JP5001239B2 - Semiconductor tunable laser - Google Patents

Description

  The present invention relates to a wavelength tunable laser, and more specifically to a semiconductor wavelength tunable laser that is an important optical component for supporting wavelength division multiplexing large capacity communication.

  In recent years, due to the explosive increase of traffic on the Internet, transmission capacity between nodes is increased by using wavelength multiplexing for transmission between nodes.

  The wavelength tunable laser is an important component indispensable for such wavelength division multiplexing transmission.

  Under such a background, a tunable laser (SSG-DBR-LD) using a super-structure grating (SSG) has been proposed (for example, Non-Patent Document 1). reference).

  FIG. 21 is a diagram illustrating a schematic configuration of the SSG-DBR-LD. SSG is a reflection type grating with the feature that the reflection intensity increases at a constant frequency interval (Free Spectral Range: FSR). In SSG-DBR-LD, the least common multiple of two FSRs is obtained by using SSG having two different FSRs. It is possible to obtain a wavelength tunable operation in the frequency region. Specifically, in order to change the oscillation wavelength, current is independently injected into the SSG region, and the refractive index of the SSG portion is changed to adjust the grating reflection peak wavelength. Refractive index modulation can be performed at high speed in about nanoseconds by current injection.

Hiroyuki Ishii et al., “Multiple-Phase Shift Super Structure Grating DBR Lasers for Broad Wavelength Tuning”, Photonics Technology Letters, IEEE, June 1993, vol.5, No. 6, pp.613-615

  However, the above-described SSG-DBR-LD has a problem that the control is complicated because it is necessary to accurately adjust the refractive index of the phase adjustment region in addition to the two SSG-DBR regions. Further, when current is injected into the SSG region, heat is generated by the element resistance at the same time, and the temperature near the grating rises. This changes the refractive index in about microseconds. As a result, in order to change the longitudinal mode wavelength of the laser, the oscillation wavelength is changed in the order of several tens of GHz. When using a wavelength tunable laser for applications such as switching the output port by combining a wavelength tunable laser and a wavelength filter, if the wavelength change as described above occurs, crosstalk will occur to the adjacent channel, making it as small as possible. There is a need.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a wavelength tunable laser having a simple wavelength control and a small wavelength change (wavelength drift) after wavelength switching.

  As described above, the wavelength drift is caused by a change in the longitudinal mode wavelength of the laser due to heat generation from the element resistance after current injection into the SSG region. Therefore, one feature of the means for solving the problem in the wavelength tunable laser according to the present invention is that the laser part and the optical feedback unit having the wavelength tunable function are independently configured and monolithically integrated. In the laser of the present invention, an optical feedback circuit (optical feedback unit) including a wavelength tunable filter selectively feeds back one mode from the multimode oscillation spectrum of the Fabry-Perot laser (laser part) to the Fabry-Perot laser. Thus, it functions as a single mode laser in which only light having the same wavelength as the fed-back light oscillates. In the wavelength tunable laser of the present invention that operates on such an operating principle, the longitudinal mode interval of the laser is determined by the cavity length of the Fabry-Perot laser portion. Therefore, the longitudinal mode of the Fabry-Perot laser (laser portion) does not change even if the refractive index of the wavelength tunable filter in the optical feedback section changes due to heat generation, and wavelength drift that causes problems in the SSG-DBR laser or the like does not occur. As the wavelength variable filter, a grating type wavelength variable filter, a ladder type wavelength variable filter, a ring resonance type wavelength variable filter, or the like can be used. When a ladder type wavelength tunable filter or a ring resonance type wavelength tunable filter, which is a transmission type filter, is used as a wavelength tunable filter, a reflector between the Fabry-Perot laser and the optical feedback circuit (two opposing mirrors of the Fabry-Perot laser) By providing a reflecting mirror at an end different from the reflecting mirror on the optical feedback circuit side), a reflective wavelength tunable filter can be obtained.

  Another feature of the means for solving the problem in the wavelength tunable laser according to the present invention is that the reflecting mirror between the Fabry-Perot laser and the optical feedback circuit is formed by dry edging. As a result, the Fabry-Perot laser and the optical feedback circuit can be monolithically integrated, and the tunable laser according to the present invention can be manufactured stably and compactly. Preferably, both of the two opposing mirrors of the Fabry-Perot laser are produced by dry edging. Thereby, the resonator length can be designed and created with the positioning accuracy of the resist mask in the edging process.

  Still another feature of the means for solving the problem in the wavelength tunable laser according to the present invention is that the longitudinal mode interval of the resonator in the Fabry-Perot laser is created in accordance with the wavelength grid interval of the output wavelength. This makes it possible to switch the wavelength of the output light (the Fabry-Perot laser oscillation wavelength) to one of the wavelength grids accurately and easily by changing only the filter characteristics of the tunable filter (frequency characteristics of the feedback light). A tunable laser can be provided. This cannot be realized with a wavelength tunable laser in which the wavelength changes in an analog manner depending on the reflection wavelength of the two DBRs and the refractive index of the phase adjustment region, such as the conventional SSG-DBR laser.

