JP3600785B2 - Waveguide type laser - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a waveguide-type laser, and more particularly, to a waveguide-type wavelength-variable laser, which is a laser light source indispensable for optical communication.
[0002]
[Prior art]
It is essential for a laser light source used for optical communication to oscillate in a single mode so that the waveform of an optical signal does not deteriorate due to propagation over a long distance. Typical lasers used for communication include a DFB (Distributed Feedback) or a DBR (Distributed Bragg Reflector) laser in which a grating is incorporated in a semiconductor laser. However, a laser made of only a semiconductor material has a problem that the oscillation wavelength shifts greatly with a change in temperature.
[0003]
Therefore, in order to use it in a DWDM system (Dense Wavelength Division Multiplexing System), which has been actively researched and developed in recent years, a device such as a built-in temperature adjustment mechanism is required, and there is a problem in cost. In order to solve these problems, there has been proposed a laser in which a grating is written in a silica-based waveguide having lower temperature dependency than a semiconductor material, and which is hybrid-integrated with a semiconductor amplifier.
[0004]
FIG. 2 is a diagram showing a typical configuration of a laser that is hybrid-integrated with a semiconductor amplifier. In the figure, reference numeral 21 denotes a silica-based waveguide, 22 denotes a fiber connection portion, 23 denotes a grating, and 24 denotes a semiconductor amplifier. This laser not only reduces the oscillation wavelength shift amount to 1/8 with temperature change, but also enables connection to a fiber indispensable for optical communication without using a lens system, thereby reducing the production cost. It became possible. However, there is a new problem that the oscillation wavelength hops for a large change in the environmental temperature.
[0005]
However, as a method of solving this problem, a fine groove is formed on a quartz-based waveguide, and a material such as a polymer whose refractive index changes in temperature with a polarity opposite to that of a semiconductor material or a quartz-based glass material is guided in the opposite direction. By flowing into the groove on the path, the temperature change of the resonator length as a whole of the laser resonator matches the temperature dependence of the grating wavelength on the quartz waveguide, suppressing the temperature dependence of the oscillation wavelength and suppressing hopping. It has been proposed and reported to suppress it (Tanaka et. Al., "Hybrid integration external cavity laser with temperature dependent dependency mode hopping", Electron Lett. 1999, 35, pp17.
[0006]
However, even with the above-described waveguide type laser, the temperature dependence of the oscillation wavelength cannot be completely eliminated, and the necessity of incorporating a temperature adjustment mechanism in the module remains. Further, lasers using these gratings have a problem in terms of cost that a special process such as microfabrication or irradiation with ultraviolet light is required to make the grating.
[0007]
Furthermore, it has not yet been reported that a single-mode laser combining a quartz waveguide and a semiconductor amplifier can only oscillate at a certain wavelength determined by the grating, and that the oscillation wavelength was greatly changed and single-mode oscillation was stably realized. Absent.
[0008]
Further, as a small-sized tunable single-mode laser, a Grating Coupled Sampled Reflector Laser (B. Broberg et. Ishii et.al., "Quasicontinuous Wavelength Tuning in Super-Structure-Granting (SSG) DBR Lasers", IEEE J. Quantum Electron, vol. However, in order to actually control the oscillation wavelength, there is a problem that fine adjustment is required while monitoring the oscillation wavelength, and there is a major problem that it is difficult to provide a function of making the oscillation wavelength independent of temperature. Was.
[0009]
On the other hand, in a DWDM system that handles optical signals of different wavelengths, there is a system problem of how to deliver each signal to a target location, and various studies have been made. In the case of a small-scale network, it is possible to make one-to-one correspondence between wavelengths and paths to be used. However, considering a network that transmits a signal from an unspecified majority to an unspecified majority, such as a telephone, all of the callers receive all signals. It is practically impossible to prepare individual wavelengths and paths up to the receiver, and it is necessary to switch the path for each section. As the path switching method, there are a method of physically changing the route of the optical signal (see Japanese Patent Laid-Open No. 5-72575) and a method of switching the wavelength of the optical signal.
