JP5099961B2 - Laser equipment - Google Patents

Description

  The present invention relates to a laser device, and more specifically, a wavelength selective laser device of a type that drives and outputs a laser having at least one wavelength selection function among laser arrays having lasers having a plurality of wavelength selection functions ( Hereinafter, it is simply referred to as a wavelength selective laser device).

  As a device for WDM (wavelength division multiplexing) optical communication, a wavelength tunable laser device that makes the wavelength of output laser light variable is known. Among conventional wavelength tunable laser devices, the wavelength selective laser device having the structure shown in FIG. 11 is excellent in wavelength stability and can obtain a wide wavelength tunable characteristic by changing the operating temperature. It attracts particular attention from such as. In this type of tunable laser device 30, a laser array in which a plurality of distributed feedback (DFB) lasers 31 are arranged in an array and output light from the plurality of DFB lasers 31 are input via each waveguide 32. In addition, an optical combiner (MMI coupler) 33 that joins them, and a semiconductor optical amplifier (SOA) 34 that amplifies the output light of the optical combiner 33, one of the output wavelengths of the plurality of DFB lasers 31. Select one to output.

  As a wavelength selection type laser apparatus currently in practical use, one having eight DFB lasers 31 shown in FIG. The resonator length of the DFB laser 31 is about 400 μm, and the length of the active layer of the SOA is about 600 to 800 μm. The light output intensity of the laser device 30 is about 40 mW at room temperature. From the viewpoint of the reliability of the DFB laser and the like, generally, a value of about 50 to 100 mA is adopted as the driving current of the DFB laser in the wavelength selection element.

  Another example of a wavelength selective laser device that has been put to practical use is known to have 16 DFB lasers as a laser array. In this laser device, the light output intensity to an optical fiber is reported to be 10 mW. Has been. The driving current of the DFB laser is 50 mA.

  In a wavelength selection type laser device, it is particularly desired to increase the bandwidth by increasing the selectable wavelength and increase the light output intensity of the laser device. For example, a wavelength selection type laser device is used as a spare light source that covers C-band (wavelength 1.53 to 1.56 μm), L-band (wavelength 1.57 to 1.60 μm), etc. of a WDM optical communication system. Therefore, it is desirable that the number of DFB lasers in the laser array is 10 or more as the device configuration, and that the output characteristics are that the optical output intensity is 20 mW or more, the spectral line width of the output wavelength Is desired to be 10 MHz or less.

“Effect of Mirror Facets on Lasing Characteristics of Distributed Feedback InGaAsP / InP Laser Diodes at 1.5 μm Range” (IEEE JOURNAL OF QUANTUM ELECTRONICS VOl.QE-20, NO3. MARCH 1984, Katsuyuki Utaka et al.) "High-Gain Array of Semiconductor Optical Amplifier Integrated With Bent Spot-Size Converter (BEND SS-SOA)" (JOURNAL OF LIGHTWAVE TECHNOLOGY, VOl. 19, NO11, NOVEMBER 2001, Yasumasa Suzaki et al.) Journal of Quantum Electronics, Vol.138, Mo11, November 2002, p1493-1502 Journal of Quantum Electronics, QE-22,7 (19861042-1051) Journal of lightwave technology vol7, No. 2 Feb., 1989, p336-339 JP 2003-69149 A

  By the way, in the laser apparatus of the above type, as the number of DFB lasers included in the laser array is increased and the number of input ports of the optical combiner is increased, the coupling loss in the optical combiner increases and the optical input intensity of the SOA decreases. There is a problem. This problem is the same for the DBR laser. For this reason, in a wide-band laser device in which the number of input ports of the optical combiner exceeds 10, it is essential to increase the signal gain of the SOA in order to obtain sufficient light output intensity. However, when the signal gain of the SOA is increased, there is a particular concern about the influence on noise characteristics and spectrum line width in the SOA. Conventionally, however, no detailed report has been made on such a problem in the wavelength selective laser device, and there is also a report on a laser device having the above-described device configuration and satisfying the above-mentioned output characteristics. Absent.