  By the way, since the refractive index of a semiconductor is generally wavelength-dependent, the longitudinal mode interval of a resonator in a Fabry-Perot laser is also slightly wavelength-dependent. In particular, an active layer having a gain necessary for laser oscillation has a large band gap and a wavelength dependency of the refractive index near the oscillation wavelength because the band gap is substantially equal to the oscillation wavelength. Such a slight change in the oscillation wavelength due to the wavelength dependence of the refractive index is an unacceptable value depending on the purpose of use.

  Accordingly, one feature of the means for solving the problem in the wavelength tunable laser according to the present invention is that the band gap of the composition of the portion where the optical confinement factor of the active layer in the Fabry-Perot laser is small or the light field other than the active layer is widened. Is configured to be large. Thereby, the wavelength dependency of the resonator in the Fabry-Perot laser can be reduced.

  Another feature of the means for solving the problem in the wavelength tunable laser according to the present invention is that a wavelength adjusting region is provided in the Fabry-Perot laser. As a result, the wavelength dependence of the resonator in the Fabry-Perot laser can be compensated, and the wavelength of the output light can be accurately and easily adjusted to the wavelength grid, and at the same time, the manufacturing margin of the tunable laser according to the present invention can be increased. Can do.

  One feature of the means for solving the problem in the wavelength tunable laser according to the present invention is that a semiconductor optical amplifier (SOA) is provided in the optical feedback circuit. Thereby, in addition to the adjustment of the reflectance of the optical feedback circuit, the light intensity of the feedback light fed back to the Fabry-Perot laser can be adjusted, and at the same time, the manufacturing margin of the wavelength tunable laser according to the present invention can be increased. it can.

  By the way, when the feedback light from the feedback circuit is excessively larger than the light intensity necessary for single mode oscillation, in addition to the resonator constituting the Fabry-Perot laser, the optical feedback circuit itself oscillates as a laser, The longitudinal mode interval is determined for the entire device including the optical feedback circuit. In such a case, the laser longitudinal mode interval becomes as narrow as about 10 GHz, and the operation becomes unstable (mode hop etc.) depending on the setting of the environmental temperature and the peak wavelength of the filter.

  Therefore, one feature of the means for solving the problem in the wavelength tunable laser according to the present invention is that the feedback light intensity is larger than the threshold intensity for causing the Fabry-Perot laser to oscillate in a single longitudinal mode, and from the Fabry-Perot laser side. In the output optical spectrum, adjustment is made so that modes other than the longitudinal mode determined by the cavity length of the Fabry-Perot laser are not observed (becomes smaller than the observed mode intensity). By controlling the laser resonator to be determined only by the Fabry-Perot laser resonator part by operating the feedback light intensity slightly larger than the threshold intensity, widen the longitudinal mode interval of the laser and stabilize the operation. Can do.

  Further, another feature of the means for solving the problem in the wavelength tunable laser according to the present invention is that a reflecting mirror between the Fabry-Perot laser and the optical feedback circuit (the optical feedback circuit side of two opposing reflecting mirrors of the Fabry-Perot laser) That is, the reflectance of the reflecting mirror is set larger than the reflectance of the other reflecting mirrors. As a result, the Fabry-Perot laser and the optical feedback circuit become independent resonators, and the entire element including the optical feedback circuit as described above is prevented from operating as a composite cavity. The operation can be stabilized.

  One feature of the means for solving the problem in the wavelength tunable laser according to the present invention is that a ladder type filter having a large wavelength tunable range is used as one wavelength tunable filter used in the optical feedback circuit. . Thereby, simpler wavelength tunable operation becomes possible.

  Further, another feature of the means for solving the problem in the wavelength tunable laser according to the present invention is that the wavelength tunable filter used in the optical feedback circuit is two or more wavelength tunable in which reflected light or transmitted light has a maximum at different frequency intervals. The filter is used. Thereby, the wavelength variable operation can be made possible.

  One feature of the means for solving the problem in the wavelength tunable laser according to the present invention is that the SOA is integrated at the output end of the Fabry-Perot laser. As a result, the output of the wavelength tunable laser according to the present invention can be increased.

  As described above, according to the present invention, it is possible to provide a wavelength tunable laser that has few wavelength drifts that could not be realized so far and is easy to control. According to the present invention, it is possible to provide a wavelength tunable laser in which wavelength drift due to a temperature change generated in the vicinity of a wavelength filter at the time of wavelength tuning is suppressed and wavelength tuning control is simple.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a configuration diagram of a wavelength tunable laser according to an embodiment of the present invention. The monolithic tunable laser shown in FIG. 1 includes a Fabry-Perot laser unit having a reflecting mirror 1 and a reflecting mirror 2 and an optical feedback circuit including a tunable filter coupled to the Fabry-Perot laser unit via the reflecting mirror 2. Integrated and oscillates in a single longitudinal mode at a wavelength selected by a tunable filter.

