JP4076145B2 - Complex coupled distributed feedback semiconductor laser - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a distributed feedback semiconductor laser, and more particularly to a complex coupled distributed feedback semiconductor laser including a diffraction grating configured by both perturbations of refractive index and gain.
[0002]
[Prior art]
A distributed feedback semiconductor laser (hereinafter referred to as DFB laser) can oscillate at a single wavelength even during modulation by utilizing black reflection by a diffraction grating fabricated in an active region. Used as a light source. DFB lasers can be classified into several types according to their diffraction gratings. In general, DFB lasers having a diffraction grating composed of a real part of a complex refractive index, that is, a perturbation of the refractive index, are index-coupled. ) DFB laser, gain-coupled DFB laser with diffraction grating consisting of imaginary part of complex index of refraction, ie gain or loss perturbation, from both real and imaginary part of complex index of refraction A DFB laser having a diffraction grating is referred to as a complex-coupled DFB laser. Further, a DFB laser having a diffraction grating having a uniform period and coupling coefficient that can be classified by the period and phase of the diffraction grating and having a uniform period and coupling coefficient over the entire resonator is replaced with a uniform diffraction grating DFB laser or λ / 4 in the middle of the resonator. A DFB laser whose phase is shifted by a minute is referred to as a λ / 4 shift DFB laser.
[0003]
In a laser, the gain of an optical waveguide that starts laser oscillation determined by the resonator structure is called a threshold gain. In a DFB laser, each mode that oscillates at a different wavelength has a different threshold gain, and the lowest threshold gain is obtained. It oscillates in the mode it has. In general, in an optical waveguide having a diffraction grating, light transmission is significantly hindered, that is, a wavelength band having a high reflectance is called a stop band (stop band). In a refractive index coupled uniform diffraction grating DFB laser, a stop band is used. Have the same threshold gain in the long-wavelength mode closest to and the short-wavelength mode closest to the stopband, so single-wavelength oscillation operation cannot be obtained, or phase asymmetry with respect to the edge reflection diffraction grating Difference in threshold gain between the mode with the lowest threshold gain and the mode with the next lowest threshold gain, even if single-wavelength operation is obtained due to asymmetry in the surrounding environment such as temperature and temperature and asymmetry due to fabrication errors Since the mode is easily changed by a modulation operation or the like, a stable single wavelength operation cannot be obtained. Therefore, in the refractive index coupled uniform diffraction grating, in general, in order to obtain the single wavelength oscillation operation, as shown in FIG. Coating films having different reflectivities (for example, an antireflection (AR) coating film 17 and a high reflection (HR) coating film 22) are applied. Further, according to the asymmetric reflectance coating film, the laser output emitted from both end faces is also asymmetric, and a large output can be obtained from the low reflectance end face side. However, as described in Non-Patent Document 1, for example, whether the wavelength at which laser oscillation occurs is the long wavelength side wavelength or the short wavelength side wavelength of the stop band is determined by the phase of the end face with respect to the diffraction grating. It is difficult to control the oscillation wavelength because the primary diffraction grating period of the 1.55 μm wavelength band is as very small as 240 nm and difficult.
[0004]
On the other hand, in a refractive index coupled λ / 4 shift DFB laser, there is usually a phase shift region at the center in the cavity axis direction, and both ends of the cavity are coated with anti-reflective coating, and the equivalent refractive index of the waveguide is increased. n_eq  If the period of the diffraction grating is Λ and the order of the diffraction grating is q,
λ_B= 2n_eqΛ / q
The threshold gain is the smallest at the black wavelength that occurs in the stopband and operates at a single wavelength, and the threshold gain difference from the mode with the next lowest threshold gain is also the refraction applied with the above-mentioned asymmetric reflectance coating film. Since it can be made larger than the rate-coupled uniform diffraction grating DFB laser, the single wavelength operation characteristic is excellent. However, in the refractive index coupled type λ / 4 shift DFB laser having the λ / 4 shift unit at the center of the resonator, the optical output from both end faces is the same, and the refractive index generally provided with the asymmetric reflectance coating film described above. The light output that can be extracted from the front is smaller than that of a coupled uniform diffraction grating DFB laser.
[0005]
In order to improve the shortcomings of the above-described refractive index coupled λ / 4 shift DFB laser, for example, in Non-Patent Document 2, in the refractive index coupled λ / 4 shift DFB laser, anti-reflective (AR) coatings are provided on both end surfaces, respectively. By applying a high reflection (HR) coating and shifting the phase shift position to the rear HR side, the front light output is maintained while maintaining the threshold gain difference between the black wavelength as the oscillation mode and the next mode at a certain value. A method of taking out large is proposed. However, this method is also affected by the phase of the end face provided with the highly reflective film with respect to the diffraction grating, and the single wavelength operation characteristics and the oscillation characteristics such as the threshold current are deteriorated.