Therefore, the inventors made prototypes of wavelength selective laser devices having various characteristics, and examined the spectral line width from the above viewpoint. Each DFB laser of the prototype laser apparatus is a standard one of conventional DFB lasers, the resonator length L is 400 μm, the product κL of the coupling coefficient κ of the diffraction grating and the resonator length L is 1.3, DFB The number N of lasers was 12, and the light reflectance of the SOA laser emission surface was 5 × 10 ⁻⁴ .

  The measured spectral line width characteristics are shown in FIG. In the figure, the relationship between the drive current of the selected DFB laser and the spectral line width at the output of the SOA is shown for both the case where the SOA drive current is 100 mA (solid line) and the case where the SOA drive current is 300 mA (dotted line). The drive current of the SOA is largely related to its signal gain. From the results shown in FIG. 6, it has been found that in the conventional wavelength selective laser apparatus, a large vibration occurs in the spectral line width depending on the magnitude of the driving current of the DFB laser. It was also found that when the SOA drive current is increased to 300 mA and the signal gain is increased to obtain an optical output intensity of 20 mW or more as the output of the laser device, the spectral line width of the output light is greatly expanded.

  A DFB laser in a conventional wavelength selection element is generally used with a drive current of 100 mA or less. Under these conditions, the spectral line width is 10 MHz even when the SOA drive current is 100 mA or less as described above. Sometimes exceeded. Here, an increase in the SOA drive current leads to an increase in noise in the SOA, which increases the absolute value of the spectral line width. In addition, the return light reflected from the exit end face of the SOA is amplified in the SOA and affects stable oscillation in the DFB laser, thereby giving vibration to the relationship between the drive current of the DFB laser and the spectral line width. Also turned out.

  In view of the above, the present invention provides output characteristics of a conventional wavelength selective laser device so that a desired light output intensity and spectral line width can be obtained even when the number of DFB lasers in the laser array is increased. Accordingly, an object of the present invention is to provide a broadband wavelength tunable laser device that can be used in the field of WDM optical communication.

  In order to achieve the above object, the inventors of the present invention prototyped a wavelength selective laser device having 12 DFB lasers, set the output light intensity to 20 mW or more, and set the spectral line width of the output light. As a result of intensive studies on the structure of the laser device necessary for suppressing the desired narrow line width, the present invention has been completed. The wavelength selective laser device of the present invention has the following configuration.

A laser apparatus according to a first aspect of the present invention includes a laser having a plurality of wavelength selection functions, an optical combiner for combining the output lights of the lasers having the plurality of wavelength selection functions, and an output light of the optical combiner A semiconductor optical amplifier (SOA) for amplifying the laser, and driving and outputting at least one of the lasers having the wavelength selection function.
The number N of coupling ports for coupling the optical combiner and the DFB laser is 10 or more and 12 or less,
The diffraction grating of the laser having the wavelength selection function has an asymmetric structure with respect to the axial center of the resonator, and the light output in the axial direction of the laser having the wavelength selection function is larger than the light output in the axial direction,
The diffraction grating of the laser having the wavelength selection function has a structure that is periodically thinned out at a front portion in the optical axis direction of the resonator,
The product (κL) of the coupling coefficient (κ) of the laser diffraction grating having the wavelength selection function and the resonator length (L) is 1.5 or more,
The SOA has a light output intensity of 20 mW or more, a SOA signal gain of 16 dB or less, and a light reflectance at a light exit end of 10 ⁻⁴ or less.

In the laser device according to the first aspect of the present invention, the light reflectivity at the light emitting end of the SOA is 10 even for a broadband laser device having 10 or more coupling ports of the optical combiner and the laser having a wavelength selection function. ^-4 or less, the output light returns to the laser having the wavelength selection function, the intensity of the return light is kept low , and a laser device having a sufficiently high light output intensity of 20 mW or more is also used for WDM optical communication. In particular, a good narrow linewidth spectrum is obtained.
In the laser apparatus according to the first aspect of the present invention, the SOA signal gain is selected to be 16 dB or less even for a broadband laser apparatus having 10 or more coupling ports between the optical combiner and the laser having a wavelength selection function. As a result, even for a laser device having a sufficiently high optical output intensity of 20 mW or more, a good narrow linewidth spectrum can be obtained when used in the field of WDM optical communication.
In the laser apparatus according to the first aspect of the present invention, a wideband laser apparatus having a coupling port number of 10 or more for the optical combiner and the laser having the wavelength selection function is also used in the diffraction grating of the laser having the wavelength selection function. With a configuration in which the product κL of the coupling coefficient κ and the resonator length L is 1.5 or more, an increase in the spectral line width is suppressed, and a laser device having a sufficiently high optical output intensity of 20 mW or more can be used for WDM optical communication. When used, a good narrow linewidth spectrum is obtained.