  As shown in FIG. 1, a gain layer having a gain at an oscillation wavelength (in this embodiment, an MQW structure using InGaAsP having a photoluminescence peak at 1.53 microns) is provided in the core layer of the waveguide of the Fabry-Perot laser section. And a passive layer having a low absorption at the oscillation wavelength (in this embodiment, an InGaAsP bulk structure having a photoluminescence peak at 1.4 microns) in the core layer of the waveguide of the wavelength tunable filter. As shown in the figure, in the wavelength tunable filter of this embodiment, the waveguide of the Fabry-Perot laser unit and the waveguide of the wavelength tunable filter use an epitaxial growth substrate having a butt joint structure. The portions other than the core layer are common, and the lower part of the core layer is an n-type InP layer on the n-type InP substrate, and the upper part of the core layer is a p-type InP layer and a p-type InGaAsP contact layer from the bottom.

  The gain layer and the reflecting mirrors 1 and 2 at both ends of the gain layer constitute a Fabry-Perot laser, and the transmission type wavelength tunable filter and the reflecting mirrors 2 and 3 at both ends of the wavelength tunable filter constitute an optical feedback circuit. When the wavelength tunable filter is a reflection type (for example, a grating type wavelength tunable filter), the reflecting mirror 3 is unnecessary and an antireflection coating is necessary.

  FIG. 2 is a diagram for explaining a configuration method of the reflecting mirror 2. In order to monolithically integrate the Fabry-Perot laser unit and the optical feedback circuit, it is important to manufacture a reflecting mirror having substantially the same reflectance in a wide wavelength range. FIG. 2A shows a cross-sectional view of a gap mirror manufactured by etching. The gap mirror was produced by inductively-coupled plasma (ICP) reactive ion etching using chlorine as an etching gas.

  FIG. 2B is a diagram showing the relationship between the gap width and the reflectance. Depending on the gap width, the reflectance can be set in the range of about 0% to 40%. When the gap is produced with a gap width of 1 micron or less, substantially the same reflectance can be obtained at a wavelength of 200 nm or more.

  FIG. 2C is a diagram showing a grating-type reflecting mirror as used in a DFB laser, and a grating-type reflecting mirror can be used as the reflecting mirror 2. However, in the case of using a grating-type reflecting mirror, it is essential to produce a grating that etches most of the active layer in order to have a large reflectance over a wide range.

  Next, the operation principle of the present element will be described with reference to FIG. In FIG. 3, the portion surrounded by the broken line is the wavelength of the laser beam to be output. FIG. 3A is a diagram showing the output from the reflecting mirror 2 of the Fabry-Perot laser of FIG. 1 when there is no optical feedback circuit, and the Fabry-Perot laser is sandwiched between the reflecting mirrors 1 and 2 as shown. Oscillates at a plurality of peak wavelengths of the Fabry-Perot resonator. FIG. 3B shows the transmission characteristics of the wavelength tunable filter. The light output from the reflecting mirror 2 of the Fabry-Perot laser is filtered by the wavelength tunable filter, then reflected by the reflecting mirror 3 having a desired reflectance, passes again through the wavelength tunable filter, and is again fed to the Fabry-Perot laser as feedback light. Incident. FIG. 3C is a diagram showing the re-incident feedback light spectrum. Therefore, the wavelength tunable laser according to the present embodiment oscillates in a single mode only at a wavelength having the highest feedback light intensity as shown in FIG. By making the peak wavelength of the feedback light variable, the wavelength tunable laser of this embodiment can obtain output light according to the longitudinal mode interval of the Fabry-Perot laser.

  For comparison, the operation principle of the conventional SSG-DBR-LD shown in FIG. 21 will be described with reference to FIG. FIG. 4A shows the output from the active layer region in the case of only the active layer region without the SSG-DBR region. As shown in the figure, it becomes a broad spontaneous emission light depending on the composition of the active layer. FIG. 4B shows a reflection spectrum from an SSG-DBR region having two front and rear different FSRs. A large reflectance can be obtained at a wavelength at which the peak wavelengths of the two SSG-DBRs coincide. FIG. 4C shows a cavity mode (longitudinal mode) obtained from the effective lengths of the active layer, the phase adjustment region, and the SSG-DBR regions on both sides. Therefore, the SSG-DBR-LD of the conventional example has the highest reflectivity as shown in FIG. 4D and oscillates in a single mode at a wavelength that matches the cavity mode (longitudinal mode).

  When both are compared, in the wavelength tunable laser of this embodiment, the oscillation wavelength is limited by the mode interval of the Fabry-Perot laser. For example, the cavity mode interval can be set at 100 GHz in the wavelength tunable laser of this embodiment, whereas it is about 20 GHz in the SSG-DBR laser. In order for the laser to stably oscillate in the single mode, the difference in the intensity of the feedback light in the cavity mode or the difference in the reflectance from the DBR region is important. Therefore, in the wavelength tunable laser according to the present embodiment, the restriction on the filter characteristics (for example, 3 dB bandwidth) is relaxed as compared with the conventional example. Further, as described above, the cavity mode of the conventional SSG-DBR laser is determined from the effective lengths of the active layer, the phase adjustment region, and the SSG-DBR regions on both sides. Therefore, when the reflection peak of the SSG-DBR is changed by current injection or the like, the refractive index of the SSG-DBR region changes due to heat generation, resulting in a change in the cavity mode wavelength and a wavelength drift. On the other hand, in the wavelength tunable laser according to the present embodiment, the cavity mode wavelength is determined only in the Fabry-Perot laser part. Therefore, even if the refractive index change due to heat generation occurs in the wavelength tunable filter part, the cavity mode wavelength changes. Will not. In addition, a slight change in the transmission peak wavelength due to heat generation does not change the transmission wavelength because the cavity mode interval is wide.

