JPS63299291A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To make an oscillation wavelength variable by mutually adjusting a refractive index of a region by a method wherein a light-emitting region and a waveguide region are integrated and a filter region equipped with a diffraction grating having a filtering function is installed in one part of the waveguide region or one part extended from the region. CONSTITUTION:A laser resonator is composed of a light-emitting region A, a non-light- emitting region (a waveguide region B and a filter region C) and a pair of reflection end faces 12, 12'; there are a number of resonance modes in accordance with a resonator length L which is an effective device length experienced when the light is propagated essentially. A wavelength width depends on a coupling coefficient and a length of a diffraction grating, on a phase shift amount, on a phase shift position and the like; it is possible to pass only one resonance wavelength lambda0 selectively. If a cycle of the diffraction grating 3 is selected, only a wavelength of a resonance wavelength lambda0 is confined strongly inside a resonator; as a result, it is possible to obtain an oscillation at a single wavelength of the resonance wavelength lambda0. By using a semiconductor laser it is possible to satisfy a requirement to operate at a single wavelength and to lengthen the resonator length; it is possible to realize a narrow oscillation line width by the long resonator length.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a semiconductor laser having a narrow oscillation linewidth and a variable oscillation wavelength.

(Prior Art) Semiconductor lasers are already in practical use as light sources for optical fiber communications because they are small, highly efficient, and highly reliable. The systems that have been put into practical use thus far take advantage of the fact that direct modulation, which is one of the major features of semiconductor lasers, is variable. There is a so-called direct intensity modulation-direct detection (DIM-DD) method in which the light is directly received by a photodiode or an avalanche photodiode after propagation. In the DIM-DD system, a distributed feedback type (DFB) is used as a light source that stably operates at a single wavelength even during high-speed modulation.
Dynamic single-wavelength lasers, such as semiconductor lasers, have been developed to reduce the dispersion effects of single-mode fibers and extend repeater spacing.

On the other hand, by actively utilizing the wave properties of light, such as its frequency and phase, for detection, the reception intensity can be greatly improved, making it possible to further lengthen the repeater interval compared to the DIM-DD method. It is. This method is called a coherent transmission method, and in recent years, it has been actively researched not only theoretically but also experimentally as a future optical communication method, and its usefulness is being confirmed (for example, T, 0koshi; J
our own of Lightwave Techno
logy, vol, LT-2, PP, 341-346
．． (see 1984).

Due to the nature of the coherent transmission system, it is essential that the line width of the light source on the transmitting side and the light source as a local oscillator on the receiving side be narrow, and that the oscillation wavelength be variable. In research at the laboratory stage so far, the main challenge has been to evaluate the potential of the system, so an external diffraction grating has been installed on a gas laser with an extremely narrow oscillation linewidth, or on a more practical ordinary semiconductor laser. However, by feeding back only a specific wavelength to the semiconductor laser, the above-mentioned high coherence and tunability of the oscillation wavelength have been achieved.

However, since the light emitting region of a semiconductor laser is extremely small with a diameter of approximately 1 μm, such a structure in which the light source and external diffraction grating are not integrated is susceptible to mechanical vibrations and thermal fluctuations (
It is obvious that this method is not practical because the desired characteristics tend to become unstable and the device becomes bulky.

One effective way to reduce the oscillation linewidth is to lengthen the laser cavity length.
An integrated semiconductor laser with a Nagai oscillator structure in which the waveguide region B was monolithically integrated was investigated by T., Fujita et al.
Low values of z have been reported (Electroni
cs Letters, vol, 21. pp, 374-
376.1985).

In FIG. 1, l is an InGaAsP light emitting layer, and 2 is an InG
InGaAsP provided on the extension of aAsP emission N1
The waveguide layer 12 is a metal film that increases the reflection efficiency at the cleavage plane.

However, in general, as the resonator becomes longer, the resonant wavelength interval also becomes narrower, which has the disadvantage that multi-wavelength oscillation tends to occur and narrow line width characteristics tend to become unstable. Furthermore, since wavelength tuning involves selecting discrete resonant wavelengths, it is not continuous and has the disadvantage of being unsuitable for practical use.

(Objects and Features of the Invention) The present invention has been made in view of the above-mentioned drawbacks of the prior art, and an object of the present invention is to provide a semiconductor laser that is used as a single wavelength light source with a narrow line width and a variable oscillation wavelength. purpose.