In a preferred aspect of the laser device according to the third aspect of the present invention, the light emitting end portion has a window structure and a bending structure, and the window length Lwindow of the window structure is 50 μm or less,
The window length Lwindow and the angle θ formed by the bent structure and the light emitting end face have the following relationship: Lwindow> 0.4464 × θ ² −13.161θ + 87.464
And the equivalent reflectivity at the light exit end is 3 × 10 ⁻⁵ or less.

  In the laser apparatus of the present invention, the above-described effects can be obtained particularly effectively by increasing the light output intensity of a laser having a wavelength selection function. The desired narrow line width is, for example, a spectral line width of 10 MHz or less.

In order to increase the light output intensity of a laser having a wavelength selection function so that the laser device of the present invention has the above-described effects,
(1) The cavity length of a laser having a wavelength selection function is 500 μm or more,
(2) The laser diffraction grating structure having a wavelength selection function has an asymmetric structure, and the light output in the front of the optical axis direction is larger than the light output in the rear of the optical axis direction.
(3) providing a reflection structure having a reflectance of 50% or more on the rear end face of the laser having a wavelength selection function;
(4) The width of the active layer of the laser having a wavelength selection function is 1.5 μm or more,
Etc.

  The laser device of the present invention has excellent output characteristics with a wide bandwidth, high wavelength stability, high optical output, and narrow spectral line width, and can be suitably used particularly in the field of WDM optical communication.

  Prior to the description of the embodiments of the present invention, the principle of the present invention will be described in order to facilitate understanding of the present invention. FIG. 10 is a graph showing the results of examining the relationship between the SOA signal gain (decibel value of the ratio of the optical output Pout and the optical input Pin) and the spectral line width of the output light for various prototype laser devices. . Note that Pout / Pin (dB) indicating the signal gain is obtained by changing the ratio of Pout and Pin in various ways (dB).

  As can be understood from FIG. 10, the spectral line width of the output light of the laser device substantially depends on the magnitude of the amplifier gain. When the amplifier gain is 16 dB or more, the spectral line width is narrowed. Could not be achieved. Therefore, in the present invention, a narrow line of the output light spectrum is obtained by adopting 16 dB or less as the SOA signal gain for a wavelength selective laser device having a laser array having a laser array N having a wavelength selection function of 10 or more. Achieving widening. Preferably, the signal light source has an amplifier gain of 13 dB or less and a spectral line width of 5 MHz or less.

In the present invention, as described above, in order to suppress the signal gain to 16 dB or less, the light output intensity of the laser having the wavelength selection function is increased. For example,
(1) The cavity length of a laser having a wavelength selection function is 500 μm or more,
(2) A structure in which the diffraction grating of a laser having a wavelength selection function has an asymmetric structure with respect to the center in the optical axis direction, and the light output in the front of the optical axis direction is greater than the light output in the rear of the optical axis direction. And
(3) providing a reflecting structure having a reflectance of 50% or more on the rear end surface opposite to the light emitting end surface of the laser having a wavelength selection function;
(4) The width of the active layer of the laser having a wavelength selection function is 1.5 μm or more,
To increase the light output intensity of a laser having a wavelength selection function. This provides a wavelength selective laser device that achieves an optical output of the laser device of 20 mW or more and a signal gain of 16 dB or less.