  The longitudinal mode interval of the resonator in the Fabry-Perot laser is created in accordance with the wavelength grid interval of the output wavelength. If the longitudinal mode interval of the Fabry-Perot laser is set to the same wavelength as the wavelength interval (ITU grid) standardized by, for example, the International Telecommunication Union (ITU), the wavelength tunable laser of this embodiment can transmit the transmission peak of the wavelength tunable filter. It becomes possible to easily change the wavelength of the output light to any wavelength on the ITU grid simply by adjusting the wavelength. At this time, since the conventional laser uses a cleavage to produce a laser, the cavity length can only be controlled within a few microns. However, if both ends of a Fabry-Perot laser are produced by an etching mirror and a gap mirror by dry etching, a submicron can be obtained. It becomes controllable at a degree.

  By the way, in general, since the refractive index of a semiconductor has a wavelength dependence, the longitudinal mode interval of the Fabry-Perot laser has a wavelength dependence, although it is slight. In particular, an active layer having a gain necessary for laser oscillation has a large wavelength dependence of the refractive index because the band gap is substantially equal to the oscillation wavelength. This is an unacceptable wavelength shift depending on the intended use.

  FIG. 5 is a diagram showing one measure for reducing the wavelength dependence of the longitudinal mode interval of the Fabry-Perot laser. FIG. 5A shows an optical waveguide with a loaded structure as an alternative to the optical waveguide of a Fabry-Perot laser. As shown in FIG. 5A, when an active layer having a multiple quantum well structure (MQW, total number of wells 10) is used, a passive layer (photoluminescence peak wavelength) having a band gap larger than that of the active layer below the active layer. 1.4 micron and 0.3 micron film thickness) shifts the light field to the passive layer. In FIG. 5A, the shaded portion is the active layer. The composition of this passive layer is InGaAsP. In FIG. 5B, the horizontal axis indicates the distance (micron) in the thickness direction, and the vertical axis indicates the light amplitude. Thus, it can be seen that the center of the light field is shifted to the passive layer side. On the other hand, in the structure of the epitaxial substrate of the laser portion in the case of using the butt joint structure used for a normal laser as shown in FIG. 6A, the active layer is at the center of the light field. In FIG. 6A, the shaded portion is the active layer. The confinement factor to the active layer in each case is 28% in the case of FIG. 5 and 38% in the case of FIG. Therefore, even when the refractive index of the active layer portion has the same wavelength dependence, the effect is reduced by reducing the confinement coefficient when viewed as the entire optical waveguide. The same thing can be achieved by reducing or thinning the number of well layers when using the MQW active layer, increasing the band gap of the barrier layer, or increasing the band gap of the SCH (Separate-confinement heterostructure) layer. By reducing the wavelength dependency of the refractive index of the entire waveguide, the problem of the wavelength dependency of the refractive index can be solved.

  FIG. 7 is a diagram showing a measure for finely adjusting the wavelength with higher accuracy and compensating for the wavelength dependence of the longitudinal mode interval of the Fabry-Perot laser. That is, since the wavelength dependence remains slightly even with the measure described above with reference to FIG. 5, more accurate wavelength adjustment is required depending on the application method of the element (for example, in a DWDM system with 50 GHz intervals, ± 2. 5 GHz), which is a solution to cope with this.

  Referring to FIG. 7, the Fabry-Perot laser portion is provided with a refractive index adjustment region for wavelength adjustment. The wavelength grid can be adjusted more accurately than this refractive index adjustment region. At this time, since only a slight adjustment of the refractive index is required, a method by applying an electric field can be considered as a method for adjusting the refractive index. In this case, an increase in wavelength drift is negligible because heat generation by applying the electric field is small. As an example, when a wavelength grid of 50 GHz intervals is considered, the resonator of the Fabry-Perot laser is about 800 microns. When a 200-micron refractive index adjustment region is provided in the cavity, the refractive index change due to electric field application is about 0.03%, so that a frequency shift of about 20 GHz can be realized. This is a sufficient value even considering the wavelength dispersion of the refractive index of the semiconductor. This value is not an absolute value of the length of the refractive index region, but is proportional to the ratio of the refractive index adjustment region in the cavity, so that sufficient adjustment is possible even when the cavity length of the Fabry-Perot laser is shortened. It is.

  In the embodiments so far, the feedback light intensity from the optical feedback circuit is adjusted by the reflectance of the reflecting mirror 3, but the present invention is not limited to this. That is, the light intensity fed back to the Fabry-Perot laser can be adjusted using a semiconductor optical amplifier (SOA) in the optical feedback circuit.

