A GAIN COUPLED DISTRIBUTED FEEDBACK SEMICONDUCTOR LASER

Abstract: A gain coupled DFB semiconductor laser with a high order grating structure is provided. By varying the order and duty cycle of the grating and shape of grooves, a predetermined ratio of gain to index coupling coefficients is obtained. Beneficially the laser includes a multiple quantum well structure with the grating etched directly through the quantum wells of the active region. Deep etching ensures strong gain coupling in the laser while usage of a high order grating reduces index coupling associated with the etching. As a result, a stable, high yield, high power and single mode operation of the DFB laser is achieved. For gratings of a particular order of diffraction, duty cycle, shape of grooves and depth of etching index coupling may be substantially reduced or eliminated. Correspondingly, a purely gain coupled DFB laser may be obtained. It is also easier to manufacture high order DFB lasers because of the larger period of gratings allowing larger fabrication tolerances. A method of producing a complex coupled DFB semiconductor laser having a predetermined ratio of gain to index coupling, and a method of obtaining a purely gain/loss coupled semiconductor DFB laser are also provided.

Description: A GAIN COUPLED DISTRIBUTED FEEDBACK SEMICONDUCTOR LASER FIELD OF THE INVENTION The invention relates to semiconductor lasers, and in particular, to complex coupled distributed feedback (DFB) semiconductor lasers having a predetermined ratio of gain to index coupling, and a method of producing a complex coupled DFB semiconductor laser having a predetermined ratio of gain to index coupling, and a method of obtaining a purely gain/loss coupled DFB semiconductor laser. BACKGROUND OF THE INVENTION Fiber optics communication systems require compact light emitting sources capable of generating single-mode, tunable, narrow linewidth radiation in the 1.3 - 1.56 μm wavelength range. Some of the existing semiconductor lasers, for example, InGaAsP DFB lasers can meet requirements for high power and proper wavelength, but a high dynamic single mode yield is difficult to achieve. Conventional index coupled DFB lasers employing an index corrugation have an inherent problem in existence of two longitudinal modes with an equal threshold gain which results in poor single mode operation as shown, for example, in the article by H. Kogelnik and C.V. Shank "Coupled-mode theory of distributed feedback lasers", J. Appl. Phys., vol. 43, no. 5, pp. 2327 - 2335, 1972. In complex coupled DFB lasers, a periodic optical gain or loss modulation in the presence or absence of conventional index corrugation along the laser cavity effectively breaks the mode degeneracy between the two Bragg modes around the stop band of the DFB lasers, and thus avoids a serious and inherent problem for conventional index coupled DFB lasers, as shown in publications by Y. Luo, Y. Nakano, K. Tada et al . "Purely gain-coupled distributed feedback semiconductor lasers", Appl . Phys . Lett., vol. 56, pp. 1620-1622, 1990 and G.P. Li, T. Makino, R. Moore et al . "Partly gain-coupled 1.55 μm strained layer multi- quantum well DFB lasers", IEEE J. Quantum Electronics, vol. QE-29, pp. 1736-1742, 1993. With an introduction of even a small amount of gain or loss coupling, the dynamic single mode yield of complex coupled DFB lasers increases drastically, whether or not there is index coupling. It effectively provides lasing predominantly on a preferred and fixed Bragg mode among the two originally degenerate ones around a stop band, regardless of random distribution of unknown laser facet phases. In in-phase gain coupled DFB lasers, the higher index region within a grating period has a higher optical modal gain, resulting in lasing mainly of the right Bragg mode with the lasing wavelength longer than the Bragg wavelength. Both theory and experiments have confirmed this conclusion. For antiphase loss coupled DFB lasers, the higher index region within a grating period experiences higher optical loss, resulting primarily in lasing of the left Bragg mode with the lasing wavelength shorter than the Bragg wavelength. One of the promising approaches for implementing gain coupling in DFB lasers is etching directly through the active multi quantum well region to form a grating which provides a distributed periodic modulation of effective modal gain along the cavity direction, e.g. J. Hong et al . "Strongly gain-coupled coolerless (-40 ~ 85°C) DFB lasers", OECC'98, Chiba, Japan. The more quantum wells are etched away within one section of the grating period, the higher the optical gain and optical confinement of that section, and as a result, the higher the effective modal gain. Unfortunately, all types of gain/loss coupled DFB lasers to some extent suffer from presence of index coupling. As a rule, strong gain coupling is also accompanied by strong index coupling. For example, in DFB lasers with a grating structure etched through the active region of the laser, the more QWs are etched away, the higher the contrast of the refractive index between the etched and un-etched sections