[0010]
Examples of proposals for a system using the latter include (M. Koka et. Al., “Design and performance of an optical path cross-connect system based on Lifetime Economics. 6, pp. 1106-1119, 1996). In these methods, it is necessary to prepare single-mode lasers having different wavelengths by the number of all output channels for each input channel, but no more than one laser is operating at a particular moment in each channel. There is a problem that an extra expensive laser must be provided.
[0011]
The present invention has been made in view of such a problem, and it is an object of the present invention to provide a waveguide type laser that omits a temperature adjustment mechanism and a fine processing or an ultraviolet irradiation process required at the time of making a grating. It is in.
[0012]
[Means for Solving the Problems]
In order to achieve such an object, the present invention provides a waveguide type laser having a built-in reflecting mirror having wavelength selectivity in a resonator, wherein a waveguide type loop mirror is used for at least one end reflecting mirror. An arrayed waveguide grating is provided in a waveguide-type loop mirror, and the difference in waveguide length between individual arrayed waveguides in the arrayed waveguide grating is not a fixed one, but is two or more. The present invention is characterized in that each array waveguide is provided.
[0015]
According to a second aspect of the present invention , there is provided a waveguide type laser having a built-in reflecting mirror having wavelength selectivity in a resonator, wherein the waveguide type loop mirror is used as at least one end reflecting mirror. Within this, an arrayed waveguide grating and a waveguide type filter having an FSR different from the FSR (Free Spectral Range) of the arrayed waveguide grating are provided.
[0016]
According to a third aspect of the present invention, in the second aspect , a waveguide filter having a variable transmission wavelength is used as the waveguide filter.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0020]
(Example 1)
FIG. 1 is a block diagram showing an embodiment of a waveguide type laser according to the present invention. In the drawing, reference numeral 11 denotes a quartz-based waveguide having a relative refractive index of 1% formed on a Si substrate, and reference numeral 12 denotes an array waveguide. Port 13 and port 14 of the arrayed waveguide grating 12 are connected to a 3 dB coupler 15 to form a waveguide loop mirror. Reference numeral 16 denotes a semiconductor amplifier, which has a high reflection coating on an end face 19 and is mounted on a quartz waveguide chip 18. Further, a groove 17 for inserting a polymer material is dug in the arrayed waveguide grating, and is designed to cancel the temperature dependence of the refractive index of the arrayed waveguide grating and the temperature dependence of the semiconductor amplifier. Have been.
[0021]
The arrayed waveguide grating 12 is composed of nine arrayed waveguides, and is designed so that the waveguide length of the m-th arrayed waveguide from the innermost waveguide to the fourth is longer by ΔL1 than the adjacent inner waveguide. The fifth through ninth arrays are designed so that the waveguide length of the arrayed waveguide is longer than the adjacent inner waveguide by ΔL2. In addition, assuming that the effective refractive index of the waveguide is n and the speed of light in a vacuum is C, the design is such that C / nΔL1 = 200 · 10 ⁹ and C / nΔL2 = 214 · 10 ⁹ . The effective refractive index in the 1.5 μm band of the Δ1% waveguide used in the example was approximately 1.45, so that ΔL1 = 1,034 μm and ΔL2 = 965 μm.
[0022]
Further, grooves having a width proportional to the length of each arrayed waveguide with respect to the innermost waveguide were formed on the nine arrayed waveguides. This groove was divided into a plurality of grooves so that the groove width would not become too large in consideration of the propagation loss, and the groove width was designed so that the sum of the groove widths was proportional to the difference in the waveguide length of the transverse waveguide. In addition, a groove crossing all nine waveguides was created so as to cancel the temperature dependence of the entire resonator length consisting of the semiconductor amplifier and the quartz waveguide used. After forming the groove, a polymer material having a temperature dependence of the refractive index opposite to that of the quartz-based waveguide or the semiconductor amplifier was selected and injected into the groove.
