavelength selectivity by a diffraction grating, a distributed reflector is not provided with a uniform pitch of the diffraction grating. There is an SSG-DBR laser using a super-periodic structure diffraction grating 22 (hereinafter, referred to as SSG) in which chirp is changed from a pitch Λa to a pitch Λb, and the chirp is repeated at a super-period Λs. As shown in a perspective view in FIG. 8, two types of SSGs having different super-periods are combined on both sides of the light emitting region 3, and inside each SSG,
A plurality of reflection modes exist with an antagonistic reflectance between the oscillation mode wavelength λa corresponding to the maximum pitch Λa and the oscillation mode wavelength λb corresponding to the minimum pitch Λb. Since the reflection mode interval is determined in proportion to the reciprocal of the super period Λs, by changing the super period Λs of the two types of SSGs, only one mode coincides and oscillation occurs. Furthermore, when a current is injected into the SSG to change the effective pitch uniformly, another mode coincides, and the oscillation wavelength changes. According to this method, the normal gain band of the semiconductor laser is 100 nm.
It is said that wavelength sweeping over almost the entire range is possible. However, in a structure in which two SSGs are provided on both sides of the light emitting region to constitute one resonator, the laser light is emitted from two SSGs.
G is performed through one of G
The output is suppressed by the high reflectivity of G,
W is at the limit.

(Prior Art 4) Although a wavelength sweep in a wider band cannot be expected as in the SSG-DBR, a DBR laser using a sampled grating is also conceivable as a reflector that can obtain substantially the same effect. However, also in this case, the structure as a laser is almost the same as the DBR laser using the SSG of FIG. 8, except that the distributed Bragg reflector is merely replaced by the sampled grating from the SSG, and the light passes through the reflector. Since the light is emitted, the disadvantage that the output is small is inevitable.

[0007]

A distributed reflection laser using a distributed reflector having only a single reflection peak as described in the prior art 1 has a large output and good single-mode characteristics. However, since the amount of change in the wavelength is only proportional to the amount of change in the refractive index, it is not possible to expect a wide band variable of several tens nm. Further, in the so-called composite resonator laser having a Y-shaped waveguide without a diffraction grating as described in the prior art 2, although a wide-band wavelength change is possible and the output is large, single-mode selectivity is insufficient. is there. Further, as described in prior arts 3 and 4, a DBR provided with SSG or sampled grating as a reflector on both sides of a light emitting region
In a laser, single mode oscillation with a large side mode suppression ratio can be obtained over a wide wavelength variable range.
There is a problem that the output is insufficient because the light is emitted through the reflector. That is, an object of the present invention is to obtain a laser having a wide-band wavelength tunable characteristic, a large output, and a high single-mode selectivity.

[0008]

Means for solving the above problems will be described below.

In the first aspect of the present invention, the following means are employed to improve the selectivity of a single mode. That is, a laser device that emits light emitted from a light emitting region through a resonance means has an output end portion (20) having partial transmission / partial reflection characteristics and a plurality of reflection wavelengths reflected at predetermined wavelength intervals. A first distributed reflector (4a) provided with a first diffraction grating which forms a resonance means for resonating light of a first mode together with the output end, and a wavelength interval different from the predetermined wavelength interval. A second distributed reflector (4b) provided with a second diffraction grating, which comprises a resonance means for resonating light of a second mode having a plurality of reflection wavelengths reflected by the output end together with the second end; A coupler (16) for coupling the light of the first and second modes, and a first and a second for controlling the phase of the light diffracted by the first and second distributed reflectors, respectively. Phase control means (17a, 17
b) and wavelength control means (18a, 18b) for controlling the wavelength by changing the refractive index of the first and second distributed reflectors.

According to the invention described in claim 2, in the two distributed reflectors 4a and 4b described in claim 1, each of the distributed reflectors 4a and 4b has a plurality of diffraction wavelengths at appropriate intervals. Means for generating a temperature distribution having different periods in the waveguides of the reflector and repeating the periods twice or more is provided.

