Abstract:
A semiconductor laser device having a semiconductor substrate, an active region formed on the semiconductor substrate and configured to radiate light having a predetermined wavelength range, a light reflecting facet and a light emitting facet positioned at opposing longitudinal ends of the active region to form a resonant cavity. A diffraction grating is positioned within the resonant cavity, and is configured to select a first portion of the radiated light for emitting from the semiconductor laser device, and an absorption region located in a vicinity of the active region and configured to selectively absorb a second portion of the radiated light, the first portion of the radiated light having a different wavelength than the second portion of the radiated light. The light emitting facet has a reflectivity value of aproximately in the range of 10%-30%.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to semiconductor laser devices, and more particularly to a distributed feedback (DFB) semiconductor laser device having excellent single longitudinal-mode lasing characteristics over a relatively wide operating temperature of the laser device, and a good resistance to external optical feedback.  
           [0003]    2. Discussion of the Background  
           [0004]    With the recent demand for increased bandwidth for data communications, optical networks and the components essential for their operation are being closely studied. To provide a light source for such optical networks, semiconductor laser devices such as the distributed feedback semiconductor laser device have been used. FIG. 8 shows a cross section of an exemplary distributed feedback semiconductor laser  800  (hereinafter, referred to as a DFB laser). As seen in this figure, the DFB laser has an active layer  801  wherein radiative recombination takes place, and a diffraction grating  803  for changing the real part and/or the imaginary part of the refractive index (complex refractive index) periodically, so that only the light having a specific wavelength is fed back for wavelength selectivity. The diffraction grating  803  is comprised of a group of periodically spaced parallel rows of grating material  805  surrounded by a cladding material  807  (typically made of InP material) to form a compound semiconductor layer that periodically differs in refractive index from the surroundings. In a DFB laser having such a diffraction grating  803  in the vicinity of its active layer  801 , the lasing wavelength λ DFB  which is emitted from the DFB laser is determined by the relation:  
           λ DFB =2 n   eff Λ,  
           [0005]    where Λ is the period of the diffraction grating as shown in FIG. 8, and n eff  is the effective refractive index of the waveguide. Thus, the period Λ of the diffraction grating and the effective refractive index n eff  of the waveguide can be adjusted to set the lasing wavelength λ DFB  independent of the peak wavelength of the optical gain of the active layer.  
           [0006]    This setting of the lasing wavelength λ DFB  independent of the peak wavelength of the optical gain of the active layer allows for essential detuning of the DFB laser device. Detuning is the process of setting the emitted lasing wavelength of a laser to a different value than the peak wavelength of the optical gain of the active layer to provide more stable laser operation over temperature changes. As is known in the art, a moderately large detuning value (that is, a large wavelength difference between the emitted lasing wavelength and the peak wavelength of the optical gain of the active layer) can improve high speed modulation or wide temperature laser performance, while too large a detuning amount degrades performance. The present inventors have recognized that the amount of detuning changes over wide temperature range because temperature dependence on the lasing wavelength is about 0.1 nm/C., while the gain peak wavelength changes at about 0.4 nm/C. Thus, for wide temperature operation, reduction of optical gain especially in the high temperature range should be carefully considered in designing the detuning.  
           [0007]    In addition to detuning, the lasing wavelength λDFB may also be set independent of the peak wavelength of the optical gain of the active layer in order to the obtain different characteristics of the semiconductor laser device. For example, when the lasing wavelength of the DFB laser is set at wavelengths shorter than the peak wavelength of the optical gain distribution, the differential gain increases to improve the DFB laser in high-speed modulation characteristics and the like. Where the lasing wavelength of the DFB laser is set approximately equal to the peak wavelength of the optical gain distribution of the active layer, the threshold current of the laser device decreases at room temperature. Still alternatively, setting λ DFB  at wavelengths longer than the peak wavelength improves operational characteristics of the DFB laser, such as output power and current injection characteristics, at higher temperatures or at a high driving current operation.  
           [0008]    The conventional DFB laser such as that disclosed in FIG. 8 can be broadly divided into a refractive index coupled type laser and a gain coupled type laser. In the. refractive index coupled DFB laser, the compound semiconductor layer constituting the diffraction grating has a bandgap energy considerably higher than the bandgap energy of the active layer and the bandgap energy of the lasing wavelength. Thus, bandgap wavelength (which is a wavelength conversion of the bandgap energy) of the diffraction grating is typically at least 100 nm shorter than the lasing wavelength and is usually within the range of 1200 nm -1300 nm if the λ DFB  is approximately 1550 nm. In the gain coupled DFB laser, the bandgap wavelength of the compound semiconductor layer constituting the diffraction grating is longer than the lasing wavelength and is typically about 1650 nm if the λ DFB  is approximately 1550 nm. FIGS. 9 a  and  9   b  show the operational characteristics of an exemplary refractive index coupled laser and gain coupled laser respectively. Each of these figures includes λe, λg, λmax, and λInP shown plotted on an abscissa which shows wavelength increasing from left to right in the figures. In this regard, λe is the selected lasing wavelength of the DFB laser  800 , λmax is the peak wavelength of the optical gain distribution of the active layer  801 , λg is the bandgap wavelength of the diffraction grating material  805 , and λInP is the bandgap wavelength of the surrounding InP material  807 . As seen in FIGS. 9 a  and  9   b , the bandgap wavelength λInP is typically 920 nm and the bandgap wavelength λg is closely related to the absorption loss of the diffraction grating which is shown by the broken curves  903  and  903 ′. Moreover, the refractive index of a material increases as the bandgap wavelength of the material increases as shown by the arrows  905 . Thus, as seen in the figures, the refractive index of the diffraction grating having the bandgap wavelength λg is generally higher than the refractive index of the surrounding Inp layer having the bandgap wavelength λInP.  