[0023]
Ports 13 and 14 of the arrayed waveguide grating are guided from the same direction into a 3 dB coupler 15 composed of an MMI (Multi Mode Interferometer), and one of the two ports on the opposite side is connected to a semiconductor amplifier mounting portion, and the other is connected to a fiber. Led to the mounting section.
[0024]
A waveguide semiconductor amplifier made of InP was used as the semiconductor amplifier. The total length of the semiconductor amplifier was about 500 μm. The opposite surface 19 which does not face the end face of the quartz waveguide is coated with an HR coating which is highly reflective in a 1.5 μm band.
[0025]
The length of the resonator composed of the waveguide loop mirror including the arrayed waveguide grating and the semiconductor amplifier was about 30 mm in vacuum. The fact that the length of one round of the resonator is approximately 30 mm in vacuum indicates that laser oscillation can be performed at intervals of about 10 GHz.
[0026]
FIG. 4 is a diagram showing characteristics of the completed arrayed waveguide grating portion. This is the transmission wavelength dependency from the port 13 to the port 14 of the arrayed waveguide grating 12. In consideration of the amplification band of the used semiconductor amplifier, the characteristics in the wavelength range of 1545 nm to 1565 nm were evaluated. As in the case of the ordinary arrayed waveguide grating, a peak having a high transmittance periodically appears repeatedly. However, since a plurality of waveguide length differences as described above are given to the arrayed waveguide, the transmittances of all peaks are not equal, and the transmittance is maximized only near one wavelength. As a result, the wavelength region where laser oscillation can be performed is limited within one peak where the above-described transmittance is maximized from the entire gain band of about 10 nm of the semiconductor amplifier.
[0027]
FIG. 5 is an enlarged view of a peak having the largest transmittance. FIG. 5 also shows guidelines at intervals of 10 GHz. This value of 10 GHz corresponds to a resonance cycle at which laser oscillation is possible. Since the transmittances next to 10 GHz are greatly different from each other, it can be understood that only the light of a single wavelength oscillates from the peak of the transmittance.
[0028]
FIG. 6 is an IL characteristic diagram when the semiconductor amplifier is driven. The oscillation threshold was 45 mA, and an optical output of 1.2 mW was obtained when 100 mA was injected. FIG. 7 shows the laser oscillation spectrum at that time. The oscillation was a single mode oscillation, and the isolation was about 40 dB. Next, the entire laser was placed in a thermostat bath filled with dry air, and the ambient temperature was changed from 5 degrees to 50 degrees to evaluate the laser oscillation characteristics. FIG. 8 shows the oscillation intensity of the laser, and FIG. 9 shows the temperature dependence of the oscillation wavelength. All are data at the time of 100 mA current injection. Due to the temperature dependence of the gain of the semiconductor amplifier, the laser oscillation intensity changed from 1.25 mW to 0.6 mW. However, the laser oscillation wavelength depends on the temperature dependence of the AWG which is the wavelength selection element and the temperature dependence of the cavity length. Since the characteristics were simultaneously designed to be temperature-dependent, almost no temperature change was observed, and the temperature was ± 2.5 GHz or less over the entire range of environmental temperature change from 5 degrees to 50 degrees.
[0029]
(Example 2)
FIG. 3 is a diagram showing a second embodiment of the waveguide laser according to the present invention. In the figure, reference numeral 31 denotes a quartz-based waveguide having a relative refractive index of 1% manufactured on a Si substrate, 32 denotes a first arrayed waveguide grating, and ports 33 and 34 of the first arrayed waveguide grating are connected. It is connected to a 3 dB coupler 35 to form a waveguide loop mirror. One exit of this waveguide-type loop mirror was guided to the fiber connection portion 40 and used as an output port of the waveguide-type laser. Further, the other exit of the waveguide type loop mirror is guided to a second arrayed waveguide grating 37, and the opposite side of the second arrayed waveguide grating is connected to a semiconductor amplifier. Further, the end face 39 is provided with a high reflection coat. Reference numeral 38 denotes a quartz-based waveguide chip.