of the grating period, and therefore the stronger the index coupling. Strong index coupling results in a number of negative effects. First, it causes longitudinal spatial hole burning inside the cavity which limits the laser power. Second, it diminishes a ratio of gain to index coupling coefficients which is a critical parameter determining a single mode yield and performance of gain/loss coupled DFB lasers, e.g. "Impact of random facet phases on the modal properties of gain-coupled DFB lasers," IEEE Journal of Selected Topics in Quantum Electronics, pp. 555-568, vol. 3, no. 2, 1997 by J. Hong et al . As a result, high efficiency single mode operation is difficult to achieve. A number of approaches have been used to eliminate or reduce strong index coupling in DFB lasers. They are as follows : 1. Etching through a small number of QWs, see e.g. "Impact of random facet phase on modal properties of partly gain-coupled DFB lasers," J. Selected Topics on Quantum Electron., pp. 555-568, vol. 3, no. 2, April 1997 by J. Hong and etc.; 2. Reducing the thickness of the top optical confinement layer, e.g. "Strongly gain- coupled coolerless DFB lasers," OECC'98, Chiba, Japan, 1998 by J. Hong and etc.; 3. Use of a quaternary material as a first regrowth material to compensate for a difference in refractive indexes between two sections of the grating; 4. Use of short cavity DFB lasers. Each approach has drawbacks which significantly affect the performance of the DFB laser. In the first approach, the grating structure cannot provide sufficient modal gain discrimination and therefore high gain coupling effect. In the second approach, the laser threshold is increased, and high temperature performance of the laser is sacrificed because of the smaller thickness of the optical confinement layer and lower confinement factor while index coupling being reduced only partly. In the third approach, the over-growth of the material is difficult to control. As a result, the grating is not well defined and preserved which leads to a risk of decreasing reliability of the laser. The fourth approach suffers from low thermal dissipation within the laser cavity, which is not acceptable for high-power and/or high temperature operation. Accordingly, there is a need in the industry for the development of alternative structures of DFB semiconductor lasers, which would provide reduced index coupling and predetermined single mode performance characteristics . SUMMARY OF THE INVENTION Thus, the present invention seeks to provide a distributed feedback complex coupled semiconductor laser which would avoid the above mentioned problems, and a method of producing such lasers . Thus, according to one aspect of the present invention there is provided a complex coupled distributed feedback semiconductor laser, comprising: (a) a substrate; (b) an active region formed thereon; (c) a high order complex coupled grating formed on the substrate, the grating having grooves along a cavity length direction and providing gain/loss modulation in the active region, (d) an excitation means for pumping the active region; wherein shape of the grooves, order and duty cycle of the grating are defined so as to provide a predetermined ratio of gain/loss to index coupling coefficients . Thus, a complex coupled semiconductor laser having a predetermined ratio of gain/loss to index coupling is provided. Beneficially, the laser has an active region comprising a multiple quantum well structure with the grating being formed by etching grooved directly through the active region. The DFB laser is either a gain coupled laser comprising a gain coupled grating, or a loss coupled laser comprising a loss coupled grating, the grooves of the grating being etched along a cavity length direction. Conveniently, the grating is a second order uniform grating or a chirped grating. Alternatively, it may be a third order grating or higher order grating. Conveniently, the grating has V-shaped grooves. Alternatively, the grooves may have a shape of a rectangular and trapezoidal cross-section. The theory states that for gratings of a particular order of diffraction, duty cycle and shape of grooves, index coupling may be eliminated under some conditions. In practice it means that it may be substantially reduced in comparison with first order grating structures, thus allowing to implement almost a purely gain/loss coupled DFB semiconductor lasers. For example, for this to occur for the second order Bragg grating, the grooves of the grating have to be of a rectangular cross-section with a duty cycle equal to 0.5, i.e. sections of the grating period having higher and lower refractive indexes have to be of equal lengths along a cavity length direction. Other known types of high order gratings, including second and third order gratings, may also be used in DFB lasers described above to reduce index coupling in such lasers. Excitation means for pumping the laser comprises electrical contacts for current injection into the active region of the laser. Alternatively, the semiconductor laser may be capable of coupling to an external optical pumping source to create