JPH02159786A - Laser device and frequency multiplex optical communication system - Google Patents

Abstract

PURPOSE:To enable a laser device of this design to oscillate laser rays of narrow spectral line in a wide wavelength band by a method wherein return rays from a first resonator to an optically active region which is optically coupled with the inside of the first resonator and other return rays from reflective regions sandwiching the optically active region in between them to the optically active region are made to resonate. CONSTITUTION:Laser rays emitted from an optically active region 14 of semiconductor laser structure are made to be incident on a beam splitter 18 provided with reflective planes 9-1 and 9-2, a half mirror 11 which discharges a part of the incident light, and end faces 13-1 and 13-2 coated with a non-reflective coating through the intermediary of a lens 12 which collimates a light flux. In this constitution, the reflective planes 9-1 and 9-2 constitute a Fabry-Perot etalon, and the reflective plate 9-1 of the beam splitter 8 and a reflective coating face 15 constitute a resonator of a Fabry-Perot laser. By this setup, a single laser ray of a narrow spectral line can be oscillated in a wide wavelength band.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a laser device, a frequency multiplexing optical communication system, and a method for setting an optical system, and particularly relates to a laser device suitable for optical communication, and a method using the device. This invention relates to a frequency multiplexing optical communication system and a method for setting an optical system.

[Conventional technology]

In frequency multiplexing coherent optical communication, a local oscillation light source is used on the receiving side as well as a transmitting light source. The function of this local oscillation light source is as follows: 1. To improve the S/N of the signal light, which has become weak after propagating through the optical fiber, by interference with this Yube oscillation light, and to improve the S/N of the signal light that has become weak after propagating through the optical fiber. It plays the role of frequency (wavelength) tuning that interferes and extracts only that signal. For this purpose, it is necessary that the spectral linewidth of the signal light source and the local oscillation light source be sufficiently narrow, that the frequency range (wavelength range) is wide enough to perform frequency multiplexing, and that the frequency (wavelength) is controllable. Until now, the oscillation wavelength has mainly been shifted by changing the temperature of the semiconductor or by changing the injection current. As an improved version of this, the electrodes of the distributed feedback semiconductor laser can be multipolarized as described in JP-A No. 61-290789 to create a structure in which the amplitude and oscillation wavelength can be controlled separately;
As described in No. 1-95592, an electrode is provided in the Bragg reflection region in addition to the photoactive region of a Bragg reflection semiconductor laser, and the wavelength of the Bragg reflection is changed by utilizing the change in refractive index caused by passing a current. , wavelength control was possible.

[Problem to be solved by the invention]

The above-mentioned conventional techniques were unable to cover a wide multiple frequency range (wavelength range) with a sufficiently narrow spectral linewidth. For example, methods that utilize temperature changes not only fail to obtain a narrow spectral linewidth, but also have problems such as higher oscillation threshold currents at high temperatures, resulting in lower output, and longer frequency (wavelength) switching. There was a problem. Further, in the multi-electrode type distributed feedback semiconductor laser, there is a problem in that the frequency (wavelength) can basically only be changed in the strong reflection wavelength range of the diffraction grating, which is about 200 GHz. In addition, in the frequency (wavelength) variable Bragg reflection type semiconductor laser, the oscillation frequency (wavelength) could be changed up to about 600 GHz, but as the current injected into the Bragg reflection region increases, the absorption loss increases and the oscillation threshold There was a problem in that as the value current increased, the spectral line width became wider.

The objects of the present invention are a laser device that maintains a narrow spectral linewidth in a wide wavelength range and oscillates at fixed frequency (wavelength) intervals, a frequency multiplexing optical communication system using the device, and settings for the optical system of the device. The purpose is to provide a method.