[0030]
The first arrayed waveguide grating 32 is composed of nine arrayed waveguides such that the waveguide length of the m-th arrayed waveguide from the innermost waveguide type is longer by ΔL3 than the length of the adjacent inner waveguide. Designed. Incidentally, the effective refractive index of the waveguide is n, when the speed of light in a vacuum is C, was designed to be ^{C / nΔL3 = 100 · 10 9} . Since the effective refractive index in the 1.5 μm band of the waveguide of Δ1% used in the example is approximately 1.45, ΔL3 = 2069 μm.
[0031]
The second arrayed waveguide grating 37 is composed of three arrayed waveguide gratings, and the waveguide length of the mth arrayed waveguide from the lowermost waveguide is longer than the length of the adjacent lower waveguide by ΔL4. It is designed to be. Incidentally, the effective refractive index of the waveguide is n, when the speed of light in a vacuum is C, designed to be ^{C / nΔL4 = 2199 · 10 9} , became ΔL4 = 94μm.
[0032]
FIG. 10 is a detailed structural diagram of the second arrayed waveguide grating. In the second arrayed waveguide grating 37, a heater 41a having a length of 4 mm × 40 μm width is provided on the uppermost arrayed waveguide type, and a 2 mm length × 40 μm width is provided on the center arrayed waveguide so that the wavelength shift operation by the heater 41 can be performed. The electrodes 42a and 42b were connected so that a heater 41b having a width of 40 μm was loaded and the heaters could be driven in series. Also, tapered portions 43 and 44 are provided on the array waveguides on both sides as compared with the central array waveguide so that the light intensity distributed to the three array waveguides 45a, 45b and 45c becomes equal. Further, in order to supplement the filter characteristics of the second arrayed waveguide grating whose transmission band wavelength is tunable, the second arrayed waveguide grating 37 is placed outside the waveguide-type loop mirror, and light circulating around the resonator is guided by the second arrayed waveguide. It was devised to pass through the wave grating twice.
[0033]
FIG. 11 is a diagram showing a change in transmission characteristics when the heater of the second arrayed waveguide is driven. With increasing voltage, the transmission band of the second arrayed waveguide could be controlled from about 1550 nm to 1560 nm.
[0034]
The waveguide type loop mirror including the above-mentioned first arrayed waveguide grating, the second arrayed waveguide grating, and a semiconductor amplifier are guided so that the resonator length is 60 mm in vacuum. The length between the waveguide loop mirror and the second arrayed waveguide grating was designed. The fact that the length of one round of the resonator is 60 mm means that the resonator frequency is 5.0 GHz.
[0035]
FIG. 12 shows the transmission characteristics of the first arrayed waveguide grating, the transmission characteristics of the second arrayed waveguide grating, and the total transmission characteristics when the resonator makes one round of the quartz waveguide portion. This is shown from 1545 nm to 1565 nm, which is the gain band of the semiconductor amplifier.
[0036]
The peaks in the transmission band of the first arrayed waveguide grating are arranged at intervals of 100 GHz and are on the ITU grid. In addition, a region where oscillation is possible is selected in the gain band of the semiconductor amplifier over about 10 nm according to the transmission characteristics of the second arrayed waveguide grating.
[0037]
Among the total transmission characteristics of the resonator composed of the waveguide-type loop mirror including the first arrayed waveguide grating and the quartz waveguide portion of the resonator including the second arrayed waveguide grating, the transmittance is highest. FIGS. 13 and 14 show enlarged views of the wavelength region. Referring to FIG. 13, it can be seen that the total transmission characteristic has a transmittance peak every 100 GHz. However, due to the characteristics of the second arrayed waveguide grating, only one peak selectively increases the transmittance. You can see that there is. Further, FIG. 14 shows that the width of one transmission peak of the total transmission characteristics is sufficiently narrow, and the longitudinal mode of the laser shifted by the resonator frequency of 5 GHz can be sufficiently suppressed.