           [0009]    [0009]FIG. 9 a  shows an exemplary refractive index coupled DFB laser wherein the DFB laser has a lasing wavelength λe of 1550 nm and bandgap wavelength λg of 1250 nm, and satisfies the relationship:  
           λg&lt;λe.  
           [0010]    Thus, the DFB laser of FIG. 9( a ) reflects λe−λg=300 nm. The DFB lasing wavelength λe is usually set within the several tens of nanometer range from the peak wavelength λmax of the optical gain distribution of the active layer. In the FIG. 9( a ), λe is located longer than λmax. With the refractive index coupled DFB laser, the absorption loss curve  903  does not cross the lasing wavelength λe and therefore absorption loss at λe is very small. Accordingly, the DFB laser of FIG. 9 a , has the advantage of a low threshold current and favorable optical output-injection current characteristics. However, as also shown in FIG. 9 a , in a refractive index coupled DFB laser, the absorption loss curve  903  also does not cross the peak wavelength of the optical gain distribution of the active layer λmax. Therefore, assuming that the absorption coefficient with respect to the lasing wavelength λe of the DFB laser is αe and the absorption coefficient with respect to the bandgap wavelength of the active layer, or the peak wavelength λmax of the optical gain distribution of the active layer, is αmax, then αe is approximately equal to αmax which is approximately equal to zero. This means that the absorption curve  903  affects neither λmax nor λe, and the peak wavelength λmax of the optical gain distribution of the active layer is not suppressed with respect to the lasing wavelength λe.  
           [0011]    More specifically, there is a problem with the refractive index coupled laser in that a side mode suppression ratio (SMSR) of adequate magnitude cannot be secured between the lasing mode at the designed lasing wavelength λe of the DFB laser and the mode around the peak wavelength λmax of the optical gain distribution of the active layer. In addition, because neither the λmax nor the λe wavelengths are affected by the absorption curve  903 , wide detuning cannot be accomplished using the refractive index coupled semiconductor laser of FIG. 9 a . That is, the absolute value of the detuning amount |λe−λmax| cannot be made greater since an increase in the absolute value of the detuning amount |λe−λmax| would result in a large gain difference between the lasing wavelength λe and λmax, and lowers the single mode properties and narrows the temperature range operation of the refractive index coupled semiconductor laser.  
           [0012]    Finally, with the refractive index coupled DFB laser of FIG. 9 a , the difference in the refractive index of the grating material  805  and the refractive index of the InP buried layer  807  is relatively small. Therefore, the physical distance between the grating material  805  and the active layer  801  of the DFB laser  800  must be reduced and, as a result, the coupling coefficient varies greatly depending on the thickness of the diffraction grating layer and the duty ratio which is expressed as W/Λ, where W is the width of one element of the diffraction grating and Λ is the pitch of the gratings. This makes it difficult to fabricate refractive index coupled DFB laser devices having the same characteristics resulting in low manufacturing yields for this type of laser.  
           [0013]    As seen in FIG. 9 b , the gain coupled DFB laser has a lasing wavelength λe of which is less than the bandgap wavelength λg of the diffraction grating layer. Specifically, the DFB laser of FIG. 9 b  has a lasing wavelength λe of 1550 nm, a bandgap wavelength λg of 1650 nm, and satisfies the relationship:  
           λe&lt;λg.  
           [0014]    Thus, this exemplary DFB laser reflects λe−λg=−100 nm. in the gain coupled DFB laser of FIG. 9 b , there is a relatively large difference between the refractive index of the grating material  805  and refractive index of the InP buried layer  807  which makes it possible to increase the distance between the grating material  805  and the active layer  801 . As a result, unlike the refractive index coupled DFB laser, the coupling coefficient of the gain coupled laser is hard to vary with the thickness of the diffraction grating layer and the duty ratio, and same-characteristic DFB lasers can be fabricated with stability thereby allowing higher production yields for this type of laser.  
           [0015]    However, as also seen in FIG. 9 b , the gain coupled DFB laser has an absorption loss curve  903 ′ that crosses the lasing wavelength λe and, therefore, absorption loss at the desired lasing wavelength λe is large resulting in a high threshold current and unfavorable optical output-injection current characteristics. Moreover, although the absorption loss curve  903 ′ also crosses the undesired wavelength of λmax, the absorption coefficient αmax is approximately equal to the absorption coefficient αe. That is, as with the refractive index coupled DFB laser, the absorption curve  903 ′ of the gain coupled DFB laser affects λmax and λe equally and the peak wavelength λmax of the optical gain distribution of the active layer is not suppressed with respect to the lasing wavelength λe resulting in a low side mode suppression ratio (SMSR). For example, in the conventional DFB lasers of FIGS. 9 a  and  9   b , the SMSR, though depending on the amount of detuning to the lasing wavelength of the DFB laser, falls within a comparatively small range of 35 and 40 dB. Also like the refractive index coupled DFB laser, since the absorption curve  903 ′ affects λmax and λe equally, wide detuning cannot be accomplished because the wider the spacing between the λmax and λe wavelengths, the smaller the gain of the desired lasing wavelength λe will be with respect to the undesired λmax. Thus, whether the λe is set shorter or longer than λmax, the absolute value of the detuning amount |λe−λmax| of conventional refractive index and gain coupled DFB lasers is limited several tens of nanometers thereby causing unfavorable single mode and temperature range characteristics for these devices.  