According to a third aspect of the present invention, in the two distributed reflectors 4a and 4b according to the first aspect, the distributed reflectors 4a and 4b are arranged such that each has a plurality of diffraction wavelengths at appropriate intervals. Have a chirped pitch that repeats with different periods.

In the invention of claim 4, the following means are employed. That is, the first distributed reflector has a structure in which a waveguide portion where a diffraction grating is present and a waveguide portion where no diffraction grating is present are repeated at least twice with a first period, and a plurality of reflections are reflected at a predetermined wavelength interval. Having a wavelength, the second
Has a structure in which a waveguide portion where a diffraction grating is present and a waveguide portion where no diffraction grating is present are repeated at least twice with a second period, and reflects at a wavelength interval different from the predetermined wavelength interval. The laser device according to claim 1, wherein the laser device has a plurality of reflection wavelengths.

In the invention described in claim 5, the heating means is used for the phase control means for controlling the phase and the wavelength control means for changing the refractive index of the distributed reflector.

[0014]

The operation will be described below for each claim. According to the first aspect of the present invention, the light emitted by the current injected into the light emitting region is split into two by the coupler, and the specific mode is selected by the first and second distributed reflectors respectively. The light is diffracted and further combined into one light by passing through the coupler. At this time, the first and second
Out of the diffracted light of the distributed reflector, laser oscillation occurs at a wavelength where both wavelengths coincide. When the refractive index of each distributed reflector is appropriately changed by the respective wavelength control means, the diffraction wavelength of each distributed reflector shifts, and coincidence occurs at a different wavelength from the above, and the oscillation wavelength changes. However,
Coherent light coupling requires Fabry-Perot mode coincidence in the two resonators, which is provided in each phase control region by the phase of the light diffracted by the first and second distributed reflectors. The first for controlling each
And the second phase control means.

According to the second aspect of the present invention, a distributed reflector having a diffraction grating having a uniform pitch is heated by a heating means, and a periodic temperature distribution is formed in the waveguide, so that an effective diffraction grating is obtained. The period can be modulated so that one reflector has multiple diffraction wavelengths at appropriate intervals. Since the Bragg wavelength of the diffraction grating having the pitch Λ is λ = 2nΛ, and the equivalent refractive index n changes with temperature, the Bragg wavelength of each diffraction grating changes from λa corresponding to the maximum temperature Ta in the temperature distribution to the minimum temperature Tb. It is distributed as a plurality of peaks in the band up to the corresponding λb. Assuming that the temperature distribution has a period of Δs, the diffraction peaks of the diffraction grating are distributed at intervals of Δλ = λ02 / 2nΛs.
If two or more diffraction peaks of two reflectors coincide at the same time, single mode oscillation does not occur, so that Δs of the two reflectors must be different.

According to the third aspect of the present invention, since the diffraction gratings themselves inside the distributed reflectors 4a and 4b have a chirp structure, they have a plurality of diffraction wavelengths, and the chirp period (super period) Λs is Since the first distributed reflector and the second distributed reflector are different from each other, oscillation occurs only in the mode having the same wavelength among the plurality of reflection modes. Further, by making full use of the wavelength control means to change the mode in which this wavelength coincides, the oscillation wavelength changes significantly.

According to the fourth aspect of the present invention, in the distributed reflectors 4a and 4b, the area where the diffraction grating exists and the area where the diffraction grating does not exist alternately with a constant period Δs, and the first distribution Since the Δs inside the Bragg reflector and the Δs inside the second distributed Bragg reflector have different values, it has the same effect as the second aspect of the present invention.

According to the fifth aspect of the present invention, since the refractive index of the waveguides of the distributed reflectors 4a and 4b and the phase control regions 11a and 11b is heated, it is inevitable in the system utilizing the plasma effect. There is no significant loss reduction or spectral line width degradation.