[0006]
In addition, when the phase condition is satisfied at the black wavelength with the λ / 4 phase shift, the reflectivity of the diffraction grating having a large kL increases, so that the two diffraction gratings existing with the phase shift interposed therebetween. Among them, the ratio of output to the diffraction grating side having a small kL increases. For example, in Non-Patent Document 3, in a λ / 4 phase shift DFB laser having antireflection coatings on both end faces, as shown in FIG. 9B, the λ / 4 phase shift point 20 is set at the center in the resonator axial direction. Is shifted forward, that is, the kL of the first diffraction grating 18 ahead of the phase shift point 20 is made smaller than the kL of the second diffraction grating 19 behind the phase shift point 20, thereby increasing the single wavelength characteristic. A method is described that improves the forward light output without degrading. Also in this example, the oscillation wavelength is a black wavelength. However, as the output asymmetry is increased, the threshold gain of the oscillation wavelength increases, the threshold current, which is the current value at which oscillation starts, increases, and the threshold gain difference from the wavelength having the next lower threshold gain decreases. Therefore, since the single wavelength characteristic is deteriorated, the asymmetry cannot be increased beyond a certain level.
[0007]
In the above example, kL is made asymmetric by changing L of the first and second diffraction gratings 18 and 19, but as shown in FIG. The kL is made asymmetric by changing k or both k and L of the two diffraction gratings 18 and 19. Thereby, the threshold gain difference is improved.
[0008]
By the way, the carrier concentration distribution of the active layer is generated by the field distribution of light in the resonator axial direction generated inside the resonator, and the equivalent refractive index of the waveguide changes accordingly, that is, the spatial axial hole burning phenomenon. As a result, the threshold gain difference between the oscillation wavelength having the lowest threshold gain and the wavelength having the next lowest threshold gain is reduced and the single-wavelength operating characteristic is deteriorated. However, in the λ / 4 phase shift DFB laser, This is a major problem when oscillating at the Bragg wavelength. Spatial axial hole burning is a big problem because the larger the kL, the higher the reflectivity, and the more concentrated the light is on the phase shift part, which limits the low threshold current operation and shortening of the cavity of the laser. The Therefore, the structures shown in FIGS. 9B and 9C illustrated so far use oscillation at the black wavelength, and spatial axial hole burning becomes a problem. In addition, when the phase shift amount is λ / 4, the phase condition is perfectly matched at the Bragg wavelength, but even if the phase shift amount is deviated from λ / 4, the phase can be resonated at a wavelength deviated from the Bragg wavelength. In the case of oscillation in a mode in which resonance is possible due to phase shift, the problem of spatial axial hole burning occurs.
[0009]
On the other hand, for example, as described in Non-Patent Document 4 and the like, the complex coupled DFB laser can operate at a single wavelength even with a uniform diffraction grating. When the perturbation of the refractive index and the gain perturbation are in phase, that is, when the refractive index portion and the high gain portion, the low refractive index portion and the low gain portion are the same, Resonance mode closest to the stopband on the long wavelength side of the stopband, where the refractive index perturbation and gain perturbation are in antiphase, that is, the high refractive index part, the low gain part, and the low refractive index part If the high gain part matches, the resonance mode is closest to the stop band on the short wavelength side of the stop band. However, the light emitted from both end faces of the resonator is basically equal because of the uniform diffraction grating. Therefore, for example, a coating film having a different reflectance must be applied to each end face. Therefore, as in the case of the refractive index coupled DFB laser, the uncertainty of the oscillation wavelength and the yield of single wavelength operation are reduced. Deterioration will occur. When the λ / 4 phase shift region is introduced into the diffraction grating, the problem of the hole burning phenomenon occurs when the Bragg wavelength becomes the oscillation wavelength.
[0010]
[Non-Patent Document 1]
“Effect of Mirror Facets on Lasing Characteristics of Distributed Feedback InGaAsP / InP Laser Diodes at 1.5 μm Range,” K. Utaka et al (IEEE J. Quantum Electron., QE-20, No. 3, pp. 236-245, March (1984)
[Non-Patent Document 2]
“Analysis of λ / 4-phase shifted DFB LD” M. Yamaguchi et al. (In 1985 Nat. Conv. Rec., IECE Japan, p. 310)
[Non-Patent Document 3]
“Asymmetric λ / 4-Shift InGaAsP / InP DFB Lasers” M. Usami et al, (IEEE J. Quantum Electron., QE-23, p.815-821, July 1987)
[Non-Patent Document 4]
“High Single-mode Yield 1.55 μm GaInAsP / InP BH-DFB Lasers with Periodic Wirelike Active Regions,” N. Nunoya et al, (The 17^th (IEEE International Semiconductor Laser Conference, pp.45-46, September 2000)
[Patent Document 1]
JP-A-11-163464
[0011]
[Problems to be solved by the invention]
As described above, the conventional refractive index coupled DFB laser and complex coupled DFB laser have the following problems when the single wavelength operation and the front output and the rear output are asymmetrical. In the case of a uniform diffraction grating DFB laser method using an asymmetric coating film, that is, when a low reflectance coating film is applied to the front end face and a high reflectance coating film is applied to the rear end face, the single wavelength characteristics of There arises a problem that the deterioration and the oscillation wavelength are uncertain. In addition, even when the asymmetric coating film is used and the λ / 4 phase shift position is placed on the high reflectivity end face behind the center of the resonator axis direction, the reflection end face is still the same as in the case of the uniform diffraction grating. Due to the phase with respect to the diffraction grating, there is a problem that the single wavelength characteristic is deteriorated and the oscillation wavelength is uncertain.