In the present invention, preferably, the output intensity of the laser having the wavelength selection function is increased in order to suppress the signal gain to 13 dB or less. For example,
(1) The cavity length of a laser having a wavelength selection function is 600 μm or more,
(2) The diffraction grating of the laser having a wavelength selection function has a structure that is asymmetric with respect to the center in the optical axis direction, and the light output in the front of the optical axis direction is about 30% larger than the light output in the rear of the optical axis direction.
(3) providing a reflecting structure having a reflectance of 80% or more on the rear end surface opposite to the light emitting end surface of the laser having a wavelength selection function;
(4) The width of the active layer of the laser having a wavelength selection function is set to 1.8 μm or more,
To increase the light output intensity of a laser having a wavelength selection function. This provides a wavelength selective laser device that achieves an optical output of the laser device of 20 mW or more and a signal gain of 13 dB or less.

  Hereinafter, with reference to the drawings, the present invention will be described in more detail based on exemplary embodiments of the present invention. FIG. 1 shows a semiconductor optical integrated device constituting a wavelength selective laser device according to a first embodiment of the present invention. The laser device 10 includes a laser array in which twelve DFB lasers 11 are arranged in an orthogonal direction to the optical axis, an optical waveguide 12 that guides output light from the DFB laser 11, and the DFB laser 11 through the optical waveguide 12. It is composed of an optical combiner (MMI coupler) 13 that receives the output light and combines it, and a semiconductor optical amplifier (SOA) 14 that amplifies the output of the MMI coupler 13, and these are integrated on a single chip. The

  Each DFB laser 11 has a phase shift region of λ / 4 in the center of the diffraction grating, and the laser oscillation threshold current Ith is 20 mA. The MMI coupler 13 is a coupler having an input number (coupling number) of 12 and an output number of 1, and combines the output lights of the twelve DFB lasers 11 received through the waveguide 12 and gives them to the SOA 13. Due to the merging in the MMI coupler 13, the light intensity of the laser device by the selected DFB laser 11 decreases to about 1/12 at the input value of the SOA 14.

  The SOA 14 has an active layer having a length of 800 μm, amplifies the light output of the MMI coupler 13, and emits output light from the light emitting end of the laser device 10 to the outside.

  FIG. 2 shows the output characteristics of the wavelength selective laser apparatus according to this embodiment in relation to the drive current (mA) of the DFB laser. In the measured output characteristics, the spectral line width (MHz) is indicated by a solid line, the optical output intensity (mW) is indicated by a broken line, and the signal gain (Pout / Pin (dB)) of the SOA is indicated by a dotted line. In FIG. 2, the driving current of the DFB laser was increased from 0, and the driving current of the SOA was set to a constant 150 mA. When the drive current Iffb of the DFB laser exceeds the threshold value, the DFB laser starts to oscillate, and the light output intensity at the emission end of the laser device increases as the drive current increases. When the driving current of the DFB laser increases to some extent, the saturation of the optical output intensity starts, and as a result, the signal gain of the SOA decreases.

  From FIG. 2, when the SOA signal gain indicated by the dotted line is 16 dB or less and the driving current Iffb of the DFB laser is 140 mA or more, the spectral line width of the output light indicated by the solid line is 10 MHz or less, and the oscillation of the spectral line width Is shown to be particularly suppressed. Further, as indicated by a broken line, it is shown that a large light output of 20 mW or more can be obtained at this time.

  FIG. 3 shows the configuration of a wavelength selective laser apparatus according to the second embodiment of the present invention. The laser device 10A of the present embodiment has a structure in which the resonator length of the DFB laser 11A is 400 μm, and the diffraction grating of the DFB laser 11A is periodically thinned out at the front portion in the optical axis direction of the resonator. However, unlike the first embodiment, other configurations are the same as those of the first embodiment.

  FIG. 4 shows a cross-sectional structure of the DFB laser 11A in this embodiment. The DFB laser 11A includes an n-InP buffer layer 22, an n-InP clad layer 23, an MQW active layer 24, a p-InP clad layer 25, and an InGaAs contact layer 26 that are sequentially formed on the n-InP substrate 21. And a diffraction grating 27 embedded in the p-InP cladding layer 25. An n-type electrode 28 and a p-type electrode 29 are formed on the bottom surface of the n-InP substrate 21 and the top surface of the InGaAs contact layer, respectively.