[0038]
FIG. 15 is a diagram illustrating an example of the IL characteristics of the completed waveguide laser. In the characteristics when 0.4 W power was applied to the heater of the second arrayed waveguide grating, the threshold value of laser oscillation was 110 mA, and a laser output of 0.45 mW was obtained when 150 mA was applied. The output light from the laser at this time is shown in FIG. The oscillation was a single mode oscillation, the side mode and the like were sufficiently suppressed, and an isolation of about 30 dB was obtained. Next, FIG. 17 shows a change in laser oscillation wavelength when 150 mA is applied to the semiconductor amplifier and power is gradually applied to the heater of the second arrayed waveguide grating. The laser oscillation wavelength discretely changes as the driving power of the heater increases, and the oscillation wavelength is controlled on the ITU grid (interval of 100 GHz ± 2.5 GHz). Except at the time of switching the oscillation wavelength, single mode oscillation was observed at all wavelengths.
[0039]
In this example, a waveguide-type loop mirror having an arrayed waveguide grating was manufactured using a silica-based waveguide, but a semiconductor waveguide such as InP, an organic material such as a polymer, and an inorganic material such as garnet were used. Wave paths may be used.
[0040]
In the present embodiment, an arrayed waveguide grating having a small number of arrayed waveguides is used as a waveguide filter having a variable transmission wavelength, but the same effect can be obtained by using a Mach-Zehnder filter, a lattice filter, a transversal filter, or the like. can get.
[0041]
In the present embodiment, the thermo-optic effect of the heater is used for adjusting the transmission wavelength of the waveguide filter. However, the adjustment can be performed by irradiation of ultraviolet rays, electron beams, or radiation. When fabricated, it can also be adjusted by current injection or the like.
[0042]
In the present embodiment, the temperature dependence of the oscillation wavelength of the waveguide laser is eliminated by using a plurality of materials having different refractive index temperature changes, but the optical waveguide portion is formed by using a plurality of materials having different linear expansion coefficients. Even the method of controlling the applied stress can eliminate the temperature dependence of the oscillation wavelength of the waveguide laser.
[0043]
In the first embodiment of the present embodiment, the array waveguides of the array waveguide grating in the waveguide type loop mirror are divided into two groups on the inside and outside, but are divided into two groups for each array waveguide. A similar effect can be obtained.
[0044]
In the first embodiment of the present embodiment, the arrayed waveguides are divided into two groups and two types of waveguide length differences are provided. However, even if three or more groups are provided and three or more types of waveguide length differences are provided, the same applies. The effect of is obtained.
[0045]
Further, in this embodiment, one waveguide grating is provided in the waveguide type loop mirror. However, a plurality of arrayed waveguide gratings can be provided, and the oscillation wavelength can be selected by the Vernier effect.
[0046]
【The invention's effect】
As described above, according to the present invention, in a waveguide type laser having a built-in reflecting mirror having wavelength selectivity in a resonator, a waveguide type loop mirror is used for at least one end reflecting mirror. An arrayed waveguide grating is provided in a loop mirror, and a difference in waveguide length between individual arrayed waveguides in the arrayed waveguide grating is not constant. Since the waveguide is provided, a single mode oscillation useful for optical communication can be performed by selecting an oscillation wavelength in the waveguide loop mirror. In addition, since an ordinary waveguide filter and process technology can be used without requiring special processes such as fine processing and ultraviolet irradiation, an inexpensive laser can be provided.
[0047]
In addition, since the transmission band wavelength of the array waveguide grating, which is a wavelength selection element, and the temperature dependence of the resonator length of the entire resonator including the semiconductor amplifier can be simultaneously athermalized, the oscillation wavelength of the completed waveguide laser can be reduced. Temperature independence is possible. Therefore, an expensive temperature control mechanism can be omitted, and an inexpensive laser module can be manufactured.
[0048]
Further, by utilizing the periodicity of the arrayed waveguide grating, it has become possible to manufacture a wavelength tunable laser having a large variable width at low cost in combination with a filter having a low wavelength selectivity but a variable wavelength.
Further, since the laser oscillation wavelength can be selected at a period determined by the periodicity of the arrayed waveguide grating, there is no need for fine adjustment such as placing the wavelength on a grid when changing the oscillation wavelength.