A feature of the present invention is that the light emitting region and the waveguide region are integrated, and a filter region having a diffraction grating with a filter function is provided on a part or an extension of the waveguide region, so that only one resonant wavelength can be detected. As a result, it is possible to stably operate at a single wavelength with a narrow oscillation linewidth, and at the same time, the oscillation wavelength can be made variable by mutually adjusting the refractive index of the filter region and the waveguide region.

(Structure and operation of the invention) The present invention will be described in detail below using the drawings.

(Example 1) FIG. 2 is a cross-sectional view of an example according to the present invention, which will be explained using an InGaAsP waveguide layer system as an example of the material. In the figure, 1 is InG forming the light emitting region A.
The aAsP light emitting layer 2 is for forming a waveguide region, and the InGaAsP waveguide layer 3 is a characteristic feature of the present invention, which has low loss for the emission wavelength and is optically coupled with the light emitting layer 1 with high efficiency. This is a band-pass type diffraction grating filter that forms a filter region C, which is a so-called phase shift type diffraction grating in which the phase of the diffraction grating is shifted near the center. The λ/4 shifted diffraction grating, which is a quarter of the wavelength, has excellent bandpass properties. Similarly, 4 is a 1-1nP substrate, 5 is a p
-InP layer, and 6 is a p-InGaAsP layer. Also,
7°8.9 is the p-side electrode of each region A, B and C,
Each is electrically isolated by a high resistance region 10 by proton implantation or etching. 11 is an n-side electrode, and 12.12° is a metal film for increasing the reflectance of the end face, which is provided with an insulating film 13 interposed therebetween. Further, in the figure, Il, , / and l indicate the respective lengths of the light emitting region A, the filter region C, and the waveguide region B, and L indicates the resonator length, respectively.

Next, the basic operation of the present invention will be explained using FIG.

The laser resonator of the present invention includes a light emitting region A, a non-optical leading region waveguide region B, a filter region C), and a pair of reflective end faces 12.
．． The resonator length, which is essentially the effective device length experienced by light propagation, is the sum of each region A, B, and C ( Il, +za+1t3), but here there are many resonance modes as shown in the figure (bl) depending on the insulating film 13 through which light is effectively transmitted.For example, L-1m and Then, the mode spacing Δλ is approximately 3, but since the light emission N1 has a gain distribution as shown in FIG. On the other hand, the phase shift type diffraction grating 3 has a steep bandpass characteristic at the center wavelength λ as shown in FIG. Although it depends on the phase shift position, etc., it can be designed to selectively pass only one resonant wavelength λ0.Therefore, the center wavelength λ can be set as the resonant wavelength (oscillation wavelength) λ near the gain peak. By selecting the period of the diffraction grating 3 to match ., only the wavelength of the resonant wavelength λ is strongly confined within the resonator.
As a result, as shown in the same figure (a), the resonant wavelength λ. single wavelength oscillation can be obtained. Moreover, since the semiconductor laser of the present invention can simultaneously satisfy single wavelength operation and a long cavity length, a narrow oscillation linewidth can be realized by increasing the cavity length.

On the other hand, the oscillation wavelength λ. meets the phase condition depending on the resonator length, and also coincides with the center wavelength λ8 of the phase shift diffraction grating 3. Now, the length and refractive index of the light emitting region A, filter region C9 and waveguide region B in FIG. 2 are L and lx, respectively.
, i3 and nl+nZ+n, and assuming that the period of the diffraction grating 30 is 8, λyaw2Δn2 and λ. = λ Akane is satisfied. Here, when a current is injected into the filter region C, n
2 decreases by Δnz, the center wavelength also becomes shorter, and if the center wavelength at that time is λ°, then λ. ≠λ゛, but at the same time, by changing the amount of current injected into each waveguide region B or light emitting region A and adjusting the refractive index n3゜n of each region, the resonant wavelength can also be changed, If the resonance wavelength at that time is λ”, then λ”. −λ″, can be maintained.Incidentally, the adjustment amounts of the refractive index are Δns and Δ
n, then the conditional expression for maintaining λ''.-λ゛3 is as follows.In other words, by changing each current amount so as to satisfy the above expression, the oscillation wavelength λ. By the way, the wavelength variable range is the refractive index change amount Δn of the filter region C,
and the gain wavelength width of the light emitting region A, but the former is generally smaller. Therefore, consider a case where a change in refractive index caused by a change in carrier density due to current injection is utilized, for example.