[Means to solve the problem]

The above object is to (1) provide a first resonator having at least two reflective surfaces; an optically active region that is coupled to the optically active region, and the return light from the first resonator to the optically active region and the return light from the reflection region installed to sandwich the optically active region resonate. a composite resonator type laser device having a first resonator configured;
(2) It has at least one first resonator having at least two reflecting surfaces, and an optical beam is provided inside the first resonator through the two outermost reflecting surfaces of the first resonator. a plurality of optically active regions coupled to the first resonator, a reflective region at one end of each of the optically active regions, and means for guiding the return light from the first resonator at the other end; (3) a frequency multiplexing laser device having a plurality of second resonators configured so that the return light from the reflection region resonates with the return light from the reflection region; a transmitting system and a receiving system, and an optical waveguide means for optically coupling the two; A frequency multiplexed optical communication system characterized by (4) passing a current through the optically active region of the laser device described in 1 above, receiving the emitted light from the laser device, and comparing the characteristics of the emitted light with the above current. This is achieved by an optical system setting method characterized by adjusting the optical axis of the laser device so that the current for starting laser oscillation is low.

In the above laser device, m is the frequency multiplicity, ne, l
Let e be the refractive index of the material constituting the first resonator and the effective spatial length of the resonator, nl, h be that of the second resonator containing the active region, δλ be the peak wavelength of the reflectance, The first and second resonators are configured to satisfy the relationship 2ntQ* 2neRe 2nele 2nJt when the reflectance is set to a wavelength difference corresponding to a reflectance having a reflectance difference that satisfies single wavelength oscillation. By determining the refractive index of the material and the effective space of the resonator = 7, single oscillation can be obtained within the entire range of multiple frequencies. Furthermore, by limiting the reflection region of the resonator to multiple frequency ranges and creating a filter with high reflectivity, complete single frequency oscillation can be obtained. Furthermore, by making the first resonator common, connecting multiple optically active regions, and adding a function to select the oscillation wavelength of the second resonator, a light source for frequency multiplexing with uniform wavelength intervals can be created. can be supplied.

Furthermore, if the first resonator is provided with a function that can change the refractive index or length, it becomes possible to change the wavelength interval of the resonant wavelengths.

[Effect]

Figure 1 shows a schematic diagram of a Fabry-Perot etalon with sharp resonant wavelength characteristics when light is incident from outside the resonator. In the figure, the Fabry-Perot etalon is composed of two parallel reflecting plates 1-1, 1-2, and there is no light emitting region therein. When light 3 is incident from the outside, the light undergoes multiple reflections, part of which is trapped and becomes trapped light 4, and part of which is transmitted from the reflector to the outside of the resonator and leaked light 5-
1.5-2, and the rest is 6 returned lights due to reciprocity. At this time, the resonance characteristics and light confinement characteristics with respect to the wavelength become the characteristics shown in FIG. 2 as the power of the confined light 4 or the power P of the returned light 6. As the reflectance of the two reflective surfaces of the Fabry-Perot etalon increases, the resonance characteristics rise sharply at the point λH, where the optical path length of the resonator is an integral multiple of 1/2 of the wavelength of the light, and light near the resonant wavelength increases. The confinement becomes stronger. Letting ne and me be the refractive index of the material constituting the Fabry-Perot etalon resonator and the effective spatial length of the resonator, the resonant wavelength λ of the Fabry-Perot etalon is expressed as follows.

2neΩ.

λH= where M is an integer. At this time, only the light of wavelengths with strong optical confinement will return to a large extent.

When the reflective region 1-3 is provided to sandwich the reflective plate 1-2 and the optically active region 7, a resonator is formed between the Fabry-Perot etalon and the reflective surface, and a laser beam is emitted. Note that one end of the optically active region 7 is the non-reflection coating surface 2. This laser has an oscillation characteristic that includes both characteristics, with one end surface reflecting the characteristics of the Fabry-Perot etalon, which has the above-mentioned sharp resonant wavelength characteristics, and one end surface reflecting the characteristics of the reflective region that makes up this end surface. will have. The case where a mirror surface is used as the latter reflective area will now be described. Basically, a laser oscillates only at a wavelength that satisfies the phase condition of a resonator formed by one end surface of the Fabry-Perot etalon and a mirror surface, that is, the condition that the round-trip phase is an integer N times 2π.