  In the center of the diffraction grating 27, a λ / 4 phase shift region 27A is provided. The diffraction grating 27 on the front side in the optical axis direction from the phase shift region 27A has a structure that is periodically thinned out. By adopting the hanging structure, the amount of feedback on the front side in the optical axis direction of the diffraction grating 27 is reduced, and the light output in the front in the optical axis direction becomes larger than the light output in the rear in the optical axis direction. A light output larger than that of the normal phase shift DFB laser used in the embodiment is obtained.

  FIG. 5 shows the output characteristics of the laser apparatus of this embodiment as in FIG. The SOA drive current was set to 150 mA as in FIG. In the region where the SOA signal gain indicated by the dotted line is 16 dB or less and the drive current Iffb of the DFB laser is 100 mA or more, the spectral line width indicated by the solid line is 10 MHz or less, and the vibration is well suppressed. I understand. Further, a large value of 20 mW or more is obtained as the optical output Pout of the laser device indicated by the broken line.

FIG. 6 shows a wavelength selective laser apparatus according to the third embodiment of the present invention. In the laser apparatus 10B according to this embodiment, the resonator length of the DFB laser 11B is 400 μm, the product κL of the coupling coefficient κ of the diffraction grating and the resonator length L is 1.4, and the SOA 14B Unlike the first embodiment, the other configurations are the same as those of the first embodiment in that the reflectance of the end face on the light emitting side is 10 ⁻⁴ or less.

In the present embodiment, a window structure made of a low refractive index semiconductor layer is formed on the end surface of the SOA 14B on the light emission side on the extension of the active layer of the SOA, and a two-layer coating film is formed on the window structure. By forming, a low light reflectance of 10 ⁻⁴ or less is realized in a band of 60 nm. The window structure is described in Patent Document 1 and Non-Patent Document 1, for example. The preferred length of the window structure in this embodiment is 50 μm or less, more preferably 30 μm or less.

FIG. 7 shows the relationship between the driving current of the DFB laser and the spectral line width of the output light in the laser apparatus, where the reflectance of the exit end face of the SOA 14B is 10 ⁻⁴ (Example) and 10 ⁻³ (Comparative Example). Both cases are shown. The drive current of the SOA was 150 mA. In the present embodiment example, it can be understood that the vibration of the spectral line width can be suppressed to a low level by suppressing the light reflectance of the exit side end face of the SOA to 10 ⁻⁴ or less. The light reflectivity of the exit side end face is preferably 3 × 10 ⁻⁵ .

  The same effect was obtained when a bending structure was adopted instead of the SOA 14B window structure in the laser apparatus of the above embodiment, and an antireflective coating was applied to the window structure. Moreover, the same effect was obtained when three-layer and four-layer antireflection coatings were applied thereto. For example, as described in Non-Patent Document 1, the bending structure is a structure in which light is incident on the output side end surface by gently bending the waveguide structure in the vicinity of the output side end surface of the SOA. . The bending angle of the preferred bending structure in this embodiment is 15 degrees or less, more preferably 10 degrees or less.

  A case where a window structure and a bending structure are adopted for the emission side end face of the SOA will be described by taking a DFB laser as an example. In the laser apparatus of FIG. 1, by changing the element temperature of the DFB laser 11, the wavelength can be varied by about 3 to 4 nm per element. If the number of elements is 12, the wavelength can be varied in the range of 36 to 48 nm, the C-band in the WDM communication system can be covered with one element, and applications such as wavelength provisioning, which is the next generation system, can be considered. . In this element, the relationship between the line width amplitude ratio and the end face reflectance when the optical output at the end face of the chip is 30 mW (20 mW at the end of the fiber when the coupling coefficient to the fiber is 2/3) is described in Non-Patent Document. The result calculated based on the theory of 1 is shown in FIG.