[0012]
On the other hand, in the method in which both end faces are made low in reflection and the position of the phase shift region is placed in front of the center of the resonator, as the asymmetry increases, the threshold current increases and the single wavelength characteristic deteriorates. In addition, when a phase shift is introduced and oscillation occurs in a resonance mode generated in the stop band due to the phase shift, axial hole burning becomes a problem.
[0013]
The present invention has been made to solve such a problem. In the DFB laser, high single wavelength oscillation characteristics are maintained regardless of the asymmetric end face coating, and light emitted from the end face of the resonator is forward and backward. The objective is to suppress the hole burning due to the introduction of phase shift.
[0014]
[Means for Solving the Problems]
  The complex coupled distributed feedback semiconductor laser according to the first aspect of the present invention (claim 1) solves the above-described problem of both the real part, ie, the refractive index and the imaginary part, ie, gain or loss, of the complex refractive index sandwiching the phase shift point or region. In a distributed feedback laser diode having two diffraction gratings configured by perturbation, the direction in which the main laser beam is emitted is the front, and the length of the first diffraction grating ahead of the phase shift point or region is L₁, The length of the second diffraction grating behind the phase shift point or region is L₂In this case, the complex coupling coefficient of the first diffraction grating ahead of the phase shift point or region is represented by k, and the refractive index coupling coefficient is represented by k._i1, Gain coupling coefficient k_g1As k₁= K_i1+ Ik_g1And the complex coupling coefficient of the second diffraction grating behind the phase shift point or region, and the refractive index coupling coefficient as k._i2, Gain coupling coefficient k_g2As k₂= K_i2+ Ik_g2When defined as
    k_i1≠ 0, k_g1≠ 0, k_i2≠ 0, k_g2≠ 0
    ｜ k₁| × L₁> | K₂| × L₂> 0
Because
k _i1 And k _g1 Or k _i2 And k _g2 , Have the same sign, oscillation occurs at a wavelength on the long wavelength side of the stop band, and k _i1 And k _g1 Or k _i2 And k _g2 When, is an opposite sign, oscillation occurs at the short wavelength side of the stop band.It is characterized by.
[0015]
  The complex coupled distributed feedback semiconductor laser of the second invention (Claim 2) is the complex coupled distributed feedback laser of the first invention, and has a length L₁The complex coupling coefficient k of the first diffraction grating of₁And length L₂The complex coupling coefficient k of the second diffraction grating₂Are the same,
    L₁> L₂
The filling,
k _i1 And k _g1 Or k _i2 And k _g2 , Have the same sign, oscillation occurs at a wavelength on the long wavelength side of the stop band, and k _i1 And k _g1 Or k _i2 And k _g2 When, is an opposite sign, oscillation occurs at the short wavelength side of the stop band.It is characterized by.
[0016]
  The complex coupled distributed feedback semiconductor laser of the third invention (Claim 3) is the complex coupled distributed feedback laser of the first invention, wherein the order of the first diffraction grating is p, and the second When the order of the diffraction grating is q,
    ｜ k₁| × L₁> | K₂| × L₂> 0
To satisfy the relationship
    p <q
age,
k _i1 And k _g1 Or k _i2 And k _g2 , Have the same sign, oscillation occurs at a wavelength on the long wavelength side of the stop band, and k _i1 And k _g1 Or k _i2 And k _g2 When, is an opposite sign, oscillation occurs at the short wavelength side of the stop band.It is characterized by.
[0017]
  The complex coupled distributed feedback semiconductor laser of the fourth invention (Claim 4) is the complex coupled distributed feedback laser of the first invention or the third invention, wherein the first diffraction grating or the second diffraction grating is used. Either one or both of the gratings are left with a grating with a mean coupling coefficient reduced to n / m by leaving the peaks of the grating at a ratio of n out of m (m> n). Configured, the average coupling coefficient satisfies the conditions of claim 1 or claim 3,
k _i1 And k _g1 Or k _i2 And k _g2 , Have the same sign, oscillation occurs at a wavelength on the long wavelength side of the stop band, and k _i1 And k _g1 Or k _i2 And k _g2 When, is an opposite sign, oscillation occurs at the short wavelength side of the stop band.It is characterized by.
[0018]
According to a fifth aspect of the present invention (Claim 5), the complex coupled distributed feedback semiconductor laser is any one of the first to fourth complex coupled distributed feedback semiconductor lasers, comprising a complex coupled diffraction grating. Is fabricated by etching part or all of the multiple quantum well active layer.
[0019]
A complex coupled distributed feedback semiconductor laser according to a sixth aspect of the invention is the complex coupled distributed feedback semiconductor laser according to any one of the first to fourth aspects, wherein the complex coupled diffraction grating is disposed on the pattern substrate. It was produced by growing an active layer.