As shown in FIG. 2, the wavelength characteristics of the reflectance have a sharp reflectance peak at the resonant wavelength λH of the Fabry-Perot etalon, reflecting the light confinement characteristics of the Fabry-Perot etalon. For this reason, the laser's oscillation wavelength selectivity becomes sharper,
The oscillation wavelength is limited to the vicinity of the resonant wavelength of the Fabry-Perot etalon. The laser oscillates at the wavelength at which the reflectance is maximum, that is, at point A, while satisfying the oscillation phase condition based on the relationship between the phase and the reflectance described above. Note that it is also possible to further provide a reflector outside the reflector 1-1, that is, on the left side of the figure, to increase the reflectance and increase the Q value.

Now, the resonant wavelength λN of the laser (the wavelength that satisfies the condition that the round-trip phase is an integer N times 2π) and the resonant wavelength λH of the Fabry-Perot etalon (the wavelength with a sharp peak of reflectance) are determined by each resonator. Depends on optical path length. nt, Qt
When is the refractive index of the material constituting the resonator including the active region of the laser and the effective spatial length of the resonator, the resonant wavelength λN of the resonator including the active region can be expressed as 2n history Q□ λN. Here, N is an integer. This λN, λN
The laser oscillation wavelength is when the two almost match.

When M and N match, all resonance points satisfy the oscillation condition, but in reality, because the optical amplification gain in the optically active region is wavelength dependent, several multimode oscillations occur. becomes. On the other hand, when the optical path length of the resonator is set so that M+1=N or M=N+1 and λN and λN match, and the following condition is satisfied, that is, when M+1=H, as we change the refraction When the ratio n2 is changed to N+1 and M+2, and further changed, the wavelengths that coincide with the resonance points of N+2 and M+3 sequentially jump. As a result, for m wavelengths, when M=N+1, oscillation occurs at only one of the minimum m resonance points. here,
For m'+1, the condition for obtaining the next lasing wavelength is N + m
+ 1 = M 10m + 1, and δλ
has the wavelength difference to obtain the reflectance difference to give single wavelength oscillation from the peak wavelength of reflectance, that is, the wavelength of the peak reflectance and the reflectance difference that satisfies the single wavelength oscillation for that reflectance. It means the difference in wavelength from the wavelength corresponding to the reflectance. Using this phenomenon, for a certain oscillation wavelength λ,
Oscillation in the wavelength range corresponding to ±m resonance points in the vicinity is suppressed.

Furthermore, for example, the injection current into the optically active region is set to 2n*
A unique oscillation wavelength can be sequentially selected in a wavelength range of Qt 2neQQ or more.

Furthermore, as can be seen from equations (1) and (2), the resonant wavelengths of the Fabry-Perot etalon and the second resonator including the optically active region match in the wavelength range covered by several m of multiplexed wavelengths. there is a possibility.

At this time, complete oscillation at a single wavelength cannot be obtained. As a way to prevent this, a reflection filter is formed on the reflection surface of the Fabry-Perot etalon, for example, by multi-coating, or a reflection filter is formed on the reflection surface of the optically active region, and the wavelength range covered by several meters of multiplexed wavelengths is applied. By increasing the reflectance of the laser beam and obtaining resonance characteristics, complete single-wavelength oscillation can be obtained.

Furthermore, a second resonator having a plurality of optically active regions is connected to a common first resonator, and further, the active regions are arranged such that the laser oscillation wavelength including each active region is different. The light from the reflection area behind the is selected by a diffraction grating and received by each receiver, and is returned to each second resonator so that the light exceeds a set value, thereby reducing the resonance of the first resonator. A light source for frequency multiplexing with equal intervals reflecting the wavelength interval can be obtained.

Furthermore, in general, the longer the first resonator is compared to the optically active region, and the greater the amount of returned light, the narrower the spectral line width becomes. In this configuration, the laser oscillates when the amount of light returned from the Fabry-Perot etalon is greatest, so the spectral linewidth becomes extremely narrow.

By using such a signal light source whose oscillation wavelength is controlled in combination with a wavelength tunable semiconductor laser device, frequency multiplex communication can be easily realized.

In the above configuration, when a plurality of reflective end faces and non-reflective end faces are used, it is necessary to apply a coating. When performing the coating, the coating can be easily performed by dividing the surface of the element into a reflective surface and a non-reflective surface.