a population inversion. Beneficially, electrodes are formed on top of the current confining region of the laser and at the bottom of the substrate. The current confining region is preferably a ridge waveguide or buried hetero-structure . By corresponding selections of semiconductor materials and type of dopings for substrate and current confining ridge, the laser is adjusted to generate light within certain wavelength ranges. Preferably, they are 1.3-1.56 μm and 0.8-0.9 μm for InP and GaAs alloys utilized as substrate materials respectively. Beneficially, the laser further comprises means for tuning a laser wavelength around a lasing mode. According to another aspect of the invention there is provided a method of producing a complex coupled DFB semiconductor laser having a predetermined ratio of gain to index coupling, the method comprising the step of forming a high order complex coupled grating on the substrate, the grating having grooves along a cavity length direction and providing gain/loss modulation in the active region, wherein shape of the grooves, order and duty cycle of the grating are defined so as to provide a predetermined ratio of gain/loss to index coupling coefficients. Beneficially the grating is formed by etching grooves through the active region, and the predetermined ratio of gain/loss to index coupling is further defined by depth of etching. According to yet another aspect of the invention there is provided a method of obtaining a purely gain/loss coupled semiconductor DFB laser, comprising the step of forming a high order complex coupled grating by etching grooves through an active region of the laser, wherein an order and duty cycle of the grating, shape of the grooves and depth of etching are defined so as to substantially eliminate index coupling within the laser. Thus, a complex coupled DFB semiconductor laser with a predetermined ratio of gain/loss to index coupling coefficients, and methods of producing such lasers are provided. The advantages of the embodiments of the present invention are as follows. By providing a predetermined ratio of gain to index coupling, it is possible to produce DFB lasers with statistically predetermined characteristics, namely efficiency, yield, single mode operation and side mode suppression ratio (SMSR) . If required strong index coupling, e.g. associated with deep etching through MQWs of the active region, may be substantially reduced or eliminated. As a result, a purely gain coupled DFB laser may be obtained. It is also easier to manufacture high order lasers because of the larger period of the gratings. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in greater detail with reference to the attached drawings, in which: Figure 1 is a schematic cross-sectional view of the DFB laser according to a first embodiment of the invention; Figure 2 is a detailed cross-sectional view of the DFB laser according to the first embodiment of the invention; Figure 3 is a schematic cross-sectional view of the DFB laser according to a second embodiment of the invention; Figure 4 is a schematic cross-sectional view of the DFB laser according to a third embodiment of the invention; Figure 5 illustrates a dependency of output laser efficiency as a function of index coupling coefficient in a DFB laser; Figure 6 illustrates a dependency of index coupling coefficient as a function of a grating duty cycle for first, second and third order gratings in a DFB laser; Figure 7 illustrates a dependency of index coupling coefficient versus grating duty cycle for a second order DFB laser; Figure 8 illustrates a dependency of gain coupling coefficient versus duty cycle for a second order DFB laser; Figure 9 illustrates a dependency of index coupling coefficient versus grating duty cycle for a third order DFB laser; and Figure 10 illustrates a dependency of gain coupling coefficient versus duty cycle for a third order DFB laser. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS STRUCTURE A schematic cross-section through a distributed feedback single mode complex coupled laser 10 according to a first embodiment of the present invention is shown in Figure 1. The device 10 comprises a substrate 12 providing a first confinement region, and active region 14 comprising a multiple quantum well structure 16 and a second order grating structure 18 defined therein, and an overlying confinement region 20. Means for excitation of the laser device are formed thereon, and include a contact to the substrate, a current confining ridge 22, and contact electrodes 24 and 26 being defined on the ridge and at the bottom of the substrate 12 respectively for current injection into the device structure. The structure of the laser has a high order grating 18 etched directly through the MQWs of the active region 14 and having predetermined parameters, which provides for a predetermined ratio of gain coupling to index coupling coefficients as will be described in detail below. The structure is shown in more detail in Figure 2, which shows an oblique cross-sectional view through the laser structure 10. A DFB semiconductor laser device 10 is fabricated from