[0020]
The complex coupled distributed feedback semiconductor laser of the seventh invention is the complex coupled distributed feedback semiconductor laser according to any one of the first to fourth inventions, wherein the complex coupled diffraction grating is above the active layer. Alternatively, it is produced by periodically providing a light absorption layer below.
[0021]
The complex coupled distributed feedback semiconductor laser of the eighth invention is the complex coupled distributed feedback semiconductor laser of any one of the first to fourth inventions, wherein the complex coupled diffraction grating is above the active layer. Alternatively, it is produced by periodically providing a current blocking layer below.
[0022]
The complex coupled distributed feedback semiconductor laser of the ninth invention is the complex coupled distributed feedback semiconductor laser according to any one of the first to fourth inventions, wherein the complex coupled diffraction grating is metal on the element surface. It is produced by periodically providing a film.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The DFB laser structure according to the embodiment of the present invention solves the problem by the following diffraction grating structure.
[0024]
The diffraction grating of the DFB laser according to the present embodiment is a complex coupling type having perturbations of a real part and an imaginary part of a complex refractive index, having both refractive index coupling and gain coupling, and has a phase shift region. In order to form a complex coupled diffraction grating, as shown in the schematic diagram of the cross section in the resonator axial direction shown in FIG. 1, for example, “1.55 μm index / gain coupled DFB lasers with strained layer multiquantum-well active grating,” GP (1) Active layer (for example, in the illustrated GaInAsP SCH-strained MQW layer 12) shown in Li et al, (Electron Lett., Vol. 28, no. 18, pp. 1726-1727, Aug. 1992.) For example, “Fabrication and characteristics of gain-coupled distributed feedback semiconductor lasers with a corrugated active layer,” Y. Luo et al, (IEEE J. Quantum Electron, vol. QE-27, no. 6, pp. 1724-1731, June 1991. (2) An active layer (for example, the illustrated GaInAsP MQW23) is formed on a pattern substrate (for example, the illustrated n-InP substrate 11). The method of forming the active layer 23a), for example, “1.55 μm Gain-Coupled Q uantum-Well Distributed Feedback Lasers with High Single-Mode Yield and Narrow Linewidth, `` B. Borchert et al, (IEEE Trans. Photon. Technol., Lett., vol. 3, no. 11, pp. 955-957, Nov 1991.) (3) an optical absorption layer (for example, the illustrated p-GaInAs light absorption layer 24) above or below the active layer (for example, the active layer 12a in the illustrated GaInAsP SCH-strained MQW layer 12). For example, “Very-low threshold, highly efficient, and low-chirp 1.55 μm complex-coupled DFB lasers with a current-blocking grating,” ZM Chuang et al, (IEEE Photon. Technol. Lett , vol. 8, no. 11, pp. 1438-1440, Nov. 1996) or above the active layer (for example, the active layer 12a in the illustrated GaInAsP SCH-strained MQW layer 12) or A current blocking layer (for example, the n-InGaAsP current blocking layer 25 shown in the figure) is periodically provided below. For example, “Cain-coupled DFB lasers with a titanium surface Bragg grating.” TW Johannes et al, (Electron. Lett., Vol. 31, no. 5, pp. 370-371, Mar. 1995.) (5) There is a method of providing a metal diffraction grating (for example, the metal diffraction grating 27 shown in the figure) on the element surface.
[0025]
The length of the first diffraction grating in front of the phase shift region, that is, in the direction of the end face from which the laser beam is extracted is L₁, The complex coupling coefficient is the refractive index coupling coefficient k_i1, Gain coupling coefficient k_g1As k₁= K_i1+ Ik_g1And the length of the rear second diffraction grating is L₂And the complex coupling coefficient for the black wavelength of the rear first diffraction grating is the refractive index coupling coefficient k._i2, Gain coupling coefficient k_g2As k₂= K_i2+ Ik_g2When
k_i1≠ 0, k_g1≠ 0, k_i2≠ 0, k_g2≠ 0
｜ k₁| × L₁> | K₂| × L₂> 0
And Further, in the above structure, | k₁| And L₁And | k₂| And L₂Threshold gain α of resonance mode caused by phase shift_{th_sh}Threshold gain α of the resonance mode outside the stop band and closest to the stop band_{th_ex}So that
α_{th_sh}> Α_{th_ex}
| K₁| And L₁And | k₂| And L₂Adjust the size. The threshold gain is, for example, “Coupled-Wave Theory of Distributed Feedback Lasers” H. Kogelnik et al, (J. Appl. Phys., Vol. 43, no. 5, pp. 2327-2335, May 1972) and “A new exact and efficient numerical matrix theory of complicated laser structures: Properties of asymmetric phase-shifted DFB lasers, ”G. Bjork et al, (J. Lightwave Technol., vol. LT-5, pp. 140-146, Jpn. 1987 ) Etc. can be obtained by the coupled wave equation or the propagation matrix method. The coupling coefficient can be adjusted by (1) changing the ratio of the peaks and valleys of the diffraction grating, (2) changing the height of the peaks of the diffraction grating, and (3) changing the waveguide width to change the equivalent refractive index. And (4) changing the waveguide thickness (the thickness of the upper and lower layers) to change the equivalent refractive index.