           [0016]    As noted above, conventional refractive index coupled and gain coupled DFB laser devices have poor SMSR and cannot achieve wide detuning thereby allowing the DFB laser device to lase in a Fabry-Perot mode at the peak wavelength λ max  rather than the designed emission wavelength λ e . The present inventors have discovered that this problem of degradation of the single-longitudinal-mode characteristic is especially noticeable when the DFB laser device operates in a wider temperature range of −40 and +85 degrees C., for example. In general, if the operating temperature of the DFB laser device changes, a change occurs in the amount of difference between the peak wavelength λ max  and the emission wavelength λ e . Thus, the absolute value of the detuning amount |λ e −λ max | is difficult to maintain within an allowable range in the wide temperature range.  
           [0017]    More specifically, the present inventors have discovered that, in a DFB laser device using an InGaAsP-based semiconductor material, the temperature dependence of the peak wavelength λ max  is around 0.4 nm/degree C., whereas the temperature dependence of the emission wavelength λ e  is around 0.1 nm/degree C. Thus, if the DFB laser device operates in the range between −40 and +85 degrees C., the detuning amount λ e −λ max  changes by about 40 nm in the temperature range, by the following calculation:  
           (0.4−0.1)×(85−(−40))=37.5  nm.    
           [0018]    Therefore, if the detuning amount is designed at 0 nm (thereby allowing the DFB laser device to lase at the peak wavelength λ max ) at a temperature of +85 degrees C. for example, at which temperature the optical gain of the DFB laser device is generally low, the detuning amount can assume as high as +40 nm at a lower temperature of −40 degrees C. This detuning amount of +40 nm causes the conventional DFB laser device to lase at the peak wavelength λ max  in the Fabry-Perot mode, i.e., not at the design emission wavelength λ e  for the DFB laser device, thereby preventing the DFB laser device from operating in the single-longitudinal-mode. Therefore, in order to achieve an excellent single-longitudinal-mode characteristic or an excellent modulation characteristic in the whole temperature range of the DFB laser device, the detuning amount should be precisely maintained within a specified range at least at a specified temperature, such as the room temperature.  
           [0019]    Prior art devices have attempted to achieve good single mode operation over a wide detuning range by suppressing the Fabry-Perot mode lasing using a low reflectivity at one of the facets of the DFB laser device. This low reflectivity is achieved by a non-reflection coating, and is generally adopted at the emission facet of the DFB laser device in order to maintain emission efficiency. The present inventors have discovered, however, that the lower reflectivity at the emission facet has a problem in that the DFB laser device suffers from a lower resistance against the external optical feedback because the light reflected from outside the cavity is likely to enter the resonant cavity of the DFB laser device through the low reflectivity emission facet. The external reflection light into the resonant cavity affects the lasing operation to generate noise and thus renders the DFB laser device unstable.  
         SUMMARY OF THE INVENTION  
         [0020]    Accordingly, one object of the present invention is to provide a semiconductor laser device and method which overcomes the above described problems.  
           [0021]    Another object of the present invention is to provide a reliable technique that effectively suppresses the Fabry-Perot mode lasing to obtain a stable operation of the DFB laser device and suppresses the degradation in the resistance against the external optical feedback for a laser device having a larger detuning amount.  
           [0022]    According to a first aspect of the invention, there is provided a semiconductor laser device having a semiconductor substrate, an active region formed on the semiconductor substrate and configured to radiate light having a predetermined wavelength range, a light reflecting facet and a light emitting facet positioned at opposing longitudinal ends of the active region to form a resonant cavity. A diffraction grating is positioned within the resonant cavity, and is configured to select a first portion of the radiated light for emitting from the semiconductor laser device, and an absorption region located in a vicinity of the active region and configured to selectively absorb a second portion of the radiated light, the first portion of the radiated light having a different wavelength than the second portion of the radiated light. The light emitting facet has a reflectivity value of aproximately in the range of 10% -30%.  
           [0023]    In one embodiment of the first aspect, the first portion of the radiated light is a single mode lasing wavelength λe and the second portion of the radiated light is a peak wavelength λ max  of an optical gain distribution of the active region. In this embodiment, the absorption region is configured to provide operational characteristics satisfying any one of the relationships: 0&lt;λe−λ abs ≦100 nm or 0&lt;λe−λ abs &lt;70 nm, where λ abs  is the bandgap wavelength of the absorption region, and λe is the single mode lasing wavelength.  
           [0024]    In another embodiment of the first aspect, the absorption region of the semiconductor laser is configured to provide operational characteristics satisfying any one of the relationships: α max &gt;αe; α max −αe≧1 cm −1 ; or α max −αe≧5 cm −1 , in terms of waveguide loss, where α max  is an absorption coefficient with respect to the peak wavelength λ max  of the optical gain distribution of the active region, and αe is an absorption coefficient with respect to the selected lasing wavelength λe. In this embodiment, the absorption region may be configured such that the absorption coefficient αe is substantially 0.  