Group III-V semiconductor materials, and comprises a heavily N-doped InP substrate 12, on which an N-doped InP buffer layer 34 of 1.5 μm thickness is defined. The first separate confinement region 35, consisting of four confinement layers 36, 38, 40 and 42 of N-doped InGaAsP with energy band gaps corresponding to wavelengths of 1.0 μm, 1.1 μm, 1.15 μm and 1.20 μm respectively, is provided over the buffer layer 34. The thickness of each confinement layer is 20 nm, and the confinement layer 36 corresponding to the 1.0 μm wavelength is adjacent to the buffer layer 34. The active region 14 overlies the confinement region 35 and comprises a multiple quantum well (MQW) structure 16 which includes four to eight 1% compressively strained N-doped or undoped InGaAsP quantum wells 44, each being 5 nm thick, separated by several N-doped or undoped InGaAsP unstrained barriers 46 with a band gap corresponding to a wavelength of 1.20 μm, each barrier being 10 nm thick. The alloy composition and layer thickness of the MQW structure 16 is tailored to have specific band gap energies to provide for lasing at a required wavelength. Increasing the number of quantum wells provides higher gain per unit length of the laser cavity. The band gap of the quantum well structure described above provides a lasing wavelength of the device at about 1.55 μm. A second separate confinement region 47, consisting of two P-doped InGaAsP confinement layers 48 and 50, having energy band gaps corresponding to 1.1 μm and 1.20 μm wavelengths respectively, is grown on top of the MQW active region 14, each layer being 20 nm thick. A grating structure 18 is defined by periodically etched grooves directly through the active region 14 and along a cavity length direction. The grooves of the grating structure 18 have a trapezoidal cross section with about 50% of total number of quantum wells of the active region 14 being etched through. In other experiments the fraction of the QWs etched through varied from about 30% to 80% depending on the requirements of laser performance. The pitch of the groove, (i.e. the period of the grating), is selected so as to define a second order grating for the lasing wavelength which is in the range from -200 nm to -250 nm. For high order gratings, the period is a multiple of the period of the first order grating, where the integer number of the multiple equals the order of the grating. Accordingly, for the second order grating, the period of the grating is equal to (200-250) nm x 2 = (400- 500) nm. A duty cycle of a grating, defined as a ratio of the length of the section 15 of the period having a higher average refractive index to the total length of the grating period 17, is equal to 0.25. Thus, the value of the duty cycle is always between 0 and 1. A P-doped InP layer 52, having a band gap wavelength smaller than the quantum well band gap wavelength, fills the grooves. A 3 nm thick etch stop layer 54 of P-doped InGaAsP, surrounded by P-doped InP buffer layer 56 at the bottom and P-doped InP buffer layer 58 at the top is formed next, the buffer layers being 100 nm and 200 nm thick correspondingly. An upper cladding layer 60 of P-type InP, followed by a highly doped P-type capping layer 62 of InGaAs for contact enhancement, having thickness 1600 nm and 200 nm correspondingly, complete the structure. Means (not shown) for controllably varying current injection and changing a temperature of the laser for tuning a laser wavelength are provided. Thus, a DFB semiconductor laser, having a high order grating 18 modulating gain in the active region 14 of the laser 10 is provided. While the laser device described above has a grating 18 formed by etching directly through the MQWs of the active region 14 of the laser 10, it is contemplated that the laser may comprise any other known type of grating having required parameters and providing gain/loss modulation in the active region. The grating may be formed either above or under the active region in the vicinity thereof and made by any of the techniques known in the industry. The laser device described above is fabricated on a N-type substrate wafer. Alternatively, a complimentary structure may be fabricated on a P-type wafer. The substrate 12 on which the laser device 10 described above is fabricated is made of InP material which results in generating a laser light within a range of 1.3-1.56 μm, corresponding to a transparency window of this material. In modifications of this embodiment, the substrate may be made of GaAs material, having a window of transparency in a shorter wavelength range of 0.8-0.9 μm, which results in generating light in this wavelength range. More precise calculation of a lasing wavelength depends also on the properties of the active region and the grating structure. While the grating structure 18 of the first embodiment has grooves having a trapezoidal cross section and a duty cycle equal to 0.25, it is also contemplated that in alternative embodiments, the grating structure described above may have any other shape of grooves, e.g. rectangular or triangular (V-shaped) , and/or a different duty cycle. A semiconductor