[0026]
At this time, the mode with the smallest threshold gain is the mode on the long wavelength side of the stopband when the refractive index and gain perturbation are in phase, and when the refractive index and gain perturbation are in reverse phase. Is a mode on the short wavelength side of the stop band.
[0027]
<Action>
The coating of the high-reflectance film on the rear end face changes the characteristics of the resulting laser depending on the phase with respect to the diffraction grating. However, for example, “High Single-mode Yield 1.55 μm GaInAsP / InP BH-DFB Lasers with Periodic Wirelike Active Regions,” N. Nunoya et al, (The 17^th Complex coupled uniform diffraction with a diffraction grating with the same phase change in gain and refractive index produced by etching the active layer described in IEEE International Semiconductor Laser Conference, pp. 45-46, Sep. 2000) In a grating DFB laser, even a uniform diffraction grating oscillates in a resonance mode on the long wavelength side of the stop band regardless of the asymmetric end-face reflectance coating, and the output ratio of the oscillation mode to the next mode, which is an index of single wavelength operation A single wavelength operation with a submode suppression ratio (SMSR) of 30 dB or more is obtained. Further, if the end face reflectance is about 30% or less due to cleavage, the oscillation wavelength is always the wavelength on the long wavelength side of the stop band regardless of the phase with respect to the diffraction grating.
[0028]
  Next, in order to perform asymmetry, the introduction of a phase shift into the diffraction grating of a complex coupled DFB laser is considered. In the case of a mode generated in a stop band that can resonate by phase shift, for example, in the case of λ / 4 phase shift, when the Bragg wavelength becomes the oscillation wavelength, the concentration of light on the phase shift portion is extremely increased and hole burning is caused. As a result, the local refractive index change accompanying the change causes the deterioration of characteristics. Considering this in reverse, even if a phase shift is introduced, if the resonance mode in the stopband that can be present does not become an oscillation mode and other wavelengths oscillate, The light intensity distribution is smooth, and spatial axial hole burning can be suppressed. In order to suppress the oscillation in the resonance mode within the stop band caused by the phase shift, the complex coupling coefficient (k₁= K_i1+ Ik_g1, K₂= K_i2+ Ik_g2) And the resonator length, ie, | k₁｜ ・ L₁Or | k₂｜ ・ L₂Make it smaller. At this time, in the mode by the phase shift, the feedback of light necessary for oscillation cannot be obtained sufficiently, and the threshold gain increases, and k_i1And k_g1Or k_i2And k_g2, Have the same sign, oscillation occurs with the wavelength on the long wavelength side of the stopband having the lowest threshold gain, and k_i1Whenk _g1 Or k_i2And k_g2When the symbols have different signs, oscillation occurs with the wavelength on the short wavelength side of the stop band having the lowest threshold gain. This is as if |₁｜ ・ L₁Or | k₂｜ ・ L₂Of the diffraction grating having a large value among them, and the light distribution inside the resonator tends to be similar to that of the uniform diffraction grating DFB laser, so that light concentration in the phase shift region occurs. Therefore, hole burning is suppressed.
[0029]
The above will be described with reference to the principle diagram of the present invention shown in FIG. In the coordinate Z, the front end face of the resonator is set to 0 along the axial direction of the resonator. For simplification, let us consider a case where both end surfaces are made non-reflective by applying an AR coating film 17 and the period and coupling coefficient of the first diffraction grating 18 and the second diffraction grating 19 are the same. First, consider the case where the diffraction grating is of a refractive index coupling type. FIG. 3 shows an example of spectrum calculation in the case of a refractive index coupled DFB laser. The phase shift position of 20 is L₁/ (L₁+ L₂) = 0.5, the mode by λ / 4 phase shift can be set at the center of the stop band.₁/ (L₁+ L₂) = 0.8, k₂・ L₂Decreases, the threshold gain of the Bragg wavelength increases, and the threshold gain of the modes on both sides of the stop band is minimized, so that oscillation does not occur at the Bragg wavelength. However, in the case of refractive index coupling, since the threshold gains of the modes on both sides of the stop band are substantially equal, single mode operation cannot be obtained as is apparent from the spectrum calculation results. Next, consider the case where the diffraction grating is of a complex coupling type in which the refractive index and gain perturbation are in phase. FIG. 4 shows a spectrum calculation result in the case of a complex coupled DFB laser. Phase shift 20 is L₁/ (L₁+ L₂) = 0.5, the mode by the λ / 4 phase shift can be formed at the center of the stop band, as in the case of the above-described refractive index coupled DFB laser. However, L₁/ (L₁+ L₂) = 0.8, k₂・ L₂When becomes smaller, the threshold gain of the mode on the long wavelength side of the stop band is minimized and single mode operation is performed as in the case of the uniform diffraction grating. That is, the operating principle of the present invention uses a phenomenon characteristic of a DFB laser using a complex coupled diffraction grating.