The wavelength variable range Δλ8 is given by Δλ8−2ΔΔn8. Here, Δn2 is the amount of change in the refractive index of the filter region C, and is related to the carrier density change ΔN by Δng −sΔN2. However, the coefficient S is S=-7Xl0-''cm' for InGaAsP-based materials.

Now, ΔN! = 2 xlO” low 3, then Δλ3
It can be seen that the oscillation wavelength can be changed over a range of = -65.8 people. Also, at this time λ. The amount of change in carrier density ΔN to obtain the change in refractive index of waveguide region B necessary to maintain =λ1 is the length of the light emitting region! .

and waveguide area length! , each it =20011rs,
If ll5=1500 and cr m, ΔNs
=2.1×10”cs+-3, which is a fully achievable value.

By the way, in the transmission characteristic of the phase shift type diffraction grating 3 shown in FIG. 3(a), some modes included in the low transmittance wavelength band on both sides of the center wavelength λ8 are reflected in opposite directions, and the reflection end face on the right side in FIG. There is a risk that the diffraction grating 3 will resonate and oscillate, but this is because the diffraction grating is 3. Alternatively, as shown in Fig. 4, the diffraction grating 3' may be formed at a slight angle of approximately 10" with respect to the direction of the light's temple, and wavelengths other than the center wavelength λ0 may be placed outside the stripe. The oscillation is suppressed by increasing the threshold of these unnecessary wavelengths by scattering them.

In the above description, the structure in which the light emitting region A, the waveguide region B, and the filter region C are arranged sequentially has been described, but the light emitting region A may be arranged in the center, and a plurality of waveguide regions B and filter regions C may be arranged. A built-in structure may also be used.

The same band-pass filter characteristics as in the first embodiment can also be achieved by using a uniform diffraction grating instead of the phase-shifted diffraction grating 3. The semiconductor laser structure in this case is basically the same as that shown in FIGS. 2 and 4, and the filter region C
The difference is that the diffraction grating is a uniform diffraction grating. The uniform diffraction grating has no phase shift, and the wavelength dependence of its transmittance is shown in Figure 5(a).The period of the zero diffraction grating is Δ
If the refractive index of the waveguide in filter region C is n2, the center wavelength λyo at which the transmittance is lowest is λ, -280 g, but due to the interference effect of light waves, wavelengths on both sides of λ3 are transmitted. There is a peak in rate. For example, the transmittance peak wavelength λ゛ on the long wavelength side is one of the resonant wavelengths λ.

By matching the gain peak wavelength and the gain peak wavelength, the wavelength λ is changed as shown in FIG. 5(d) similarly to FIG. Single wavelength oscillation with a narrow linewidth can be obtained. As described in FIG. 2, this wavelength can be tuned by adjusting the refractive index of each region.

(Example 2) FIG. 6 shows a second example according to the present invention, and the difference from Example 1 is that a distributed reflector is used as a reflector for the resonator of a semiconductor laser instead of the facet surface formed between the folds or etched. (hereinafter referred to as rDBRJ) is provided with a region D'. DBR region and D are InGaAsP
A diffraction grating 15.15' is formed on the external waveguide 14.14°, and wavelengths around the center wavelength λ determined by the period of the diffraction grating 15.15' are selectively reflected in the reflection direction. A laser resonator is formed by providing it on both sides (FIG. 6 shows D B RfiI regions on both sides,
D' is shown). D B R area and D'
A feature of using this method is that there is no need to physically break the waveguide, so output light can be extracted through the waveguide, and therefore the semiconductor laser according to the present invention can be incorporated into an optical integrated circuit. This optical integrated circuit is important in coherent transmission for integration with an external modulator on the transmitting side, mixing with signal light on the receiving side, and integrating a light receiver.

FIG. 7 shows the wavelength relationship of each parameter in this example. It is basically the same as Fig. 3, but the DBR area and D
'' uses Bragg reflection, so the reflectance has wavelength dependence and the band is finite.

Therefore, the reflection wavelength λ. It is necessary to adjust the wavelength to near the center wavelength λ and the gain peak wavelength, but by changing the refractive index of the provided waveguide layer by current injection, voltage, etc.
The wavelength variable width can also be widened as in the previous embodiment.

The main characteristics of the semiconductor laser according to the present invention are that it has a narrow linewidth and the oscillation wavelength is variable.
1 In FIG. 6, by adjusting only the waveguide refractive index n2 of the filter region C, the center wavelength λ8 is adjusted to the resonance wavelength λ.

In this case, which resonant wavelength λ
. oscillation stops because it has not passed through either. That is, the resonant wavelength λ. When viewed at a wavelength of , the loss of the resonator is modulated, and it can also function as a Q-switched light source suitable for so-called high-speed operation.