           [0025]    In yet another embodiment of the first aspect of the present invention, the active region, wavelength selecting structure, and absorption region are configured to provide operational characteristics satisfying any one of the relationships λ abs &lt;λ max &lt;λe, or the relationship λ max &lt;λ abs &lt;λe, where λ abs  is the bandgap wavelength of the absorption region, λ max  is the peak wavelength of an optical gain distribution of the active region, and λe is the single mode lasing wavelength.  
           [0026]    The absorption region may be provided by the diffraction grating or a selective absorption semiconductor layer. Moreover, the light emitting facet may be approximately in the range of 10-20%, or approximately equal to 10%. Also, the light reflecting facet may be approximately 90%. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:  
         [0028]    [0028]FIG. 1 is a partially sectional perspective view showing the structure of a semiconductor layer device according to a first embodiment of the present invention;  
         [0029]    [0029]FIG. 2 is a sectional view of the semiconductor laser device taken along the arrowed line I-I of FIG. 1;  
         [0030]    [0030]FIGS. 3 a  and  3   b  are a wavelength graphs showing the operational characteristics of DFB laser devices according to the first embodiment of the present invention.  
         [0031]    FIGS.  4 ( a )- 4 ( e ) are sectional views depicting the steps of fabricating the semiconductor laser device according to the first embodiment of the present invention;  
         [0032]    [0032]FIG. 5 is a partially sectional perspective view showing the structure of a semiconductor laser device according to a second embodiment of the present invention;  
         [0033]    [0033]FIG. 6 is a sectional view of the semiconductor laser device taken along the arrowed line III-III of FIG. 5.  
         [0034]    [0034]FIG. 7 is a wavelength graph showing the operational characteristics of DFB laser devices according to the second embodiment of the present invention;  
         [0035]    [0035]FIG. 8 is a cross section view of a conventional DFB laser device; and  
         [0036]    [0036]FIGS. 9 a  and  9   b  are wavelength graphs showing the operational characteristics of conventional refractive index coupled and gain coupled DFB lasers. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]    Referring now to the drawings wherein like elements are represented by the same or similar reference designations throughout, and more particularly to FIGS. 1 and 2, there is shown a semiconductor laser device  40  for providing selective light absorption in accordance with a first embodiment of the present invention. FIG. 1 is a partially sectional perspective view showing the structure of a semiconductor laser device according to the first embodiment of the present invention, and FIG. 2 is a sectional view of the semiconductor laser device taken along the arrowed line I-I of FIG. 1.  
         [0038]    The semiconductor device of FIGS. 1 and 2 is a buried heterojunction type DFB laser device including an n-InP substrate  42  having a 1-μm-thick n-InP buffer layer  44 , an active layer or active region  46 , and a 200-nm-thick p-InP spacer layer  48  sequentially stacked on the substrate  42 . Buffer layer  44  serves both as a buffer layer by the n-InP material and a under cladding layer, while the active layer  46  is a separate confinement multiple quantum well (SCH-MQW) structure. As best seen in FIG. 2, a diffraction grating  50  of a GaInAsP material is periodically formed within the p-InP spacer layer  48  substantially along the entire length of active layer  46 . However, the diffraction grating  50  may be formed over a portion of the entire length of active layer  46  as shown by the phantom grating material in FIG. 2. The diffraction grating  50  of the embodiment of FIGS. 1 and 2 has a film thickness “th” of 20 nm, a period “Λ” of 240 nm, and selects a laser beam having a lasing wavelength of 1550 nm to be emitted by the semiconductor laser device  40 .  
         [0039]    A top portion of the n-InP substrate  42 , the n-InP buffer layer  44 , the active layer  46 , and the p-InP spacer layer  48  having the diffraction grating  50  buried therein form a laminated structure which is etched into mesa stripes so that the active layer  46  has a width of approximately 1.5 μm. Current block structures each including a p-InP layer  54  and an n-InP layer  56  are formed on both sides of the mesa stripes. The DFB laser device  40  also has a 2-μm-thick p-InP upper cladding layer  58  and a heavily doped p-GaInAs contact layer  60  sequentially stacked on the spacer layer  48  and blocking layers. Also included is a p-side electrode  62  made of a Ti/Pt/Au laminated metal film over the contact layer  60 , and an n-side electrode  64  made of AuGeNi on the bottom surface of the substrate  42 .  
         [0040]    It is to be understood that the device of FIGS. 1 and 2 is for exemplary purposes only as many variations of the structure of the laser device  40  will be readily apparent to one having ordinary skill in the art. For example, the material composition and layer thicknesses described may be changed without deviating from the principles of the present invention. Thus, the embodiment of FIGS. 1 and 2 are exemplary for a better understanding of the present invention and the present invention is not limited to these illustrations.  