FIG. 8 is a diagram illustrating a configuration example of a wavelength tunable laser using a ladder type wavelength tunable filter as a wavelength tunable filter. As the reflecting mirror 2, a gap mirror having the structure shown in FIG. 2A was produced by an ICP-RIE etching apparatus using chlorine gas. Further, the reflecting mirrors 1 and 3 have the cleavage plane as a reflection point. The Fabry-Perot laser is a resonator part sandwiched between the reflecting mirrors 1 and 2. The optical feedback circuit has a phase adjustment region, a ladder-type wavelength variable filter, and a semiconductor optical amplifier. The laser active layer of the fabricated device is an n-InP layer on an n-InP substrate, an active layer of InGaAsP / InP multiple quantum well structure (MQW) (photoluminescence peak wavelength 1.53 μm), and SCH (up and down of the active layer). Separate-confinement heterostructure) layer. Next, a SiO ₂ film is formed by sputtering, and is removed by etching except for a portion that becomes a gain region (a Fabry-Perot laser portion and a semiconductor optical amplifier portion). Further, the active layer is removed using the patterned SiO ₂ film as a mask. . Next, a 1.4Q composition InGaAsP optical waveguide layer was butt-joint grown by selective growth, and finally the SiO ₂ layer was removed to grow a p-InP layer and a p ⁺ -InGaAs layer on the entire substrate.

  The ladder-type wavelength tunable filter shown in FIG. 8 includes two waveguides that perform input and output, optical couplers arranged at equal intervals, and a connection waveguide array that connects the optical couplers. At this time, the length of the adjacent connection waveguide array is decreased by ΔS. At this time, the transmission peak wavelength is expressed by the following equation.

n _eff is the effective refractive index of the waveguide, and m ₀ is the diffraction order. This peak wavelength can be changed by injecting a current into the electrode 1 or 2 to change the refractive index of the input / output waveguide.

  The feedback light intensity of the optical feedback circuit can be adjusted by the product of the amount of current injected into the semiconductor optical amplifier and the reflectance of the reflecting mirror 3. Therefore, even if the loss of the tunable filter is large and the Fabry-Perot laser cannot be controlled to single longitudinal mode oscillation (single wavelength oscillation), the feedback above the threshold for single longitudinal mode oscillation can be achieved by using a semiconductor optical amplifier. Light intensity can be obtained.

FIG. 9 shows a reflection spectrum of feedback light from the optical feedback circuit of the wavelength tunable laser shown in FIG. In this device, m ₀ was set to 60. In this case, as is apparent from the equation (1), this filter has a transmission peak wavelength even in the adjacent diffraction orders (m ₀ ± 1). When the oscillation spectrum from the Fabry-Perot laser has a large wavelength dependence, the optical feedback intensity is small at the reflection peak on both sides, so that oscillation does not occur at the peak of (m ₀ ± 1) and there is no problem in operation. . In this case, the wavelength variable operation is performed based on the operation principle described above with reference to FIG.

  FIG. 10 shows that the optical intensity of the feedback light from the optical feedback circuit to the Fabry-Perot laser in the wavelength tunable laser is larger than the threshold intensity (in this case, 40 microwatts) for oscillating the single longitudinal mode, and the Fabry-Perot laser side In the output light spectrum from, the feedback light intensity at the peak wavelength (so that it is smaller than the observed light intensity) is observed so that modes other than the longitudinal mode determined by the cavity length of the Fabry-Perot laser are not observed. The oscillation spectrum of output light when adjusted to 60 microwatts is shown. As shown in the figure, the light intensity increases at intervals of about 0.8 nm according to the longitudinal mode interval of the Fabry-Perot laser. When the feedback light intensity is controlled in this way, the desired frequency interval (100 GHz is obtained by changing only the center wavelength of the wavelength tunable filter by adjusting to the output wavelength interval (in this case, 100 GHz interval) as described above. ) To change the wavelength. As described above, since the wavelength tunable laser according to the present invention is not an analog wavelength tunable operation as in the conventional example, the wavelength control method can be simplified.

In addition, when the reflecting mirror 2 is composed of a gap mirror and the gap interval is changed to change the reflectance of the reflecting mirror 2, the higher the reflectance of the reflecting mirror 2, the more the influence of the composite cavity is affected even when the feedback light intensity is larger. Not observed. When the reflectance R ₂ of the reflecting mirror ₂ is larger than the reflectances R ₁ and R ₃ of the reflecting mirrors 1 and 3 (R ₂ > R ₁ and R ₂ > R ₃ ), the influence of the composite cavity is observed. Was not.

  The wavelength tunable laser according to the present invention operates as a wavelength tunable laser even when the reflectance of the reflecting mirror 3 is set to be greater than 0% and 3% or less and extremely close to 0. FIG. 11 is a diagram for explaining the operating principle when the reflectance of the reflecting mirror 3 is set very close to zero. FIG. 11 (a) is a diagram showing the output from the reflecting mirror 2 of the Fabry-Perot laser when there is no optical feedback circuit, as in FIG. 3 (a), and shows a plurality of peak wavelengths. . Since the reflectivity of the reflecting mirror 3 is very close to 0, the light incident on the Fabry-Perot laser is incident on the filter by the spontaneous emission light from the SOA, not the feedback light from the Fabry-Perot laser. FIG. 11B shows spontaneous emission light from the SOA after passing through the wavelength tunable filter. In this case, if the intensity of spontaneously emitted light from the SOA is sufficiently large, it is possible to select light having only one wavelength from among the oscillation wavelengths of the Fabry-Perot laser as shown in FIG.