laser 100 according to a second embodiment shown in Fig. 3 is similar to that of the first embodiment except for the order and the duty cycle of the grating. Instead of the second order grating in the first embodiment, the second embodiment includes a third order grating having a duty cycle of 1:3, i.e. the length of the section of the grating period 115 with higher refractive index is equal to 1/3 of the total grating period 117. The grooves are etched through the active region 114 in a way similar to that described above. The period of the third order grating is within a range of (600-750) nm which is 3 times that of the first order grating (or selected lasing wavelength correspondingly) . The shape of the grating grooves is of trapezoidal cross section with about 50% of total number of QWs being etched away (with fraction of etched QWs varying from about 30% to 80% in other experiments) . To facilitate comparison with Figure 1, corresponding layers of the structure shown in Fig. 3 are denoted by the same reference numerals, incremented by 100. The semiconductor laser of the embodiments described above are semiconductor diode laser structures, i.e., have contacts 24 and 26 for electrical excitation of the active region by current injection. It is also contemplated that a semiconductor laser device 200 of a third embodiment, shown in Fig. 4, may be provided with optical pumping means 230, replacing corresponding electrical contacts 24 and 26 of the first embodiment, e.g. by providing population inversion with suitable optical coupling to a another light source on the substrate. The laser 200 according to the third embodiment comprises an excitation means 230 for pumping of the active region 214 of the laser, and means for tuning a laser wavelength around the left and the right Bragg modes correspondingly (not shown) . The rest of the structure of the laser device 200 is similar to that of the first embodiment described above. It comprises a substrate 212 providing a first separate confinement region, an active region 214 comprising a MQW structure 216 and a high order grating structure 218 defined therein, an overlying confinement region 220 and a confining ridge 222. The grating structure 218 may comprise a second order grating as described in the first embodiment or a third order grating structure as described in the second embodiment. Alternatively, it may also comprise any other known type of high order grating having suitable parameters, the grating being uniform or chirped. In the embodiments described above the semiconductor laser is a ridge waveguide laser device. It is also contemplated that a semiconductor laser device in alternative embodiments may be a buried heterostructure device. A buried heterostructure laser may also comprise the high order grating mentioned above, the grating being uniform or chirped. OPERATION Principles of operation, demonstrated on a complex coupled DFB laser 10 of the first embodiment, are as follows. For a complex coupled DFB laser, the efficiency of the laser decreases with the increase of index coupling. This is mainly due to the effect of spatial hole burning presented inside the DFB laser cavity. The higher the index coupling, the larger the spatial hole burning, and thus, the lower the external efficiency of the laser. Curve 70 in Fig. 5 shows the output facet efficiency of a DFB laser with a first order grating as a function of index coupling coefficient. It is assumed that a product of gain coupling by cavity length equals KgL = 0.6, the laser has 2% reflection from both front and back facets, and zero facet phases. It follows from Fig. 5 that the efficiency of an AR/AR coated DFB laser drops to less that 0.2 when the index coupling reaches an amplitude of 4-6 (which is common for a first order grating laser with several QWs etched away) . Correspondingly, the performance of the laser is substantially reduced. For a rectangular grating, it has been shown that the m-th order index coupling coefficient κ m is determined by the first order index coupling coefficient Kη. and a multiplication factor which is related to the Bragg scattering order m, see formula (1) below and a publication by W. Streifer, D. R. Scifres and R. Burnham in "Coupling coefficient for distributed feedback single- and double-heterostructure diode lasers," IEEE J. of Quantum Electronics, vol. QE-11, No. 11, pp. 867-873, 1975. In (1), W stands for a section of the period of the grating having a higher average refractive index as designated by numerals 15, 115 and 215 in Figures 1, 3 and 4 correspondingly, and Λ stands for the grating period as designated by numerals 17, 117 and 217 in the same figures, thus W/Λ defining a duty cycle of the grating: κ m = Kι * sin ( m π W / 2 Λ) / m (1) For a grating of an arbitrary shape of grooves and duty cycle calculations can be made using various computational techniques known in the industry. It is noted that the term "complex coupling coefficient" (including index and gain coupling coefficients as its real and imaginary parts) usually relates to a first order grating laser. It is defined in the absence of