         [0041]    In order to evaluate the DFB laser device  40 , wafers having the laminated structure of FIGS. 1 and 2 were subjected to coating the front facet to form a low-reflective film of about 10% and at the rear facet to form a high-reflectance film of about 90%, and measured for the laser characteristics thereof. Based on these measurements, FIG. 3 a  shows exemplary operational characteristics of a DFB laser according to the embodiment of FIGS. 1 and 2 of the present invention. As seen in FIG. 3 a , the DFB laser  40  has a lasing wavelength λe of approximately 1550 nm, and the diffraction grating  50  has a bandgap wavelength λg of approximately 1510 nm, and a λ max  of 1530 nm at a temperature of 25 degrees C. In a preferred embodiment, GaInAsP having a λ g  of approximately 1510 nm is used to form the diffraction grating  50 . Thus, the difference in the lasing wavelength and the bandgap wavelength of the diffraction grating in FIGS. 1 and 2 is approximately 40 nm. More generally, a laser device in accordance with the present invention reflects a difference λe−λg of between 0 and 100 nm as noted in FIG. 3 a  (λe−λg=0˜100 nm). Moreover, the peak wavelength λ of the active layer  46  is approximately 1530 nm in the optical gain distribution curve  301 . Therefore the DFB laser of FIG. 3 a  satisfies the relationship:  
         λg&lt;λ max &lt;λe.  
         [0042]    As with the prior art figures discussed in the Background section above, FIG. 3 a  depicts an absorption curve  303  as well as an arrow  305  showing the refractive index of the grating material  50  and the active layer  46 . The present inventors have discovered that the a DFB laser of FIGS. 1 and 2 having the operational characteristics of FIG. 3 a  provides several advantages over the prior art DFB lasers.  
         [0043]    First, the semiconductor laser device of FIGS. 1 and 2 provides selective absorption of the undesirable peak gain wavelength λ max . As described above, the bandgap wavelength of a material is closely related to the wavelength absorption characteristics of the material. Thus, in the embodiment of FIGS. 1 and 2, the diffraction grating material  50  is selected to have a bandgap wavelength λg that will provide an absorption curve  303  that crosses the peak wavelength λ max  but does not cross the lasing wavelength λe. That is, the absorption at wavelength λe is preferably 0. More generally, however, according to the embodiment of FIGS. 1 and 2, the diffraction grating is constructed such that an absorption coefficient α max  is greater than an absorption coefficient αe. For example, the diffraction grating  50  is preferably constructed such that α max −αe is greater than or equal to 1 cm −1  in terms of waveguide loss, and more preferably greater than or equal to or equal to 5 cm −1  in terms of waveguide loss.  
         [0044]    It is preferable that the relationship α max &gt;α e  hold for the whole operating temperature range of the DFB laser. However, it is sufficient that the relationship α max &gt;α e  hold in a part of the operating temperature range, such as a lower temperature range below zero degree C. In such a case, the DFB laser device can suppress the Fabry-Perot lasing at around λ max  in the lower temperature range.  
         [0045]    In addition, by providing such selective absorption of the peak wavelength λ max , a laser device of FIGS. 1 and 2 simultaneously provides the benefits of both the refractive index coupled and the gain coupled DFB lasers described in the Background section above. That is, as with the refractive index coupled DFB laser described in FIG. 9 a , the selective absorption loss curve  303  of the present invention does not cross the lasing wavelength λe. Therefore, the absorption loss at λe is very small and the DFB laser depicted in FIG. 3 a  has a low threshold, and favorable optical output-injection current characteristics and higher output power. Specifically, the lasing efficiency of the present invention was compared with that of the conventional type DFB laser devices, revealing that the absorption with respect to the lasing wavelength of the diffraction grating  50  was sufficiently lower, and that the threshold current was as low as 9 mA in the present embodiment.  
         [0046]    Moreover, as with the gain coupled DFB laser described in FIG. 9 b , the laser having the characteristics of FIG. 3 a  has a relatively large difference between the refractive index of the grating material  50  and refractive index of the InP buried layer  48 . In a practical exemplification, the diffraction grating  20  has a refractive index of 3.49 whereas the embedding InP layer  22  has a refractive index of 3.17, whereby the difference in the refractive index is as large as 0.32. Although a larger difference is preferable, 0.25 or above should be adopted in a practical view point. As mentioned, this makes it possible to vary the duty ratio and increase the distance between the grating material  50  and the active layer without varying the coupling coefficient of the laser device of the present invention. Therefore, even if the p-InP spacer layer  48  is increased in thickness to separate the diffraction grating  50  away from the active layer  46 , it is possible to obtain a diffraction grating coupling coefficient of adequate magnitude. Accordingly, the tolerance in the crystal growing process of the fabrication process is alleviated to allow higher production yields of a laser device in accordance with the present invention.  
         [0047]    The DFB laser device of FIGS. 1 and 2 provides advantages not offered by either of the conventional laser devices described above. First, since the DFB laser  40  according to the present invention selectively absorbs the peak wavelength λ max , the side mode suppression ratio (SMSR) is significantly better than that of the prior art devices. Specifically, the DFB laser device  40  was found to have stable single mode lasing characteristics and offered a side mode suppression ratio as large as 45-50 dB. As mentioned above, conventional DFB laser devices offered a SMSR generally limited to around 35-40 dB. Moreover, because the selective absorption of the invention of FIGS. 1 and 2 provides an absorption of λ max  that is greater than the absorption of λe, wide detuning can be accomplished using the semiconductor laser of FIG. 3 a . That is, the absolute value of the detuning amount |λe−λ max | can be made greater since the selective absorption and high SMSR can be used to maintain single-mode properties of the longitudinal mode and suppress the gain at the peak wavelength λ max . In this regard, it is noted that, by way of the present invention, the present inventors have discovered a device which can achieve wide detuning and can also be manufactured with high production yields due to the spacing between the active and grating layers as described above. Finally, since wide detuning can be achieved, the semiconductor laser device according to the present invention can provide high output power over a wide temperature range. Thus, the semiconductor laser of the first embodiment can maintain favorable single mode properties even though detuning is increased.  