Further, in the laser configuration having the above-mentioned hybrid configuration of the first resonator and the second resonator, setting up the optical system is quite difficult. However, by setting the optical system so that a current flows through the optically active region to cause laser oscillation, and adjusting it so that the threshold current value is the lowest, the optimal setting state can be obtained. become.

〔Example〕

Example 1 An embodiment of the present invention will be described using FIG. 3.

Reflective surfaces 9-1 and 9-2 for forming the passive Fabry-Perot etalon shown in FIG. The light emitted from the photoactive region 14 of the semiconductor laser structure is input into a beam splitter 8 having 13-1 and 13-2 via a lens 12 for collimating the light beam. In this configuration, the reflective surface 9-1 of the beam splinter 8
．． 9-2 constitutes a Fabry-Perot etalon, and the reflective surface 9-1 of the beam splitter 8 and the reflective coating surface 15 constitute a resonator of the Fabry-Perot laser.

As the beam splitter 8, a beam splitter made of BK7 glass with a transmittance of 50% and a reflectance of 50% is used.
The reflective surfaces 9-1 and 9-2 were coated with Au.

The interval between the reflective surfaces was 3 mm.

The distance between the end surfaces 13-1 and 13-2 coated with anti-reflection coating was 2 mm. In the optically active region 14, InG
One end face of a 1.5 μm band Fabry-Perot laser made of aAsP/InP is coated with SiO anti-reflection coating to form an end face 16 coated with anti-reflection coating, and the other end face is coated with a two-layer film of S10/a-8i. A reflective coating was applied to form a reflective coating surface 15. The light emitted from the anti-reflection coated end face 16 of the Fabry-Perot laser is converted into parallel light by the lens 12, enters the beam splitter 8, and is reflected by the half mirror 11. The light is multi-reflected by the reflective surface 9-1, 9-2. do. The reflectance of the half mirror is 40%.

The settings of the optical system will be explained using FIG. 3(b). First, a current is applied to the optically active region 14 to make it active, and after the emitted light is collimated into parallel light by the lens 12, the light is incident on the beam splitter 8. The light emitted from the beam splitter 8 is received by a light monitor 34 and fed back to the optical characteristic evaluation device 35, where it is compared with the current injected into the photoactive region.

The axis of the beam splitter 8 is adjusted by the axis adjustment device 36 so that the threshold current value for starting laser oscillation is the lowest.

The wavelength width δ is 50% of the peak of the characteristics of the beam splitter 8, which is the Fabry-Perot etalon of this embodiment.
λ is 0. O3 nm, and the wavelength interval of the resonance peak is 0.
It is 26 nm. Furthermore, the wavelength spacing of the resonant wavelengths of the second resonator length is 0.223 nm, and the wavelength spacing of the resonance characteristics of the Fabry-Perot etalon is 0.26n+n and 0.037n.
m different settings. When the injection current of the optically active region was set, oscillation occurred in a substantially single mode at a wavelength of 1542 nm. Next, when the injection current was increased by 2 mA, the oscillation wavelength jumped to the longer wavelength side by 0.26 nm.

For normal lasers, 0. Since it is a jump of O2nm, 1
A jump of three times the wavelength was confirmed. Furthermore, the spectral line width was several tens of minutes lower than the IMI (z) of a normal laser, and the effect of narrowing the spectrum was also confirmed.

Example 2 An example of a light source that oscillates at equally spaced wavelengths will be explained using FIG. 4. In this configuration, the passive Fabry-Perot etalon 8 in Example 1 is used in common, and four optically active regions are arranged. Since the Fabry-Perot etalon is common to each of the optically active regions, four lasers are constructed which oscillate at discrete resonant wavelengths specific to the Fabry-Perot etalon 8. Now, as a wavelength control method, the light emitted from the back surface of the optically active region 14-1.14-2.14-3.14-4 is controlled by the lens 17-1.17-2.17-3.17-
4 and enters a common diffraction grating 18 set behind it. This may alternatively be a surface grating. A light receiving element 19-1, 19-2 for monitoring is placed at a position corresponding to a diffraction angle of a desired wavelength for this diffracted light.
．． 19-3. 19-4 is installed, and the control circuit 20 is set so that the received light power exceeds a certain set value or reaches the maximum.
-1.20-2.20-3.20-4 to feedback control the injection current to the optically active region. The oscillation spectrum is shown in FIG. By providing wavelength selectivity in this way, Fabry-Perot crystals with fine and uniform wavelength spacing can be created.
The resonant wavelength of the etalon can be selected and each optically active region 14-1.14-2.14-3.14-
4 can be oscillated at four different wavelengths.