In Non-Patent Document 3, calculation is performed on a model in which a DFB laser and an SOA are directly connected one by one. However, in the element as shown in FIG. Analysis that takes into account is necessary. In the analysis, the optical output from the DFB laser 11 is 15 mW, the number of array elements is 12, the loss at the MMI coupler 13 is 1/12, the waveguide loss is 1 dB, and the optical output from the chip end face that has passed through the SOA 14 is 30 mW. It is assumed that the SOA has such a gain. The line width vibration factor here is the ratio of the amplitude of the spectral line width vibration that occurs when the oscillation wavelength of the DFB laser changes to the spectral line width of the original DFB laser. For example, when the DFB laser inherent spectral line width is 8MH _Z, if the amplitude is in 8MH _Z, the vibration rate is 100%. Generally spectral line width required in Longhaul system of the WDM communication system is said below 10 MHz _Z. Spectral line width of the DFB laser itself is 5～10MH _Z, given that there are variations by driving conditions, the vibration rate of the amplitude good 50% or less. In this case, as can be understood from FIG. 13, the reflectance of the end face needs to be about 3 × 10 ⁻⁵ or less.

By forming the AR film on the end face, the reflectance of the emission side end face can be reduced. In this case, in order to realize an extremely low reflectivity of the order of 10 ⁻⁵ , it is necessary to control the refractive index and film thickness of the film with high accuracy. However, even if active control is performed using an in-situ monitor during film formation, in-plane variations of film forming apparatuses such as PCVD apparatuses and sputtering apparatuses, and the wavelength band of ultra-low reflection films are considered. The formation of such an AR film is not realistic, and the actual AR film has a reflectance of about 3 × 10 ⁻³ .

  Therefore, a window structure that eliminates the waveguide structure in the vicinity of the end face (Non-patent Document 4), a curved waveguide structure that tilts the incident angle of the waveguide to the end face (Non-Patent Document 5), or a structure that combines them is known. It has been.

  FIG. 14 shows the calculation result of the relationship between the effect of reducing the reflectance by the bent waveguide + window structure and the spot size. In the calculation, the reflectance of the end face was set to 1 in order to estimate only the reduction effect. As can be understood from FIG. 14, the effect of reducing the reflectance increases as the spot size is increased. However, because of the coupling to the fiber and the efficiency of the SOA, it is necessary to make the waveguide a single transverse mode. Therefore, in the optical semiconductor element for optical communication, the upper limit of the active layer width of the SOA is about 2.3 μm. Moreover, it is not practical to set the active layer width to 1.2 μm or less from the viewpoint of the electrical resistance of the device and the light output. The range of the active layer width corresponds to approximately 1.0 to 1.3 μm in terms of spot size.

  FIG. 15 shows the relationship between the effect of reducing the reflectance by the bent waveguide + window structure and the window length Lwindow when the bending angle is in the range of 4 degrees to 12 degrees. Here, in order to estimate only the reduction effect by this structure, the reflectance at the chip end face is set to 1. Further, as shown in FIG. 14, since the reflectance increases when the spot size is small, 1.0 μm, which is the spot size at which the highest reflectance can be obtained within the practical spot size range, was selected.

In order to obtain the required extremely low reflectivity of 3 × 10 ⁻⁵ or less in the range of the bending angle shown in the figure, the reflectivity of AR is about 3 × 10 ⁻³ , and thus the reflectivity reduction effect of 10 ⁻² or less is required. From FIG. 15, the relationship between the bending angle and the reflectance for obtaining the reflectance reduction effect of 10 ⁻² or less can be derived, and the relationship is shown in FIG. From FIG. 16, the conditions for obtaining the required reflectance are as follows: the window length is Lwindow and the bending angle is θ.
Lwindow> 0.4464 × θ ² -13.161θ + 87.464
It becomes.

  The longer the window length Lwindow, the more the reflectance reduction effect can be obtained. However, if the window is too long, the light spreads when propagating in the vicinity of the window, and the phenomenon that the FFP is disturbed by hitting the top of the chip occurs. It becomes a factor which reduces a coupling coefficient (patent document 1). Therefore, the window length is preferably 50 μm or less.