         [0048]    More specifically, the tested samples having the structure of FIGS. 1 and 2 exhibited a threshold current of about 26 mA and had a detuning amount around zero nm at a high temperature of 85 degrees C. The samples also had a detuning amount of about +40 nm at a lower temperature of −40 degrees C., and exhibited an excellent single mode yield of 85% even with the relatively high reflectivity of 10% adopted at the emission facet. This is considered due to the suppression of the Fabry-Perot mode lasing by the function of the diffraction grating  20  selectively absorbing the vicinity of the peak wavelength λ max . Moreover, the reflectivity of 10% at the emission facet provides a higher optical output power of 0.32 watts/ampere and a higher resistance to external feedback noise. Thus, the laser device having the structure of FIGS. 1 and 2 provides good single mode operation over a wide detuning range, while maintaining good resistance to noise caused by external feedback. That is, in the present invention, even if the absolute value of the detuning amount |λ e −λ max | changes depending on the operating temperature to assume a value more than 40 nm at the maximum thereof, the DFB laser device has an excellent single-longitudinal-mode characteristic or an excellent modulation characteristic. Moreover, the DFB laser operates at a high optical output power and has an excellent resistance against the external optical feedback.  
         [0049]    More generally, the present inventors conducted experiments on a plurality of specimens for each of six samples of the DFB laser device of FIG. 2. Each specimen had a reflectivity of 1% (first sample), 5% (second sample), 10% (third sample), 20% (fourth sample), 30% (fifth sample) or 50% (sixth sample) at the emission facet, and a reflectivity of 90% (first to sixth samples) at the rear facet on the samples were coated with a high reflection coat, except for the fifth sample having a reflectivity of 30%, which was an as-cleaved emission facet wherein the emission facet is not coated. The samples having the respective reflectivities were subjected to measurements or calculations of the optical output efficiency (watts/ampere), the product yield (%) with respect to the single mode characteristic (“single mode yield”), the product yield (%) with respect to the resistance against the external optical feedback (“resistance yield”), and the overall product yield. In this regard, the term “single mode yield” as used herein means the ratio of number of specimens of DFB laser having a SMSR of 35 dB or more in the whole temperature range between −40 and +85 degrees with respect to the total number of the specimens having a SMSR of 35 dB or more at 25 degrees C. Moreover, the term “resistance yield” as used herein means the number of specimens of DFB laser having a relative intensity noise (RIN) of −120 dB/Hz or less with respect to the total number of the specimens, in the case that external reflection has an intensity of −15 dB, or about 3% of the intensity of the original laser. Finally, the term “overall yield” means the product of the single mode yield and the resistance yield.  
         [0050]    The results are shown in Table 1 as follows.  
                                   TABLE 1                       Sample       efficiency   single mode   resistance           No.   reflectivity   (W/A)   yield   yield   overall yield                   1          1%   0.37         95%         18%         17.1%       2    5   0.34   91   50   45.5       3   10   0.32   85   80   68.0       4   20   0.30   77   86   66.2       5   30   0.28   68   90   61.2       6   50   0.25   45   98   44.1                  
 
         [0051]    Based on these experiments, the present inventors discovered that a larger reflectivity of the emission facet reduces the optical output efficiency and the single mode yield, but increases the resistance yield. Moreover, the overall yield obtained by multiplication of the single mode yield by the resistance yield has a higher value in the range of the reflectivity of the emission facet between 10% and 30%. Finally, it was discovered that the range of reflectivity of the emission facet should reside between 10% and 20% for obtaining an optical output efficiency of 0.3 watts/ampere or higher.  
         [0052]    While the above description provides an example of a DFB laser device having a λ g  less than both λ e  and λ max , an absorption coefficient α max  greater than the absorption coefficient αe can also be achieved by setting the bandgap wavelength λ g  of the diffraction grating  50  to a value between the peak wavelength of the optical gain distribution and the lasing wavelength 1550 nm of the DFB laser  40  as shown in FIG. 3 b . While these characteristics realize the benefits of the present invention, the absorption caused by the tails of the band edges also occurs on the lasing wavelength λ e  as seen in FIG. 3 b . Thus, the threshold current increases and the lasing efficiency decreases, which is generally undesirable. It is to be noted, however, that the concept of selective absorption is not limited to selectively absorbing λ max . Thus, if λ g &lt;λ max &lt;λ e  holds, as shown in FIG. 3A, an excellent temperature characteristic can be obtained, whereby the DFB laser device achieves a higher optical output power at a higher temperature range or a higher injection current range. For example, if λ e −λ max =20 nm, the difference between λ e  and λ g  may be set for satisfying the relationship λ e −λ g =40 nm. On the other hand, if λ max &lt;λ g &lt;λ e  holds, then the absorption coefficient α max  for the peak wavelength can be set at a larger value, whereby a Fabry-Perot lasing can be suppressed. In this case, for example, λ e −λ max  may be set at 30 nm, whereas λ g −λ max  may be set between 10 nm and 20 nm.  