[0049]
Such a single-mode laser whose oscillation wavelength is independent of temperature and a single-mode laser whose laser oscillation wavelength can be varied over a wide range have conventionally been very expensive. However, according to the present invention, a low-cost and small waveguide is used. A single-mode laser can be provided, which greatly contributes to the construction of a WDM optical system in terms of price, and also contributes to miniaturization of devices such as exchangers.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a waveguide laser according to a first embodiment.
FIG. 2 is a configuration diagram of a conventional waveguide laser which is a comparative example.
FIG. 3 is a configuration diagram of a waveguide laser according to a second embodiment.
FIG. 4 is a graph showing transmission characteristics of an arrayed waveguide type portion of the waveguide type laser according to the first embodiment.
FIG. 5 is a graph showing in detail a portion having a high transmittance among transmission characteristics of an arrayed waveguide type portion in the waveguide type laser according to the first embodiment.
FIG. 6 is a graph showing IL characteristics of the waveguide laser according to the first embodiment.
FIG. 7 is a graph showing an oscillation wavelength characteristic of the waveguide laser of the first embodiment.
FIG. 8 is a graph showing the temperature dependence of the laser oscillation intensity of the waveguide laser according to the first embodiment.
FIG. 9 is a graph showing the temperature dependence of the laser oscillation wavelength of the waveguide laser according to the first embodiment.
FIG. 10 is a configuration diagram showing in detail a second arrayed waveguide type structure of the waveguide type laser according to the second embodiment.
FIG. 11 is a graph showing a waveguide-type wavelength shift operation of a second array among the waveguide-type lasers of the second embodiment.
FIG. 12 shows a transmission spectrum of a waveguide type loop mirror including a first arrayed waveguide type, a transmission spectrum of a second arrayed waveguide type, and resonance of the waveguide type laser of the second embodiment. 6 is a graph in which transmission spectra of a quartz waveguide type portion of the vessel are superimposed.
FIG. 13 is a partially enlarged view of the spectrum diagram of FIG. 12;
FIG. 14 is a partially enlarged view of the spectrum diagram of FIG. 13;
FIG. 15 is a graph showing IL characteristics of the waveguide laser according to the second embodiment.
FIG. 16 is a graph showing the oscillation wavelength characteristics of the waveguide laser according to the second embodiment.
FIG. 17 is a graph showing an oscillation wavelength variable operation of the waveguide laser according to the second embodiment.
[Explanation of symbols]
11, 31 silica-based waveguide 12 arrayed waveguide grating 13, 14 input / output port 15, 353 dB coupler 16, 36 of arrayed waveguide grating semiconductor amplifier 17 groove 18 into which polymer flows, 38, quartz-based waveguide chip 19, 39 End surfaces 20 and 40 of semiconductor amplifier coated with high reflection fiber Fiber connection 21 Silica-based waveguide chip 22 Fiber connection 23 Grating,
24 Semiconductor amplifier 32 First arrayed waveguide grating 33, 34 Input / output port 37 of first arrayed waveguide grating Second arrayed waveguide type 41a, 41b, 41c Heater 41a, 42b Electrode 43, 44 Tapered portion 45a, 45b, 45c Array waveguide

Claims (3)

In a waveguide laser incorporating a reflector having wavelength selectivity in a resonator, a waveguide loop mirror is used for at least one end of the reflector, and an arrayed waveguide grating is provided in the waveguide loop mirror . The waveguide length difference given to each array waveguide in the arrayed waveguide grating is not one type, and two or more types of waveguide length differences are given to each array waveguide. Waveguide type laser. In a waveguide laser incorporating a reflector having wavelength selectivity in a resonator, a waveguide loop mirror is used for at least one end of the reflector, and an arrayed waveguide grating is provided in the waveguide loop mirror. waveguide laser you characterized by comprising a waveguide filter with different FSR is the FSR of the arrayed waveguide grating. 3. The waveguide laser according to claim 2 , wherein a waveguide filter having a variable transmission wavelength is used as the waveguide filter.

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