In any of the above inventions, the laser device has the output end portion 20 having partial transmission / partial reflection characteristics, so that the emitted light is emitted with sufficient intensity.

[0020]

【Example】

(Embodiment 1) An embodiment of the present invention according to claims 1 and 3 will be described with reference to FIGS. 1 and 2. FIG. FIG. 1 (A)
FIG. 1B is a top view, and FIG. First, the manufacturing method will be described. 1. On a p-type InP substrate 1,
An InGaAsP active layer 2 (not shown) having an energy gap of 55 μm is grown, and all of the active layer 2 is removed except for the waveguide portion of the light emitting region 3 by etching. Subsequently, a pitch of 6 is added to a portion to be the distributed reflector 4a.
A diffraction grating 5a chirped from 2460 ° to 2380 ° at a period of 7.5 μm, and a diffraction grating 5b chirped from 2460 ° to 2380 ° at a period of 75 μm at a portion to be the distributed reflector 4b, both using an electron beam drawing method. Then, in the portion where the active layer 2 was removed, 1.
After growing an InGaAsP guide layer 6 having an energy gap of 3 μm band, an InP cladding layer 7 is formed on the entire surface.
Grow.

The structure of this diffraction grating (called a super-periodic structure)
Is schematically shown in FIG. The super-periodic grating has a plurality of diffraction wavelengths at an interval inversely proportional to Δs in a wavelength range from a diffraction wavelength λa corresponding to the maximum pitch Δa to a diffraction wavelength Λb corresponding to the minimum pitch Λb.
Thereafter, a Y-shaped waveguide coupled by the coupling section 16 is formed by mesa etching, and this waveguide is
P current blocking layer and p-type current blocking layer (not shown)
To perform current confinement and lateral confinement of light. Further, an n-type electrode 10a for injecting a current into the light emitting region 3, and a wavelength control means 1 for changing the refractive index of the distributed reflectors 4a, 4b and the phase control regions 11a, 11b.
8a, 18b and n-type electrodes 10b, 10c necessary for flowing current for the phase control means 17a, 17b are formed on the upper surfaces of the respective cladding layers by vapor deposition. Thereafter, the substrate is polished to a thickness of 100 μm, and a p-type electrode 10d is formed on the polished surface by vapor deposition and alloying. In the case of the resonating means in which the distributed reflectors 4a and 4b are the reflecting surfaces on one side of the resonator and the output end 20 is the reflecting surface on the other side, the effective optical length of the resonating means is the distributed reflectors 4a and 4b. When a plurality of resonance means are formed using two distributed reflectors, it is necessary to match the Fabry-Perot modes inherent to the two resonance means.
In this embodiment, an electrode is provided on the upper surface of the waveguide in the phase control region, and current is injected as phase control means 17a and 17b to change the refractive index of the waveguide in the phase control regions 11a and 11b. The phase control regions 11a and 11a are heated by an Au thin film resistor as in the invention according to claim 5.
The refractive index of the waveguide b can be changed.

(Embodiment 2) Next, an embodiment of the present invention will be described with reference to FIG. FIG. 3 (A)
FIG. 3B is a top view, and FIG. The distributed reflectors 4a and 4b according to the first aspect of the present invention each have a diffraction grating having a uniform pitch Λ, these waveguides have different periods, and the period is repeated twice or more. In order to generate a temperature distribution, the wavelength control means 18a, 18a,
An Au thin-film resistor 13 is formed via an electrode 10c for 8b and an insulating film 12 of SiO2 for insulation. A
The u thin film resistor 13 is composed of two pad portions for connecting wires for passing a current and a stripe portion that generates heat. However, in the first distributed reflector 4a and the second distributed reflector 4b, it is necessary to change the cycle of the calorific value distribution of the Au thin film resistor 13 so that the cycle of the generated temperature distribution is different. . In the example shown here, a large number of Au thin film resistors 13 are arranged at different intervals, respectively, so that the calorific value distribution is set at different intervals.