[0030]
Looking at a calculation example of the optical power density distribution inside the resonator when the phase shift position in the complex coupled DFB laser shown in FIG. 5 is changed, when trying to obtain the same forward output, the λ / 4 phase shift is L in the center₁/ (L₁+ L₂) = 0.5 or L with a λ / 4 phase shift unit placed forward to increase the forward output₁/ (L₁+ L₂) = 0.4, the mode in the stop band caused by the λ / 4 phase shift, that is, the black wavelength becomes the resonance mode, so that the concentration of light on the phase shift portion increases. The same applies to the refractive index coupled DFB laser. On the other hand, L₁/ (L₁+ L₂) = 0.8, since oscillation occurs at a wavelength outside the stop band as shown in FIG. 4, there is no extreme concentration of light on the phase shift portion, and the distribution is close to a flat surface with gentle relief. . At this time, an output ratio substantially the same as the output ratio of the front and rear end faces when the phase shift is inserted from the front can be obtained. Further, as can be seen from FIG. 5, when the output from the front end face is made larger than the output from the rear end face, the phase shift position is placed rearward, that is, | k₂｜ ・ L₂| K₁｜ ・ L₁Smaller than.
[0031]
As described above, in the case of a complex coupled diffraction grating, a single wavelength operation is possible even in the case of a uniform diffraction grating, and since it is not easily affected by end face reflection, an antireflection (AR) coating film on the end face is not necessarily perfect. It is not necessary that the reflectivity be less than the cleavage plane. In addition, since the phase shift exists, the resonator structure becomes asymmetric, and the ratio of light emitted from the end surface opposite to the side on which the product of the complex coupling coefficient and the resonator length is reduced increases. The phase shift is for making the resonance structure asymmetric, and since the resonance mode due to the phase shift is not oscillated, the phase shift amount does not necessarily have to be λ / 4.
[0032]
Next, the structure of the diffraction grating will be described. Consider a case in which the product of the coupling coefficient and the diffraction grating length of the second diffraction grating 19 is reduced, in particular, the coupling coefficient is reduced. In the first method, the coupling coefficient can be reduced by lowering the height of the peak of the diffraction grating closer to the second diffraction grating 19 than the first diffraction grating 18. However, it is considered difficult to accurately produce the two regions unless they are produced separately. In the second method, the coupling coefficient can be reduced by shifting the ratio of the peaks and valleys of the diffraction grating from 1: 1. According to this method, two diffraction gratings having different coupling coefficients can be manufactured at the same time. However, in the case of a primary diffraction grating having a wavelength band of 1.55 μm, for example, the diffraction grating period is 240 nm, and it is difficult to accurately control the ratio of peaks and valleys. As a third alternative method, a first-order diffraction grating is desirable in order to obtain excellent laser characteristics, but the principle of operation does not change even with a higher-order diffraction grating, so the product of the coupling coefficient and the diffraction grating length. A high-order diffraction grating may be used in order to reduce the size. According to this method, for example, a first-order diffraction grating and a second-order diffraction grating can be formed at the same time, and processing controllability is improved. Considering another fourth method, the diffraction grating is reduced by reducing the average coupling coefficient to n / m by a method of leaving the peaks of the diffraction grating at a ratio of n out of m (m> n) or the like. K · L may be adjusted.
[0033]
Here, based on FIGS. 6-8, the Example of this invention is described.
[0034]
<First embodiment>
FIG. 6 is a sectional view showing a structure of a complex coupled distributed feedback semiconductor laser according to a first embodiment of the present invention (corresponding to claims 1 and 2). In FIG. 6, 11 is an InP substrate, 12 is a SCH-strained MQW layer comprising an InGaAsP multilayer strained quantum well (MQW) active layer having an emission wavelength of 1.55 μm and a SCH layer having a band gap wavelength of 1.2 μm sandwiching the active layer, 13 Is a p-InP cladding layer, 14 is a p-contact layer, 15 is a p-side electrode, 16 is an n-side electrode, 17 is an AR coating film, and 18 and 19 are first and second diffraction gratings.
[0035]
Since the first diffraction grating 18 and the second diffraction grating 19 are formed by etching the SCH-strained MQW layer 12 with a period (Λ) of 240 nm and burying and regrowing it with InP, the first diffraction grating 18 and the second diffraction grating 19 are affected by the perturbation of refractive index and gain. Diffraction gratings are constructed, each having the same complex coupling coefficient k₁And k₂Have Refractive index coupling coefficient is approximately 360 cm^-1Although it is difficult to accurately estimate the gain coupling coefficient, it is about several to several tens of centimeters.^-1It becomes. In this embodiment, the diffraction grating is produced by (1) etching the MQW layer (active layer) shown in FIG. 1 [corresponding to claim 5]. (2) A method of forming an active layer on a pattern substrate (corresponding to claim 6), a method of periodically providing a light absorption layer above or below the active layer (corresponding to claim 7), and (4) A complex coupled diffraction grating is formed by a method of periodically providing a current blocking layer above or below the active layer [corresponding to claim 8] or a method of [5] surface metal diffraction grating [corresponding to claim 9]. It may be realized. The length of the first diffraction grating 18 in front of the λ / 4 phase shift region 20 is set to L₁= 160 μm, and the length of the second diffraction grating 19 behind is L₂= 40 μm, and the λ / 4 phase shift region is
｜ k₁| × L₁> | K₂| × L₂> 0
It is formed to satisfy.