  The reflectance reduction effect of the 7-degree bent waveguide structure + window structure was calculated. FIG. 17 shows a schematic diagram of a 7-degree bending + window structure. In order to reduce the reflectivity, as shown in FIG. 17B, the extension of the waveguide is embedded with a material having the same refractive index as that of the clad. With this structure, light is diffracted at the window and the mode is expanded, so that the coupling coefficient between the reflected light from the end face and the waveguide can be reduced. By inclining the waveguide with respect to the end face, the traveling direction of the reflected light deviates from the axial direction of the waveguide as shown in FIG. 17A, so that the reflectance is further reduced as compared with the window structure alone. be able to.

In order to simplify the calculation, the propagation light is a Gaussian beam, and the calculation is performed by obtaining the coupling efficiency. This calculation model is shown in FIG. The horizontal direction of the element is x, the vertical (perpendicular to the paper surface) direction is y, and the direction perpendicular to the end face is z. FIG. 18 shows a horizontal section. The SOA waveguide is bent at an angle θ with respect to the end face of the chip, and light is incident obliquely on the end face. Η _x is obtained by obtaining the coupling coefficient of the reflected light from the end face to the SOA waveguide, and the effect of reducing the reflectance is estimated from this.

ただし、あるｚでのスポットサイズω、曲率半径Ｒはビームウェストでのスポットサイズω₀（電界強度１／ｅでの半幅）を用いて以下のように表される。

この積分を行うために、ｘ₁，ｚ₁，ｘ₂，ｚ₂をｘで表現する必要がある。 In order to obtain the coupling coefficient, a virtual Gaussian beam E ₂ emitted from the position folded at the end face is considered, and the coupling coefficient between E ₂ and the SOA waveguide may be obtained. Actually, the coupling between the outgoing light E ₁ and the virtual beam E ₂ is obtained. When considering the combination of two beams, it can be obtained by performing integration on an arbitrary plane between the beams. As shown in FIG. 18, a coordinate system is considered in which each beam emission point is the origin, the traveling direction is zi = 1, 2 axes, and the horizontal direction is xi = 1, 2 axes. At this time, the electrolytic distribution of the beam is expressed as follows.

However, the spot size ω and the curvature radius R at a certain z are expressed as follows using the spot size ω ₀ (half width at the electric field intensity 1 / e) at the beam waist.

In order to perform this integration, it is necessary to express x ₁ , z ₁ , x ₂ , and z ₂ with x.

From FIG. 8, x ₁ , z ₁ , x ₂ and z ₂ are expressed as x and z as follows.
x ₁ = cos θ (6)
z ₁ = Lwindow + x · sin θ (7)
x ₂ = x · cos (−θ) (8)
z ₂ = −Lwindow + x · sin (−θ) (9)
Coupling efficiency in the horizontal direction at the tip end faces the formula (1) - can be obtained eta _x is substituted into (8) Equation (5). Further, the coupling efficiency η _{y in the} vertical direction can be obtained in the same manner as a normal window structure. The equivalent end face reflectivity R _f is R _f = η _x · η _y · R _AR (10) where the AR coating film reflectivity is R _AR.
Thus, the effect of the seven-degree bending + window structure can be estimated.

  In the above, the upper limit of the bending angle is not particularly provided, but it is preferable that the bending angle is 12 degrees or less from the viewpoint of assembly and miniaturization. The angle of 12 degrees or less is a preferable angle from the viewpoint of radiation loss in the bent waveguide portion when bent to a necessary angle.

  The effect is verified as follows. First, the spot size is obtained from the full width at half maximum of the far field image (FFP) of the output light. The window length Lwindow is obtained by performing FIB processing and photographing with an electronic telescope. Further, the incident angle to the end face can be easily obtained.

  FIG. 8 shows a wavelength selective laser apparatus according to the fourth embodiment of the present invention. The laser device 10C according to the present embodiment has the first feature in that the resonator length of the DFB laser 11C is 400 μm and the product κL of the coupling coefficient κ of the diffraction grating and the resonator length L is 3. Unlike the embodiment, other configurations are the same as those of the first embodiment.

  FIG. 9 shows the relationship between the drive current of the DFB and the spectral line width of the output light in the laser apparatus when κL is both 3.0 (Example) and 1.0 (Comparative Example). In this embodiment, by selecting κL as 3.0, the average value and vibration of the spectral line width of the output light are greatly improved.