FIG. 4 shows an example of the temperature distribution of the waveguides of the distributed reflectors 4a and 4b according to the second embodiment of the present invention, in which the Au thin film resistor 13 has a thickness of 1000 °, a stripe width of 10 μm, and a length of 100 μm. Shows simulation results by numerical analysis when power is applied by 0.2 W or 0.4 W, respectively. The coordinates on the horizontal axis indicate relative positions. If the temperature difference is 10 ° C., the effective Bragg reflection wavelength differs by about 1.1 nm.

(Embodiment 3) Next, an embodiment of the present invention will be described with reference to FIG. By forming the diffraction grating inside the distributed reflector as a sampled grating whose schematic diagram is shown in FIG. 5A, the same function as the invention described in claim 2 can be obtained. FIG. 5B shows the sampled grating shown in FIG. 5A with Λ = 2350２３, equivalent refractive index n = 3.6875, Λs = 45 μm, Z = 5 μ.
m is a diagram showing calculated values of reflection characteristics of the distributed reflector when the area length of the distributed reflector is 500 μm. 1.
This indicates that a mode selection of about 60 nm with 55 μm as the center is possible. Super period of this diffraction grating Λ
First and second distributed reflectors having different s are formed.

(Embodiment 4) Next, an embodiment of the first, second and fifth aspects of the present invention will be described with reference to FIG.
FIG. 6A is a top view of the distributed reflector and the phase control region, and FIG. Distributed reflector 4b
The Au thin film resistor 13 is not shown in order to make the upper part of FIG. In the embodiment described above, the distributed reflector 4
In order to control the refractive indices of the waveguides a and 4b and the phase control regions 11a and 11b, a current injection electrode is formed above the waveguide and current is injected. However, in this case, there remains a problem that the spectral line width is deteriorated due to an increase in loss and fluctuation of the refractive index. Therefore, Au thin film resistors 14a and 14b are formed as heating means 14 on the upper surfaces of the phase control regions 11a and 11b and the distributed reflectors 4a and 4b via an insulating film 12 of a SiO2 film for insulation. In this embodiment, as in the second embodiment, an Au thin-film resistor 13 is further provided via an insulating film 12 of SiO2 and heated to generate a temperature distribution in the waveguide. The Au thin film resistor 14a serves as wavelength control means 18a and 18b for controlling the wavelength by changing the refractive index of the distributed reflector to change the refractive index of the distributed reflector. The Au thin film resistor 14
b is phase control means 17a and 17b for controlling the phase of light diffracted by the distributed reflector, respectively.
As a result, the threshold current 15
A high output of 10 mW was obtained at mA and 100 mA, and a variable width of 40 nm or more was secured. Moreover, since the change in the refractive index due to heat hardly increases the loss of the waveguide, the drive current increases by at most 2 even in the wavelength sweep in the constant output operation.
It was reduced to about a percentage. Further, the side mode suppression ratio was always 30 dB or more, and good single mode characteristics were obtained. As described above, the scale in any of the figures is arbitrary unless specified, and the width of the waveguide, the shape of the diffraction grating,
Fine parts such as the stripe width of the thin film resistor are highlighted. In addition, the method of forming the composite resonator is not limited to the Y-shaped coupler, but also includes a method of forming and coupling a reflection surface on a semiconductor substrate using a crystal plane, and a method of using a hybrid method. It is intended to fall within the scope of the claims of the present invention.

[0026]

According to the first, second, third, and fourth aspects of the present invention, it is possible to select a broadband oscillation wavelength mode comparable to the entire gain band of the semiconductor laser, and to output a normal semiconductor laser. High output comparable to that obtained. According to the fifth aspect of the present invention, since the heating means is used as the wavelength control means and the phase control means, problems such as an increase in loss and a deterioration in spectral line width do not occur.