In addition, in the above embodiment, a direct coupling structure was used as an optical coupling method between the light emitting layer l and the waveguide layer 2, but the large
It can be applied to other bonding methods including e-optical cavity (LOG) structure. In particular, in a multiple quantum well structure, even if the structure is the same throughout the resonator,
The parts other than the light emitting region A can be used as a low-loss waveguide as is, and the fabrication is correspondingly easier. Although the stripe structure that confines light in the lateral direction was not specifically mentioned, it can be applied to any lateral mode confinement structure including a buried structure. Regarding semiconductor materials, in the above description InGa
Although the AsP/InP system has been described, GaAs/AlG
aAs, InGaAlAs/InP.

It is also applicable to compound semiconductor crystals such as AIGaAsb/GaSb. Furthermore, although current injection has been described as a method for adjusting the refractive index of the waveguide, it is also possible to use an electro-optic effect by applying a voltage.

(Effects of the Invention) As explained above, the semiconductor laser according to the present invention, which has a built-in bandpass type diffraction grating filter in the resonator, suffers from the problem that occurs when the length of the resonator is lengthened in order to narrow the oscillation line width. Multi-wavelength oscillation can be suppressed. That is, it is possible to realize a semiconductor laser with a narrow oscillation linewidth, stable single-wavelength operation, and variable oscillation wavelength. Therefore, it is promising as a light source for coherent transmission and other optical measurements.
The effect is extremely large.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing a structural example of a semiconductor laser having a conventional Nagai shaker, FIG. 2 is a cross-sectional view of a semiconductor laser of Example 1 according to the present invention, and FIGS. 3 (a), (b), (C), and (d). is a characteristic diagram showing the wavelength dependence of each parameter of the structure in the first embodiment of the present invention, and FIG. 4 is a characteristic diagram showing the wavelength dependence of each parameter of the structure in the first embodiment of the present invention. 5(a), (b), (C), and (6) show the wavelength dependence of each parameter when a normal diffraction grating is used in the filter region in the first embodiment of the present invention. 6 is a cross-sectional view of a semiconductor laser according to a second embodiment of the present invention, and FIGS. FIG. 3 is a characteristic diagram showing the wavelength dependence of parameters. 1 = -InGaAsP light emitting layer, 2 = 4nGaAsP
Waveguide layer, 3... phase shift type diffraction grating, 4...n
-1nP substrate, 5...p-InP layer, 6-p-In
GaAsP cap layer, 7.8.9... p-side electrode, 1
0... Electrical insulation region, 11... N-side electrode, 12...
...Metal film, 13...Insulating film, 14, 14'-InG
aAsP external waveguide, 15.15°...diffraction grating.

Claims (2)

[Claims] (1) A light-emitting region having a light-emitting layer, a waveguide region having a waveguide layer coupled with high efficiency to at least one side of the light-emitting layer, and a waveguide region having a part of the waveguide layer or an extension of the waveguide. A laser resonator is formed by integrating a filter region having a diffraction grating waiting for a pass-through filter function on the same substrate and providing a pair of reflective end faces, and the light emitting region, the waveguide region, and the filter are integrated on the same substrate. Each of the regions is electrically separated and provided with an electrode, and the refractive index of at least the waveguide region and the filter region is varied by applying a voltage or current to the electrode, thereby making the oscillation wavelength variable. A semiconductor laser configured to extract single-wavelength oscillation output light having a narrow linewidth and a wavelength matching a center wavelength of the filter region determined in accordance with a set refractive index. (2) a light emitting region having a light emitting layer; a waveguide region having a waveguide layer coupled with high efficiency to at least one side of the light emitting layer; and a waveguide region having a part of the waveguide layer or an extension of the waveguide. A filter region having a diffraction grating having a pass-through filter function is the same as a distributed reflector region having an external waveguide layer and a reflective diffraction grating on an extension of at least one of the filter region and the light emitting region. A laser resonator is formed by integrating the laser resonator on a substrate, and the light emitting region, the waveguide region, and the filter region are electrically separated from each other by electrodes, and a voltage or current is applied to the electrodes. causing a change in the refractive index of at least the waveguide region and the filter region to make the oscillation wavelength variable;
A semiconductor laser configured to extract single-wavelength oscillation output light with a narrow linewidth corresponding to a center wavelength of the filter region determined in accordance with the set refractive index through the external waveguide.