  FIG. 12 shows an output spectrum when the feedback light intensity is 10 milliwatts. In this case, the laser resonator is also formed between the reflecting mirrors 2 and 3 and between the reflecting mirrors 1 and 3 in FIG. 8, and as shown in FIG. Is observed. When the longitudinal mode interval is narrow, phenomena such as mode hopping in which the oscillation wavelength suddenly moves to the adjacent peak due to fluctuations in the transmission peak wavelength of the wavelength tunable filter and slight changes in the operating temperature environment are observed. Not suitable as a variable laser.

In this way, a tunable laser that is stable and easy to control can be realized by using the ladder filter as an optical filter. However, depending on the length of the Fabry-Perot laser, the injection current, or the setting of the diffraction order of the ladder filter (m ₀ ± In some cases, the reflection peak of 1) has a gain that can be sufficiently oscillated. In this case, oscillation occurs at a plurality of wavelengths. In particular, narrowing the transmission peak band of the ladder filter is important for improving the wavelength selectivity, but this requires an increase in the diffraction order. At the same time, it is necessary to solve this problem because the transmission peak interval due to adjacent diffraction orders becomes narrow. In order to avoid this, it is important for the stable operation of the device to chirp the waveguide length difference (ΔS) in each connecting waveguide to produce a wavelength filter with a single transmission peak. is there.

  FIG. 13 is a diagram showing a configuration of a wavelength tunable laser including a Fabry-Perot laser and a chirped ladder filter (chirped ladder filter). The chirp ladder filter is coupled to the Fabry-Perot laser via the reflector 2 and the SOA. The chirped ladder filter has a plurality of MMI couplers each connected to one of the arrayed waveguides, and each MMI coupler is connected to an adjacent MMI coupler by a connection waveguide. A reflecting mirror 3 is provided at the end and the end of each arrayed waveguide connected to the MMI coupler. In the chirp ladder filter, the filter wavelengths are the same at the time of interference at each MMI coupler, and the diffraction orders at the front and rear couplers are set to be different.

  The operation principle of the filter portion will be described with reference to the lower diagram of FIG. The light incident on the MMI coupler 1 is bifurcated at a certain intensity ratio (15:85 in this case). The light output to the upper arrayed waveguide is reflected by the terminal reflecting mirror 3-k and is incident on the MMI coupler 1 again. On the other hand, the light output to the lower connection waveguide is branched again by the MMI coupler 2. Considering only the interference between adjacent array waveguides, the light is reflected by the reflecting mirror 3- (k + 1) and again enters the MMI coupler 1 through the MMI coupler 2 and the connection waveguide. At this time, interference occurs in the MMI coupler 1. The center wavelength at this time is expressed by the following equation.

Here, the upper array waveguide length is l ₁ , the connection waveguide length is l ₂ , the lower array waveguide length is l ₃ , the refractive index is n _eff , and the refractive index of the MMI coupler 2 is n _MMI , MMI coupler. L _{eff. Assuming MMI} , the effective length ΔS _eff is

It is.

  At this time, the diffraction orders at each interference stage are as follows.

Here, γ is the chirping intensity, i is the arrayed waveguide number, and m _MAX is the maximum diffraction order. Thus, if you set the interference wave in each MMI coupler at both sides of the peak, but the diffraction orders are different it is constant and l ₀ is suppressed.

FIG. 14 is a diagram showing filter characteristics obtained by calculation assuming that l ₀ is 1.53 microns, m _N = 72, and γ = 3. As can be seen, the transmission peak near 1.505 microns or 1.555 microns is small. When an optical filter having a single peak is used as described above, there is no problem of multimode oscillation as described above, and a wider wavelength variable region can be stably obtained.

  Even with a chirped ladder filter, it is necessary to change the refractive index by forming an electrode on the connection waveguide in order to change the oscillation wavelength. In this case, in order to make the wavelength variable amount constant at each interference stage, It is necessary to change the electrode length in accordance with the change of the diffraction order. Specifically, the electrode length is changed according to the following equation.

Here, L _k is the electrode length at the k-th diffraction stage, and L _MAX is the electrode length at the diffraction stage having the maximum diffraction order. By setting the electrode length in this way, a large wavelength variable range of 50 nm could be obtained.