[0036]
Although the active layer may be a bulk, a strained MQW layer is used here because the light output characteristics are improved by using a strained MQW layer. The laser structure is a buried heterostructure in which both sides of the waveguide are buried with a current blocking layer or a semi-insulator layer in which semiconductor layers having different polarities are stacked. However, the principle of the present invention has no problem even with a ridge structure or other waveguide structures.
[0037]
With the structure of the first embodiment, the mode having the lowest threshold gain is not the black wavelength due to the λ / 4 phase shift, but the mode on the long wavelength side of the stop band, so that the hole burning effect is suppressed. Moreover, it is a single wavelength operation and a submode suppression ratio (SMSR) of 35 dB or more is obtained. Furthermore, because of the asymmetric structure, the output from the first diffraction grating 18 side, that is, the output from the emission end face side becomes larger than the output from the second diffraction grating 19 side, ie, the rear end face side, and is 8 or more. An output ratio is obtained. Although this embodiment is an example using a GaInAsP quantum well on an InP substrate 11, it is obvious that the present invention can be applied in principle even when using a substrate made of a material other than InP such as GaAs.
[0038]
<Second embodiment>
FIG. 7 shows a second embodiment of the present invention (corresponding to claims 1 and 3). In the second embodiment, the second diffraction grating in the structural diagram of the first embodiment shown in FIG. The order of 19 is made higher than the order of the first diffraction grating 18.
[0039]
Specifically, in the second embodiment, the period of the first diffraction grating 18 (Λ₁) Is 240 nm, which is the primary diffraction grating period of the 1.5 μm wavelength band, and the period (Λ₂) Was 480 nm, which is the second-order diffraction grating period. The length of the first diffraction grating 18 is set to L with the λ / 4 phase shift region 20 in between.₁= 120 μm, and the length of the second diffraction grating 19 is L₂= 80 μm, and the λ / 4 phase shift region 20 is
｜ k₁| × L₁> | K₂| × L₂> 0
It is formed to satisfy. Thereby, the same effect as the first embodiment can be obtained.
[0040]
<Third embodiment>
FIG. 8 shows a third embodiment of the present invention (corresponding to claims 1 and 4). In the third embodiment, the second diffraction grating in the structural diagram of the first embodiment shown in FIG. The average coupling coefficient is lowered by setting 19 of 19 peaks to n out of m.
[0041]
Specifically, in the third embodiment, the first diffraction grating 18 and the second diffraction grating 19 have the same period of 240 nm, but one of the three peaks of the second diffraction grating 19 is removed. . The length of the first diffraction grating 18 is set to L with the λ / 4 phase shift region 20 in between.₁= 140 μm, and the length of the second diffraction grating 19 is L₂= 60 μm, and the λ / 4 phase shift region 20 is
｜ k₁| × L₁> | K₂| × L₂> 0
It is formed to satisfy. Thereby, the same effect as the first embodiment can be obtained.
[0042]
【The invention's effect】
As described above, according to the complex coupled distributed feedback semiconductor laser of the present invention, the output of both end faces can be output as a non-reflective coating without using an asymmetric end face coating film without significant deterioration of the submode suppression ratio. Since the asymmetry can be made, the instability of the oscillation wavelength due to the uncertainty of the position of the highly reflective end face coating with respect to the diffraction grating can be removed. Furthermore, it is possible to realize a good light source for use in an integrated optical circuit configuration in which end face coating is difficult. Further, the end face non-reflective coating is not necessarily required, and the end face coating process can be eliminated, which is effective in reducing the cost. Furthermore, since the resonance mode caused by the phase shift is not used, spectral hole burning can be suppressed, and stable characteristics can be obtained even at high output.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a complex coupled diffraction grating.
FIG. 2 is a diagram illustrating the principle of the invention.
FIG. 3 is a diagram for explaining an oscillation wavelength (in the case of a refractive index coupled DFB laser).
FIG. 4 is a diagram for explaining an oscillation wavelength (in the case of a complex coupled DFB laser).
FIG. 5 is a calculation example of the power density distribution of light inside the resonator.
FIG. 6 is a diagram for explaining a complex coupled distributed feedback semiconductor laser according to a first embodiment of the present invention.
FIG. 7 is a diagram for explaining a complex coupled distributed feedback semiconductor laser according to a second embodiment of the present invention.
FIG. 8 is a diagram for explaining a complex coupled distributed feedback semiconductor laser according to a third embodiment of the present invention.