[Brief description of the drawings]

FIG. 1 is a top view and a sectional view of an embodiment according to claims 1 and 3 of the present invention.

FIG. 2 is a schematic diagram illustrating a diffraction grating structure according to claims 1 and 3 of the present invention.

FIG. 3 is a top view and a cross-sectional view of the first and second embodiments of the present invention.

FIG. 4 is an example of a simulation result of a temperature distribution in a waveguide generated by a heating unit.

FIG. 5 is a schematic diagram of an embodiment according to claims 1 and 4 of the present invention, and a diagram showing calculated values.

FIG. 6 is a top view and a sectional view of the first, second, and fifth embodiments of the present invention.

FIG. 7 is a diagram illustrating a conventional technique.

FIG. 8 is a diagram illustrating a conventional technique.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Active layer 3 Light emitting area 4a Distributed reflector 4b Distributed reflector 5a Diffraction grating 5b Diffraction grating 6 Guide layer 7 Cladding layer 10a Electrode 10b Electrode 10c Electrode 10d Electrode 11a Phase control area 11a Phase control area 12 Insulating film 13 Au thin film resistor 14 Heating means 14a Au thin film resistor 14b Au thin film resistor 16 Coupler. 17a Phase control means 17b Phase control means 18a Wavelength control means 18b Wavelength control means 20 Output end 21 Wavelength control area 22 Super periodic structure diffraction grating.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-289982 (JP, A) JP-A-64-49293 (JP, A) JP-A-3-286587 (JP, A) JP-A 64-64 54790 (JP, A) JP-A-58-206183 (JP, A) JP-A-62-287683 (JP, A) JP-A-63-160391 (JP, A) (58) Fields investigated (Int. ⁷ , DB name) H01S 5/00-5/50 JICST file (JOIS)

Claims (5)

(57) [Claims] 1. A laser device for emitting light emitted from a light emitting region through a resonance means, comprising: an output end having partial transmission / partial reflection characteristics; and a plurality of reflections reflecting at predetermined wavelength intervals. First with wavelength
A first distributed reflector (4a) provided with a first diffraction grating, which constitutes a resonance means for resonating light of the first mode together with the output end, and reflects at a wavelength interval different from the predetermined wavelength interval. A second distributed reflector (4b) provided with a second diffraction grating that constitutes a resonance unit for resonating light of a second mode having a plurality of reflection wavelengths together with the output end; A coupler (16) for coupling the light of the second mode, and first and second phase controls for controlling the phases of the light diffracted by the first and second distributed reflectors, respectively. Means (17a, 17b), and changing the refractive index of the first and second distributed reflectors to the first
Wavelength of both the mode light and the second mode light
A laser device comprising wavelength control means (18a, 18b) for controlling the wavelength so that laser oscillation occurs at the same wavelength. 2. A distributed reflector comprising: a first distributed reflector having a first period on a waveguide; and means for generating a temperature distribution in which the period is repeated at least twice. 2. The laser device according to claim 1, further comprising means for generating a temperature distribution having a second period on the waveguide and repeating the period two or more times. 3. The first diffraction grating has a chirped pitch that repeats with a first period, and the second diffraction grating has a second pitch.
2. The laser device according to claim 1, wherein the laser device has a chirp-like pitch that repeats at a period of: 4. The first distributed reflector has a structure in which a waveguide portion where a diffraction grating is present and a waveguide portion where no diffraction grating is present are repeated twice or more with a first period, and a predetermined wavelength interval is provided.
In has a plurality of reflection wavelengths for reflecting, with a structure in which a waveguide section in which the second distributed reflector does not exist with the existing waveguide portion of the diffraction grating is repeated two or more times with a second cycle Different from the predetermined wavelength interval
With multiple reflection wavelengths reflected at different wavelength intervals
2. The laser device according to claim 1, wherein: 5. A laser device according to claim 1, wherein a heating means is used for said phase control means and said wavelength control means.