radiation within the laser cavity and depends on the parameters of the grating and material gain/loss modulation. The coupling coefficient defined in this way describes coupling between forward and backward waves inside the laser cavity which is caused only by index and gain modulation present in the laser structure. However, a radiation mode present in high order DFB lasers causes an additional coupling between the waves, which is normally a complex number. Therefore a complex coupling coefficient for high order grating lasers should be defined by summation of two types of coupling coefficients, namely, the one caused by index and gain modulation, and the other caused by coupling through the radiation mode, both of them being complex numbers. The additional coupling due to radiation mode results in corresponding inputs into index and gain coupling coefficients, whose effect is similar to index and gain/loss coupling in the first order grating. For consistency in terminology throughout the specification, the terms "complex coupling coefficient", "index coupling coefficient" and "gain/loss coupling coefficient" when used with regard to higher order gratings will take into account additional coupling caused by radiation mode. The calculated results of index coupling coefficient as a function of a grating duty cycle for a first, second and third order grating in a DFB laser (curves 72, 74 and 76 correspondingly) are shown in Fig 6. The shape of grating grooves is assumed to be of a rectangular cross section. It is seen from Fig.6 that index coupling can be significantly reduced by introducing high order grating structures. Within a practical range of duty cycle (0.1 < W/Λ < 0.9), index coupling is much higher for the first order grating (curve 72) than for the second and third order gratings (curves 74 and 76 respectively), and it never becomes zero. Usage of second and third order gratings reduces index coupling by several times (curves 74 and 76) , and under particular conditions may theoretically eliminate it completely (see points A, B and C in Fig. 6 where index coupling reaches zero) . For practical applications, taking into consideration deviations of the grating shape from a rectangular cross section and other limiting factors, it means that index coupling may be substantially reduced. For example, as follows from (1) and Fig. 6 (curve 74), for a second order grating having a 0.5 duty cycle, index coupling coefficient is zero (point B) for an even order Bragg scattering process and is inversely proportional to Bragg order m for an odd order Bragg scattering process. A well defined rectangular grating structure may be conveniently obtained by dry etching technique widely known in the industry. As a result, an index coupling can be efficiently reduced or eliminated, and a purely gain coupled DFB laser can be easily manufactured. Figure 7 shows a calculated dependency of index coupling coefficient versus grating duty cycle for a second order grating having a rectangular cross section of grooves. Curves 80, 82, 84 and 86 correspond to material gain in the active region equal 0, 400, 800 and 1200 l/cm respectively. It is seen that the higher the gain in the active region, the lower the index coupling in the laser. This is due to the reduced refractive index contrast between two sections of the grating period caused by carrier injection into active QWs having high material gain. It also follows from Fig. 7 that in high order DFB lasers index coupling may be reduced more effectively by varying a duty cycle of the grating than a material gain in the active region. Fig. 8 shows a dependency of gain coupling coefficient versus duty cycle for a DFB laser having a second order grating with a rectangular cross section of grooves. Curves 90, 92, 94 and 96 correspond to material gain in the active region equal to 0, 400, 800 and 1200 l/cm respectively. It is seen that a significant gain coupling coefficient is present, which leads to an increased single mode yield and improved SMSR similar to the gain coupling or loss coupling effect in first order DFB lasers. It also follows from Figures 7 and 8 that the ratio of gain to index coupling coefficients depends on both duty cycle and material gain modulation along the cavity. Therefore it is possible to reduce index coupling coefficient and to obtain a high gain to index coupling ratio, which is a key parameter for DFB lasers. The higher the gain to index coupling ratio, the higher the single mode yield and side mode suppression ratio (SMSR) in the laser. Figure 9 illustrates a dependency of index coupling coefficient versus grating duty cycle for a third order DFB laser. The grating has a rectangular cross- section of grooves, and curves 150, 152, 154 and 156 correspond to material gain in the active region equal to 0, 400, 800 and 1200 l/cm respectively. The results of Fig. 9 for third order grating DFB lasers are similar to that of Fig. 7 for the second order grating DFB lasers, i.e. the higher gain in the active region the lower index coupling of the laser. In contrast to Fig. 7, there are two values of a duty cycle for third