FIG. 9 is a diagram illustrating a conventional technique.
[Explanation of symbols]
11 n-InP substrate
12 GaInAsP SCH-strained MQW layer
13 p-InP cladding layer
14 p-GaInAs contact layer
15 p-side electrode
16 n-side electrode
17 AR coating film
18 Diffraction grating 1
19 Diffraction grating 2
20 Phase shift
21 GaInAsP guide layer
22 HR coating film
23 GaInAsP MQW
24 p-GaInAs light absorption layer
25 n-InGaAsP current blocking layer
26 i-InP
27 Metal diffraction grating

Claims (9)

The main laser beam is emitted from a distributed feedback laser diode with two diffraction gratings composed of a perturbation of both the real part, that is, the refractive index, and the imaginary part, that is, gain or loss, of the complex refractive index across the phase shift point or region. In this case, the length of the first diffraction grating ahead of the phase shift point or region is L ₁ , and the length of the second diffraction grating behind the phase shift point or region is L _2. The complex coupling coefficient of the first diffraction grating in front of the phase shift point or region is defined as k ₁ = k _i1 + ik _g1 where the refractive index coupling coefficient is k _i1 and the gain coupling coefficient is k _g1. When the complex coupling coefficient of the second diffraction grating behind the point or region is defined as k ₂ = k _i2 + ik _g2 where the refractive index coupling coefficient is k _i2 and the gain coupling coefficient is k _g2 ,
k _i1 ≠ 0, k _g1 ≠ 0, k _i2 ≠ 0, k _g2 ≠ 0
| K ₁ | × L ₁ > | k ₂ | × L ₂ > 0
Because
When k _i1 and k _g1 or k _i2 and k _g2 have the same sign, oscillation occurs at the wavelength on the long wavelength side of the stop band, and k _i1 and k _g1 , or k _i2 and k _g2 are different. A complex coupled distributed feedback semiconductor laser characterized in that oscillation occurs at a wavelength on the short wavelength side of the stop band when the code is used . A complex coupling distributed feedback laser according to claim 1, length a complex coupling coefficient k ₁ of the first diffraction grating of L _1, the second complex coupling coefficient k of the diffraction grating of length L _{2 2} is the same,
L ₁ > L ₂
The filling,
When k _i1 and k _g1 or k _i2 and k _g2 have the same sign, oscillation occurs at the wavelength on the long wavelength side of the stop band, and k _i1 and k _g1 , or k _i2 and k _g2 are different. A complex coupled distributed feedback semiconductor laser characterized in that oscillation occurs at a wavelength on the short wavelength side of the stop band when the code is used . The complex coupled distributed feedback laser according to claim 1, wherein the order of the first diffraction grating is p and the order of the second diffraction grating is q.
| K ₁ | × L ₁ > | k ₂ | × L ₂ > 0
To satisfy the relationship
p <q
age,
When k _i1 and k _g1 or k _i2 and k _g2 have the same sign, oscillation occurs at the wavelength on the long wavelength side of the stop band, and k _i1 and k _g1 , or k _i2 and k _g2 are different. A complex coupled distributed feedback semiconductor laser characterized in that oscillation occurs at a wavelength on the short wavelength side of the stop band when the code is used . 4. The complex coupled distributed feedback laser according to claim 1, wherein either one or both of the first diffraction grating and the second diffraction grating has m peaks of the diffraction grating. It is comprised by the diffraction grating which reduced the average coupling coefficient to n / m by leaving it in a ratio of n pieces (m> n), and the average coupling coefficient is according to claim 1 or claim 3. Meet the requirements,
When k _i1 and k _g1 or k _i2 and k _g2 have the same sign, oscillation occurs at the wavelength on the long wavelength side of the stop band, and k _i1 and k _g1 , or k _i2 and k _g2 are different. A complex coupled distributed feedback semiconductor laser characterized in that oscillation occurs at a wavelength on the short wavelength side of the stop band when the code is used .   5. The complex coupled distributed feedback semiconductor laser according to claim 1, wherein the complex coupled diffraction grating is fabricated by etching a part or all of the multiple quantum well active layer. A complex coupled distributed feedback semiconductor laser.   5. The complex coupled distributed feedback semiconductor laser according to claim 1, wherein the complex coupled diffraction grating is fabricated by growing an active layer on a pattern substrate. Complex coupled distributed feedback semiconductor laser.   5. The complex coupled distributed feedback semiconductor laser according to claim 1, wherein the complex coupled diffraction grating is formed by periodically providing a light absorption layer above or below the active layer. A complex coupled distributed feedback semiconductor laser characterized by the above.   5. The complex coupled distributed feedback semiconductor laser according to claim 1, wherein the complex coupled diffraction grating is formed by periodically providing a current blocking layer above or below the active layer. A complex coupled distributed feedback semiconductor laser characterized by the above.   5. The complex coupled distributed feedback semiconductor laser according to claim 1, wherein the complex coupled diffraction grating is fabricated by periodically providing a metal film on the element surface. Complex coupled distributed feedback semiconductor laser.

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