         [0053]    [0053]FIGS. 4 a - 4   e  are sectional views showing the method steps of fabricating the DFB laser device  40  of FIGS. 1 and 2 of the present invention. FIGS. 4 a - 4   c  show cross sections taken along the arrowed line I-I of FIG. 1, while FIGS. 4 d  and  4   e  show cross sections taken along the arrowed line II-II of FIG. 1.  
         [0054]    As seen in FIG. 4 a , the process begins with an n-InP substrate  12  on which a 1-μm-thick n-InP buffer layer  14 , MQW-SCH active layers  16 , a 200-nm-thick p-InP spacer layer  18 , and a 20-nm-thick GaInAsP diffraction grating layer  20 A are sequentially stacked. Each layer is epitaxially grown on an n-InP substrate  12  in succession, in a metal-organic chemical vapor deposition (MOCVD) system at a growth temperature of 600° C. to form the laminated structure shown in FIG. 4 a . An electron beam (EB) resist is applied on the diffraction grating layer  20 A with a thickness of approximately 100 nm and the resist layer is patterned according to conventional techniques to form a diffraction grating pattern  21  having a period A of approximately 240 nm. Thereafter, etching is performed in a dry etching system with the diffraction grating pattern  21  as the mask, whereby trenches  23  penetrating the diffraction grating layer  20 A are formed to expose the p-InP spacer layer  18  at the trench bottoms. This forms a diffraction grating  20  as shown in FIG. 4 b.    
         [0055]    The diffraction grating pattern  21  is then removed, and, as shown in FIG. 4 c , a p-InP first cladding layer  22  to bury the diffraction grating  20  is re-grown in the MOCVD system. Thereafter, a SiN x  film is formed over the p-InP first cladding layer  22  in a plasma CVD system. Then, using a photolithography and reactive ion etching system (RIE), the SiN x  film is processed into a stripe to form a SiN x  film mask  25  as seen in FIG. 4 d . Subsequently, using the SiN x  film mask  25  as the etching mask, the p-InP first cladding layer  22  including the diffraction grating  20 , the p-InP spacer layer  18 , the active layer  16 , the n-InP buffer layer  14 , and a top portion of the n-InP substrate  12  are etched into mesa stripes with an active layer width of the order of 1.5 μm. Then, using the SiN x  film mask  25  as the selective growth mask, a p-InP layer  24  and an n-InP layer  26  are selectively grown in succession. This forms current block structures on both sides of the mesa stripes as shown in FIG. 4 d.    
         [0056]    Next, the SiN x  film mask  25  is removed before a 2-μm-thick p-InP second cladding layer  28  and a contact layer  30 , or a GaInAs layer that is heavily doped to make an ohmic contact with a p-side electrode  32 , are epitaxially grown as shown in FIG. 4E. The n-InP substrate  12  is polished at its bottom surface to a substrate thickness of on the order of 120 μm. Then, a Ti/Pt/Au laminated metal film is formed as the p-side electrode  32  over the contact layer  30 . On the bottom surface of the substrate is formed an AuGeNi film as an n-side electrode  34 .  
         [0057]    The wafer having the above-described laminated structure can be cleaved into a chip before coating the emission facet with a reflection coat having a reflectivity of 10% and the rear facet with a reflection coat having a reflectivity of 90%, and bonded by die bonding and wire-bonding to form the DFB laser device shown in FIGS. 1 and 2. As mentioned above, by using a material for diffraction grating  20  that has a bandgap wavelength close to that of the active layer material  16 , the difference in refractive index of the diffraction grating  20  and the surrounding InP layer  22  is increased. This allows a desired refractive index coupling coefficient to be obtained even if the diffraction grating  20  is separated farther from the active layer  16  than in the conventional DFB laser devices. Accordingly, the tolerance in the crystal growing process and in the fabrication process of FIGS. 4 a - 4   d  is eased to allow stable crystal growth.  
         [0058]    [0058]FIGS. 5 and 6 show a semiconductor laser device in accordance with a second embodiment of the present invention. FIG. 5 is a partially sectional perspective view showing the structure of a semiconductor laser device according to the second embodiment of the present invention, and FIG. 6 is a sectional view of the semiconductor laser device taken along the arrowed line III-III of FIG. 5. In the DFB laser device  40  of the embodiment 1, the absorption region of the laser device is provided in the diffraction grating  20 . On the other hand, the semiconductor laser device  40 ′ of the second embodiment, though constituted likewise as a buried hetero-junction type DFB laser device with the designed lasing wavelength of 1550 nm, includes a selective absorption layer  45 A, aside from the diffraction grating for selectively absorbing the light in the mode of the peak wavelength of the optical gain distribution of the active layer.  