Further, FIG. 6 shows an example of a coherent optical communication system using this light source. Reference numeral 27 in the figure is a frequency multiplexing light source in the embodiment, in which a modulation element 21 is arranged for each light having a different oscillation wavelength, and the signal light group is transmitted to one optical fiber 2.
Combine with 2. The propagated light is multiplexed in the receiving system 23 with the light of the wavelength tunable laser shown in the first embodiment by the optical coupler 25.

This interference light is restored to an electrical signal by a light receiver 26. At this time, by using a Fabry-Perot etalon 8 having the same resonance characteristics as the transmitting system = 20- as the local oscillation laser of the signal receiving system 23, the embodiment 1
As shown in Figure 2, the oscillation wavelength can be easily changed to correspond to the resonance wavelength of the Fabry-Perot etalon.
The signal light to be received can be selected.

Example 3 Several possible configurations are shown as modifications of Example 1. FIGS. 7(a) and 7(b) show configuration diagrams when a coupler is used instead of the half mirror of the beam splitter. 7th
Figure (a) shows an optical waveguide type coupler 28, which includes, for example, Sio2, TiO2 optical waveguide, InP/I
An optical waveguide of nGaAsP, a leading waveguide made of doped SiO2, etc. are used. Further, an optical fiber type coupler 29 is shown in FIG. 7(b). At this time, a reflective coating is applied to the reflective surface 9 that is in contact with the optically active region. A reflective coating is applied to one end face on the output side, and the reflective surface 9
Further, another end face 13 on the emission side is coated with an anti-reflection coating. The same applies when using the optical fiber coupler 29.

Furthermore, an example of a configuration in the case of monolithic formation is shown in FIGS. 7(c) and 7(d). A passive orthogonal leading waveguide 30-1, 30-2 is arranged in the optically active region 14, and a mirror 31 that appropriately reflects light is provided at the intersection thereof. Optical waveguide 3
0-2 constitute the Fabry-Perot etalon. Figure 7 (
d) is a configuration in which the corresponding parts of the laser and Fabry-Perot etalon configurations of Example 2 are integrated as shown in FIG. 7(c). Such a configuration would avoid mechanical configuration problems. In the case of integration, the reflective surface 9 and the end surface 13 coated with anti-reflection coating are arranged so as to be separated, as in the configuration shown in FIG. 7(d). Then, one type of coating is applied to one end face, and different coatings are used depending on the end face.

Example 4 A reflection filter using multi-coating is formed on the reflection surface of the Fabry-Perot etalon, or a reflection filter is formed on the reflection surface of the optically active region, and resonance occurs only in the wavelength range covered by several meters of multiplexed wavelengths. This shows a configuration in which the characteristics are obtained and oscillation is performed at a completely single wavelength in the entire wavelength range.

FIG. 8 shows a configuration diagram in the case where the covering wavelength range is about 2 nm in Example 1, and a reflection filter is installed in the reflection region outside the optically active region. In the case of optically active semiconductor materials, it is several tens of nanometers. A distributed black reflective laser configuration 32 with a single-sided cleavage plane is utilized as the optically active region and reflective filter combination. A black reflective region 33 is connected to the optically active region 14 of this laser.

However, the cleavage plane 37 is coated with an anti-reflection coating.

The wavelength range in which this distributed Bragg reflection has a high reflectance is
The reflectance is about 2 nm, and the reflectance in other optically active wavelength ranges is extremely low. As a result, oscillation occurs at only one wavelength in the high reflection wavelength range.

〔Effect of the invention〕

According to the present invention, since it is a composite resonator, it is possible to cause laser oscillation with a single narrow spectral linewidth within a wide wavelength range. Furthermore, in frequency division multiplexing communication of coherent optical communication which requires a narrow spectral linewidth, it is possible to make signal lights of a plurality of wavelengths equally spaced so as not to interfere with each other. Furthermore, it becomes possible to easily cover a wide wavelength tunable range required for a local oscillation laser on the receiving side, and has characteristics suitable for frequency division multiplexing of coherent optical communication.