  FIG. 15 is a diagram illustrating a configuration example of a wavelength tunable laser using a ring resonator as a wavelength tunable filter. In the configuration example shown in FIG. 15, a ring resonator is used as a wavelength tunable filter having a periodic transmission peak, but other than this, a grating such as an SSG as in the conventional example can also be manufactured. Normally, when a ring resonator or a grating is operated as a wavelength tunable filter using current injection into a semiconductor, the wavelength tunable range is about 10 nm or less in one ring resonator or grating, unlike a ladder type tunable filter. However, a large wavelength tunable range can be obtained by combining two or more periodic filters having slightly different repetition periods (FSRs). FIG. 15 shows a configuration example in which wavelength tunable filters having periodic transmission peaks are connected in series. However, wavelength tunable filters may be connected in parallel to be coupled to the phase adjustment region.

  FIG. 16 is a diagram for explaining a method for setting the FSR of the ring resonator of the wavelength tunable laser shown in FIG. In the configuration example of the wavelength tunable laser shown in FIG. 15, the longitudinal mode interval of the laser is 100 GHz. On the other hand, the FSR of the ring resonator was designed around 500 GHz. FIG. 16A shows the transmittance (reflectance) of the wavelength tunable filter when the FSRs of the two ring resonators are 530 GHz and 560 GHz. FIG. 16B is an enlarged view of a part of FIG. When the transmittance is observed at a wavelength (optical frequency) that is a multiple of 100 GHz of the longitudinal mode interval of the Fabry-Perot laser, if the relative optical frequency oscillates at 0 GHz, the adjacent peak of the ring resonator at 500 GHz, 1000, or 1100 GHz Is observed. Therefore, in order to reduce the transmission peak of these frequencies, 530 GHz and 560 GHz which do not coincide with a multiple of 100 are set. With such a setting, it is possible to obtain a large feedback light intensity difference of 3 dB or more at an optical frequency that matches the longitudinal mode interval of the Fabry-Perot laser.

  FIG. 17 is a diagram showing the wavelength tunable characteristics of a wavelength tunable laser manufactured by applying the method for setting the FSR of the ring resonator described with reference to FIG. In FIG. 17, the oscillation spectrum when the wavelength is tuned is overwritten. As in the previous embodiments, the output light from the Fabry-Perot laser is fed back to the Fabry-Perot laser only with light having a wavelength that matches the resonance wavelength of the two ring resonators, so that single-mode oscillation can be obtained. Further, by controlling the refractive indexes of the two ring resonators, the oscillation wavelength can be freely selected at 100 GHz intervals of the longitudinal mode interval of the Fabry-Perot laser as illustrated.

  FIG. 18 is a diagram showing a result of measuring the oscillation wavelength of the wavelength tunable laser of the present embodiment when the resonance wavelength of the ring resonator of the present embodiment is switched at a high speed on the order of nanoseconds. For the measurement, a Mach-Zehnder interferometer having an FSR of 150 GHz was used. The current amplitude to the ring resonator was 8 mA. As shown in FIG. 18, the wavelength drift was 1 GHz or less, and it was confirmed that stable operation was achieved even when the resonance wavelength was switched at a high speed on the order of nanoseconds. The wavelength tunable filter is not limited to a ring resonator type tunable filter, but the same effect can be obtained by configuring a tunable filter with a grating type tunable filter or a ladder type tunable filter. It is done.

  For reference, FIG. 19A shows a configuration example in which the optical feedback circuit portion operates as a laser, and FIG. 19B shows a change in output wavelength in the configuration of FIG. 19A. As shown in FIG. 19B, it can be seen that the wavelength is changed by about 7 GHz despite the same current injection of 8 mA. This is because the longitudinal mode wavelength of the laser is changed due to a change in refractive index due to heat generation caused by current injection because the ring resonator is in the laser resonator. In the case of the wavelength tunable laser according to the present invention, since the longitudinal mode is determined by the Fabry-Perot laser, the heat generated in the vicinity of the ring resonator does not affect the longitudinal mode wavelength, so that stable operation as shown in FIG. 18 is possible.

  FIG. 20 shows a reflection mirror 1 having a configuration example of a wavelength tunable laser using the ring resonator shown in FIG. 15 manufactured by etching in the same manner as the reflection mirror 2 and a semiconductor optical amplifier at the output end (left side) of the Fabry-Perot laser. It is a figure which shows the structural example which formed. In the case of the configuration example shown in FIG. 20, since the reflecting mirrors on both sides of the Fabry-Perot laser are formed by dry etching, the resonator length can be controlled with higher accuracy than in the case of using cleavage. Since a semiconductor optical amplifier is provided on the laser beam output side, an output light intensity of +14 dBm can be obtained.

  Although the embodiments of the present invention have been described above, the active layer is not limited to InGaAsP, and InGaAs, AlGaInAs, GaInNAs, or the like may be used. The wavelength band is not limited to the 1.55 μm band, and may be a long wavelength band such as a 1.3 μm band or other wavelength bands.