  Although the present invention has been described based on the preferred embodiment, the laser apparatus of the present invention is not limited to the configuration of the above embodiment, and various modifications can be made from the configuration of the above embodiment. Further, modifications and changes are also included in the scope of the present invention. Although the DFB laser is used in the above embodiment, the present invention can provide the same effect even if it is a laser having a wavelength selection function, such as a distributed reflection (DBR) laser.

1 is a plan view of a wavelength selective laser device according to a first embodiment of the present invention. The graph which shows the relationship between the drive current and optical output characteristic of the DFB laser in a 1st embodiment. The top view of the wavelength selection type laser apparatus which concerns on the 2nd Example of this invention. Sectional drawing which shows the detail of the wavelength diffraction grating of the DFB laser in the laser apparatus of FIG. The graph which shows the relationship between the drive current and optical output characteristic of the DFB laser in a 2nd example. The top view of the wavelength selection type laser apparatus which concerns on the 3rd Embodiment of this invention. The graph which shows the relationship between the drive current of the DFB laser in 3rd Example, and a spectral line width. The top view of the wavelength selection type laser apparatus which concerns on the 4th Example of this invention. The graph which shows the relationship between the drive current of DFB laser in 4th Example, and a spectral line width. The graph which shows the relationship between the signal gain of SOA and spectrum line width in the prototype wavelength selective type laser apparatus. The top view of the conventional wavelength selection type laser apparatus. The graph which shows the relationship between the drive current of a DFB laser in a conventional wavelength selection type laser apparatus, and a spectral line width. The graph which shows the relationship between the amplitude rate of the line | wire width obtained by calculation, and an end surface reflectance. The graph which shows the relationship between the spot size in a bending structure + window structure, and the reduction effect of a reflectance. The graph which shows the relationship between the window length in a bending structure + window structure, and the reduction effect of a reflectance. The graph which shows the relationship between window length and a bending angle. (A) And (b) is a top view which shows typically the waveguide and SOA in a 7-degree bending + window structure, respectively. The schematic horizontal direction sectional drawing for calculating | requiring the coupling coefficient of the reflected light in 7 degree bending structure + window structure.

Explanation of symbols

10, 10A, 10B, 10C: Wavelength selective laser device 11, 11A, 11B, 11C: DFB laser 12: Waveguide 13: Optical combiner (MMI coupler)
14, 14B: Semiconductor optical amplifier (SOA)

Claims (5)

A laser having a plurality of wavelength selection functions, an optical combiner for combining the output lights of the lasers having the plurality of wavelength selection functions, and a semiconductor optical amplifier (SOA) for amplifying the output light of the optical combiner, In a laser device of a type that drives and outputs at least one of the lasers having the wavelength selection function,
The coupling port number N for coupling the optical combiner and the laser having the wavelength selection function is 10 or more and 12 or less,
The diffraction grating of the laser having the wavelength selection function has an asymmetric structure with respect to the axial center of the resonator, and the light output in the axial direction of the laser having the wavelength selection function is larger than the light output in the axial direction,
The diffraction grating of the laser having the wavelength selection function has a structure that is periodically thinned out at a front portion in the optical axis direction of the resonator,
The product (κL) of the coupling coefficient (κ) of the laser diffraction grating having the wavelength selection function and the resonator length (L) is 1.5 or more,
A laser apparatus, wherein the SOA has a light output intensity of 20 mW or more, a signal gain of the SOA of 16 dB or less, and a light reflectance at a light emission end of 10 ⁻⁴ or less.   The laser device according to claim 1, wherein a spectral line width of output light at the light emitting end is 10 MHz or less.   The laser device according to claim 1, wherein a window structure is formed at the light emitting end portion.   The laser apparatus according to claim 1, wherein a bent structure is formed at the light emitting end portion. The light emitting end has a window structure and a bending structure, and the window length Lwindow of the window structure is 50 μm or less,
The window length Lwindow and the angle θ formed by the bent structure and the light emitting end face have the following relationship: Lwindow> 0.4464 × θ ² -13.161θ + 87.464
The laser device according to claim 1, wherein an equivalent reflectance of the light emitting end portion is 3 × 10 ⁻⁵ or less.

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