[Brief explanation of the drawing]

Fig. 1 is a conceptual diagram for explaining the present invention, Fig. 2 is a diagram showing the basic characteristics of the present invention, Fig. 3 is a block diagram of the first embodiment, and Fig. 4 is a diagram of the frequency multiplexed light source of the second embodiment. 5 is a spectrum diagram of the emitted light, FIG. 6 is a diagram of the applied system configuration of Examples 1 and 2, FIG. 7 is a diagram of the configuration of other embodiments of the present invention, and FIG. 8 is a diagram of the reflection filter. It is a block diagram of the example used. 1-1.1-2...Reflector plate 1-3...Reflection area 2...
・Non-reflection coating surface 3...Light 4...Confined light 5-1.5
-2...Leak light 6...Return light 7, ]4.14-1.
14-2.14-3.14-4 ・Optically active = 23 Active region 8...Beam splitter 9.9-1.9-2...Reflecting surface 1 Half mirror 12.17-1. , 17-2.17-3.17-4・
・Lens 13.13-1.13-2.16・End face 15・
・Reflection coating surface 18 ・Diffraction grating 19.19-1.19-2.19-3.19-4...
Light receiving element 20.20-1.20-2.20-3.20-
4...Current control circuit 21...Modulation element 22
...Optical fiber 23...Reception system 24.
・・Converging lens 25 ・・Coupler 26 ・Photoreceiver 27 ・Frequency multiplexing light source 28 ・Optical waveguide type coupler 29 ・・Optical fiber coupler 30-1, 30-2 ・Optical waveguide 31 Mirror

Claims (1)

[Claims] 1. A first resonator having at least two reflective surfaces;
an optically active region that is optically coupled to the interior of the first resonator without going through the two outermost reflective surfaces of the first resonator; and the first resonance to the optically active region. A composite resonator type laser device comprising a second resonator configured so that the return light from the device resonates with the return light from the reflection region disposed to sandwich the optically active region. 2, m is an integer, n_e, l_e are the refractive index of the material constituting the first resonator and the effective spatial length of the resonator, n_l
, l_l is the refractive index of the material constituting the second resonator and the effective spatial length of the resonator, δλ is the peak wavelength of the reflectance, and has a reflectance difference that satisfies single wavelength oscillation with respect to the reflectance. When the wavelength difference corresponds to the reflectance, when 1/2n_ll_l<1/2n_el_e,
δλ<λ^2/2n_el_e−λ^2/2n_ll_
l<λ^2/(m+1)2n_ll_lAlso, when 1/2n_el_e<1/2n_ll_l, δ
λ<λ^2/2n_ll_l−λ^2/2n_el_e
A claim characterized in that the refractive index of the material constituting the first and second resonators and the effective spatial length of the resonators are determined so as to satisfy the relationship <λ^2/(m+1)2n_el_e. 1. The laser device according to 1. 3. The laser device according to claim 1 is provided on the same substrate, at least one surface of the substrate is provided with the reflective surface, and the surface for extracting light is provided on a surface different from the surface having the reflective surface. Characteristic laser equipment. 4. It has at least one first resonator having at least two reflective surfaces, and optically enters the inside of the first resonator without going through the two outermost reflective surfaces of the first resonator. a plurality of optically active regions to be coupled; a reflective region at one end of each of the optically active regions; and means for guiding return light from the first resonator at the other end; A frequency multiplexing laser device comprising a plurality of second resonators configured so that the return light from the reflection region resonates with the return light from the reflection region. 5. A transmitting system and a receiving system each equipped with the laser device according to claim 1, and an optical waveguide means for optically coupling the two, the effective A frequency multiplexing optical communication system characterized by nearly matching lengths. 6. Applying a current to the optically active region of the laser device according to claim 1, receiving the emitted light from the laser device, and comparing the characteristics of the emitted light with the above-mentioned current, and determining whether the current for starting laser oscillation is low. A method for setting an optical system, the method comprising adjusting the optical axis of the laser device so that