It is a block diagram of the wavelength tunable laser concerning one Embodiment of this invention. It is a figure for demonstrating the structural method of the reflective mirror 2 of FIG. It is a figure explaining the principle of operation of the wavelength variable laser shown in FIG. FIG. 22 is a reference diagram for explaining an operation principle in the case of the SSG-DBR-LD of the conventional example shown in FIG. 21. It is a figure for demonstrating the field distribution of the light at the time of making the optical waveguide of a Fabry-Perot laser into a loading type structure. It is a figure for demonstrating the field distribution of the light at the time of making the optical waveguide of a Fabry-Perot laser into a butt joint structure. It is a figure which shows the structure of the Fabry-Perot laser at the time of providing the refractive index adjustment area | region in a Fabry-Perot laser. It is a figure which shows the structural example of the wavelength variable laser provided with the semiconductor amplifier and using the ladder type wavelength variable filter as a wavelength variable filter. It is a figure which shows the reflection spectrum from a ladder type | mold wavelength variable filter at the time of seeing the optical feedback circuit side from the reflective mirror. In a wavelength tunable laser, the optical intensity of the feedback light from the optical feedback circuit to the Fabry-Perot laser is larger than the threshold intensity for causing single longitudinal mode oscillation, and the Fabry-Perot laser resonator in the output light spectrum from the Fabry-Perot laser side It is a figure which shows the oscillation spectrum of output light at the time of adjusting feedback light intensity so that modes other than the longitudinal mode determined by length may not be observed. It is a figure explaining the principle of operation when the reflectance of reflecting mirror 3 is brought close to 0. FIG. 9 is a diagram showing an oscillation spectrum when the optical feedback light intensity is increased in the wavelength tunable laser shown in FIG. 8. It is a figure which shows the structure of a wavelength tunable laser provided with the Fabry-Perot laser and the chirped ladder filter. It is a figure which shows the reflection spectrum at the time of introduce | transducing chirping into a reflection type ladder filter. It is a figure which shows the structural example of the wavelength tunable laser which used the ring resonator as a wavelength tunable filter. FIG. 16 is a diagram for explaining a method for setting the FSR of the ring resonator of the wavelength tunable laser shown in FIG. 15. It is a figure which shows the wavelength variable characteristic of the wavelength variable laser shown in FIG. FIG. 16 is a diagram illustrating a result of measuring an oscillation wavelength of the wavelength tunable laser according to the present embodiment when the resonance wavelength of the ring resonator illustrated in FIG. 15 is switched at a high speed on the order of nanoseconds. It is a reference figure for demonstrating the case where an optical feedback circuit part is operated as a laser. FIG. 16 is a diagram showing a modification of the configuration example of the wavelength tunable laser using the ring resonator shown in FIG. 15. It is a figure which shows schematic structure of the conventional SSG-DBR-LD.

Claims (8)

A tunable laser that oscillates in a single longitudinal mode at a wavelength selected by a tunable filter,
A Fabry-Perot laser having a first reflecting mirror and a second reflecting mirror;
An optical feedback circuit optically coupled to the Fabry-Perot laser and feeding back light to the Fabry-Perot laser side in accordance with light from the Fabry-Perot laser through the second reflecting mirror ;
With
The Fabry-Perot laser and the optical feedback circuit are monolithically integrated,
The second reflecting mirror is a reflecting mirror formed by etching ,
The optical feedback circuit includes a ladder-type wavelength tunable filter including an arrayed waveguide including a plurality of waveguides having different optical path lengths, and a plurality of couplers branching the waveguides, and the ladder-type wavelength tunable filter. A plurality of third reflecting mirrors provided corresponding to the respective waveguides and reflecting light having a light intensity corresponding to the reflectivity toward the respective couplers;
The optical feedback circuit further includes a semiconductor optical amplifier capable of adjusting the light intensity of the light reflected from each of the third reflecting mirrors and fed back to the Fabry-Perot laser side via the ladder-type wavelength tunable filter. A tunable laser characterized by that.   The wavelength of the Fabry-Perot laser resonator of the Fabry-Perot laser can be tunable at a desired frequency interval by changing only the center wavelength of the tunable filter by adjusting the longitudinal mode interval to the output wavelength interval. Tunable laser.   The waveguide structure of the Fabry-Perot laser has a wavelength dependency of the longitudinal mode interval of the Fabry-Perot laser by forming a laminated structure of an active layer having a gain in the oscillation wavelength band and an optical waveguide layer having no gain. 3. The wavelength tunable laser according to claim 1, wherein the wavelength tunable laser is reduced.   4. The wavelength tunable laser according to claim 1, wherein a refractive index adjustment region for wavelength adjustment is provided in the Fabry-Perot laser, and adjustment is possible at a desired frequency interval. The intensity of the feedback light from the optical feedback circuit is larger than a threshold intensity for oscillating in a single longitudinal mode, and in the output light spectrum from the Fabry-Perot laser side, other than the longitudinal mode determined by the cavity length of the Fabry-Perot laser 5. The wavelength tunable laser according to claim 1, wherein the feedback light intensity is adjusted so that the mode is smaller than the observed intensity. 6. The wavelength tunable laser according to claim 1, wherein a reflectance of the second reflecting mirror is set to be larger than a reflectance of another reflecting mirror. The wavelength tunable laser according to claim 1, wherein a reflectance of the third reflecting mirror is greater than 0% and 3% or less. 9. The wavelength tunable laser according to claim 1, wherein a high output can be achieved by integrating a second semiconductor optical amplifier at the output end.

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