order gratings at which the coupling coefficient changes its sign (point D and F in Fig. 9) . This is mainly due to the constructive and destructive interference of partially reflected waves coming from two adjacent discontinuity points taken along the grating period. The higher the order of the grating, the more frequently the coupling coefficient changes its sign with an increase of a duty cycle. The period of a third order grating is larger, and therefore the ratio of index coupling to gain coupling coefficients is easier to control from a fabrication point of view. The major difference lies, however, in the magnitude of index coupling. The third order grating exhibits smaller maximum index coupling coefficient than that in second order grating DFB lasers. Figure 10 illustrates a dependency of gain coupling coefficient versus duty cycle for a DFB laser having a third order grating of a rectangular cross section. Curves 160, 162, 164 and 166 correspond to material gain in the active region equal to 0, 400, 800 and 1200 l/cm respectively. The results of Fig. 10 are similar to that of Fig. 8 for second order gratings except for the peak of index coupling coefficient being smaller for third order grating lasers. As a result a peak ratio of gain to index coupling coefficients is higher, which is beneficial for obtaining a higher single mode yield and SMSR. Thus, by using a high order grating in a DFB laser, index coupling may be substantially reduced or eliminated for a grating of a particular order of diffraction, duty cycle and shape of grooves. As a result a predetermined index coupling to gain coupling ratio may be achieved, and therefore the laser may be optimized for high power, high temperature performance and single mode operation. FABRICATION Fabrication of the DFB semiconductor laser 10 according to the first embodiment shown in Fig. 1, proceeds in four stages as follows : 1. first epitaxial growth of substrate and multiple quantum well structure; 2. patterning of the grating structure; 3. second epitaxial growth of the overlying layers ; and 4. completion of the laser fabrication (e.g. ridge formation, contacts) . The prepared substrate 12 is loaded promptly into a commercially available CVD growth chamber, and a buffer layer 14 of InP followed by the first confinement region 35, including four layers of InGaAsP, is grown. The active region 14, comprising eight 1% compressively strained P-doped InGaAsP quantum wells 44, separated by seven P-doped InGaAsP unstrained barriers 46, is grown next . The wafer is then removed from the growth chamber and processed so as to form photolithographically a second grating structure 18 by periodically etched grooves through the active region 14. First, a dielectric such a Si0 (not shown) is grown on the surface of the wafer, and the groove pattern is created in the dielectric layer. The grooves are etched using reactive ion etching or wet chemical etching process. The residual dielectric is then removed. Using known crystal growth techniques, for example, a metal oxide chemical vapor deposition, an InP layer 52 is grown in the grooves. Etch stop layer 54 of InGaAsP grown between two buffer layers 56 and 58 of InP, followed by cladding layer 60 of InP and capping layer 62 of InGaAs complete the structure. Laser fabricating is then completed using a standard process. For example, to form a rectangular ridge waveguide 22 perpendicular to the grooves of the grating structure 18, a ridge mask is provided on the substrate, and the ridge is formed by etching through the capping layer 62 and top cladding layer 60, the ridges being 2 μm nominal width. The top electrode 24 is defined by the mask used in the metalization step and created in the lift-off process. The front facet of the composite complex coupled DFB laser is AR-coated (anti-reflection coated) . The back facet may be AR-coated or as-cleaved for DFB laser with a large stop band width, or HR-coated (high-reflection coated) for efficient DFB laser with a small stop band width. Alternatively, after the second regrowth, when a current confining region is formed on the active region, a buried heterostructure may also be grown. For the embodiments of the laser device described above, comprising different order gratings, a phase mask generated by Electron Beam (EB) lithography or the direct EB writing on wafer may be used as an alternative to a wet etching process. The grating structure may also be formed in the layer adjacent to the active region by a holographic exposure technique. Thus, beneficially the structure disclosed herein is made by two step metal organic chemical vapor deposition growth and ridge waveguide processing step, and provides a significantly reduced index coupling and predetermined gain to index coupling ratio while maintaining the excellent dynamic single mode operation inherent for the complex coupled DFB laser. Thus, it will be appreciated that, while specific embodiments of the invention are described in detail above, numerous variations and modifications of these embodiments fall within the scope of the invention as defined in the following claims.