         [0059]    Specifically, the DFB laser device  40 ′ includes a 1-μm-thick n-InP buffer layer  44 ′, a 5-nm-thick InGaAs selective absorption layer  45 A, a 100-nm-thick n-InP spacer layer  45 B, MQW-SCH active layers  46 ′, a 100-nm-thick p-InP spacer layer  48 ′, a diffraction grating  50 ′ including a 30-nm-thick GaInAsP layer having a period Λ of 240 nm, and a p-InP first cladding layer  52 ′ having the diffraction grating  50 ′ buried therein. The elements of the diffraction grating  50 ′ may extend along the entire length of the active layer  46 ′, or may extend along a portion of the length of the active layer  46 ′ as shown by the phantom elements in FIG. 6. As with the embodiment of FIG. 1, these layers are epitaxially grown on an n-InP substrate  42 ′ in succession by an MOCVD or similar method. The selective absorption layer  45 A is a “quantized” structure which, for purposes of this invention, means the thickness of the selective absorption layer  45 A is reduced to a size on the order of quantum mechanical wavelengths of electrons to develop the quantum effect. The thickness of selective absorption layer  45 A is controlled so that the absorption edge wavelength (equivalent to bandgap wavelength) falls at a desired wavelength as will be further described below.  
         [0060]    A top portion of the n-InP substrate  42 ′, and the constituents of the laminated structure, i.e., the n-InP buffer layer  44 ′, the selective absorption layer  45 A, the n-InP spacer layer  45 B, the active layer  46 ′, the p-InP spacer layer  48 ′, the diffraction grating  50 ′, and the p-InP first cladding layer  52 ′ having the diffraction grating  50 ′ buried therein, are etched into mesa stripes so that the active layer  46 ′ has a width of approximately 1.5 μm. Then, current block structures each including a p-InP layer  54 ′ and an n-InP layer  56 ′ are formed on both sides of the mesa stripes. Furthermore, the semiconductor laser device  40 ′ has a 2-μm-thick p-InP second cladding layer  58 ′ and a heavily doped p-GaInAs contact layer  60 ′ over the first InP cladding layer  52 ′ and the n-InP layer  56 ′. The laser device of the second embodiment also includes a p-side electrode  62 ′ including a Ti/Pt/Au laminated metal film over the contact layer  60 ′, and an n-side electrode  64 ′ made of AuGeNi on the bottom surface of the substrate  42 ′. As with the first embodiment, the semiconductor laser device  40 ′ includes a light emission facet having a reflectivity of 10-30% and a light reflecting facet of 90%.  
         [0061]    As with the embodiment of FIGS. 1 and 2 of the present invention, it will be understood by one of ordinary skill in the art that the second embodiment of the present invention shown in FIGS. 5 and 6 is exemplary only and the compositions, film thicknesses, and the like of the compound semiconductor layers may be changed without deviating from the principles of the present invention. For example, while embodiment of FIGS. 5 and 6 shows the selective absorption layer  45 A opposed to the diffraction grating across the active layer, it may be arranged on the same side as the diffraction grating. Although, it is noted that the opposite side arrangement has a higher degree of flexibility in design since the distance from the active layer can be arbitrarily selected. In addition, while the selective absorption layer  45 A is shown as a single layer in FIGS. 5 and 6, multiple-quantum-well layers may be formed to achieve a greater difference in absorption coefficient as will be described below.  
         [0062]    [0062]FIG. 7 shows exemplary operational characteristics of a DFB laser according to the embodiment of FIGS. 5 and 6 of the present invention. As seen in FIG. 7, the DFB laser  40 ′ has a lasing wavelength λe of approximately 1550 nm, the diffraction grating  50 ′ has a bandgap wavelength λg of approximately 1200nm, and the peak wavelength λ max  of the active layer  46 ′ is approximately 1530 nm in the optical gain distribution curve  701 . Thus, the diffraction grating  50 ′ is sufficiently transparent to the peak wavelength of the optical gain distribution of the active layer  46 ′, and to the designed lasing wavelength of the DFB laser device  40 ′. In addition, a bandgap wavelength λ sel  of the selective absorption layer  45 A is approximately 1540 nm. Therefore the DFB laser of FIG. 7 satisfies the relationship:  
         λ max &lt;λ sel &lt;λe.  
         [0063]    [0063]FIG. 7 also depicts an absorption curve  707  of the selective absorption layer, as well as the absorption curve  703  and increasing refractive index shown by arrow  705 . A complete discussion of the laser device having the selective absorption region can be found in U.S. patent application Ser. No. 09/906,842, the entire contents of which is incorporated herein by reference. As with the embodiment of FIG. 1, the second embodiment of the present invention shown in FIGS. 5 and 6 provides excellent single mode operation over a wide detuning range, while having good resistance to external feedback.  
         [0064]    The first and second embodiments of the present invention have been described above in the context of a laser device having a lasing wavelength longer than a peak wavelength of the optical gain distribution. As described in the Background section above, this provides improved operational characteristics such as high power light intensity output and current injection characteristics at higher temperatures. However, the benefits of present invention may be realized by providing a laser device of the first or second embodiment wherein the lasing wavelength λe is shorter than λ max . This allows increased differential gain at high frequencies and provides favorable high-speed modulation characteristics for a laser device constructed according to the first and second embodiments. Moreover, as is understood by one of ordinary skill in the art, the difference in the wavelength values λe and λ max  (i.e., detuning value) may be set to any value, limited only by the selectivity of the absorption region, to realize the benefits of the present invention. Specifically, where a steep absorption curve is achievable, the lasing wavelength λe may be set very close to the λ max  in order to obtain a low threshold current characteristic for the laser of the first or second embodiments. Moreover, while the invention has been described with respect to a lasing wavelength of 1550 nm, it is to be understood that other lasing wavelengths may be used in realizing the benefits of the present invention.  
         [0065]    Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.