Claims: WHAT IS CLAIMED IS: 1. A complex coupled distributed feedback semiconductor laser, comprising: (a) a substrate; (b) an active region formed thereon; (c) a high order complex coupled grating formed on the substrate, the grating having grooves along a cavity length direction and providing gain/loss modulation in the active region, and (d) an excitation means for pumping the active region; wherein shape of the grooves, order and duty cycle of the grating are defined so as to provide a predetermined ratio of gain/loss to index coupling coefficients . 2. A laser of claim 1 wherein the active region comprises a multiple quantum well structure. 3. A laser of claim 2 having the grating formed by etching grooves through the active region, wherein the predetermined ratio of gain/loss to index coupling is further defined by depth of etching. 4. A laser of claim 1 wherein the laser is a gain coupled laser comprising a gain coupled grating. 5. A laser of claim 1 wherein the laser is a loss coupled laser comprising a loss coupled grating. 6. A laser of claim 1 wherein the grating is a second order grating. 7. A laser of claim 1 wherein the grating is a third order grating. 8. A laser of claim 1 wherein the grating is one of the uniform grating and chirped grating. 9. A laser of claim 1 wherein the grooves are V- shaped. 10. A laser of claim 1 wherein the grooves have one of a rectangular and trapezoidal cross-section shape. 11. A laser of claim 1 wherein the means for pumping the active region comprise electrical contacts for current injection into the active region. 12. A laser of claim 1 wherein the current confining region is formed on the active region. 13. A laser of claim 12 wherein the current confining region is a ridge waveguide. 14. A laser of claim 12 wherein the current confining region is a buried hetero-structure . 15. A laser of claim 1 wherein the means for pumping the active region comprise an external optical pumping source. 16. A laser of claim 13 wherein the substrate is P-type and the ridge is N-type. 17. A laser of claim 13 wherein the substrate is N-type and the ridge is P-type. 18. A laser of claim 13 wherein the substrate is InP. 19. A laser of claim 18 capable of generating light in the wavelength range of 1.3-1.56 micrometers. 20. A laser of claim 13 wherein the substrate is GaAs . 21. A laser of claim 20 capable of generating light in the wavelength range of 0.8-0.9 micrometers. 22. A laser of claim 1 further comprising means for tuning a laser wavelength around a lasing mode. 23. A laser of claim 6 wherein the second order grating comprises a first section of higher index of refraction and a second section of lower index of refraction, the sections having a rectangular cross- section and equal lengths along the cavity length direction. 24. A method of producing a complex coupled DFB semiconductor laser having a predetermined ratio of gain to index coupling, the method comprising the step of forming a high order complex coupled grating on the substrate, the grating having grooves along a cavity length direction and providing gain/loss modulation in the active region, wherein shape of the grooves, order and duty cycle of the grating are defined so as to provide a predetermined ratio of gain/loss to index coupling coefficients . 25. A method of claim 24 wherein the grating is formed by etching grooves through the active region and the predetermined ratio of gain/loss to index coupling is further defined by depth of etching. 26. A method of obtaining a purely gain/loss coupled semiconductor DFB laser, comprising the step of forming a high order complex coupled grating by etching grooves through an active region of the laser, wherein an order and duty cycle of the grating, shape of the grooves and depth of etching are defined so as to substantially eliminate index coupling within the laser.

Inventors: HONG JIN, MAKINO TOSHIHIKO, HONG, JIN, MAKINO, TOSHIHIKO
Applicants: NORTEL NETWORKS CORP, NORTEL NETWORKS CORPORATION
Application year: 1999
Main classification: H01S5
IPC codes: H01S5