Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
  • H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
  • H01S5/185—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only horizontal cavities, e.g. horizontal cavity surface-emitting lasers [HCSEL]
  • H01S5/187—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only horizontal cavities, e.g. horizontal cavity surface-emitting lasers [HCSEL] using Bragg reflection
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
  • H01S5/065—Mode locking; Mode suppression; Mode selection ; Self pulsating
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
  • H01S5/12—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region the resonator having a periodic structure, e.g. in distributed feedback [DFB] lasers
  • H01S5/1228—DFB lasers with a complex coupled grating, e.g. gain or loss coupling
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/40—Arrangement of two or more semiconductor lasers, not provided for in groups H01S5/02 - H01S5/30
  • H01S5/42—Arrays of surface emitting lasers

Definitions

• TITLE Method and Apparatus for Suppression of Spatial-Hole Burning in
• This invention relates generally to the field of telecommunications and in particular to optical signal based telecommunication systems. Most particularly, this invention relates to lasers, such as semiconductor diode lasers, for generating pump and carrier signals for such optical telecommunication systems.
• lasers such as semiconductor diode lasers
• a number of different laser sources are currently available as optical signal sources for telecommunications. These include various forms of fixed, switchable or tunable wavelength lasers, such as Fabry-Perot, Distributed Bragg Reflector (DBR), Vertical Cavity Surface Emitting Lasers (VCSEL) and Distributed Feedback (DFB) designs.
• DBR Distributed Bragg Reflector
• VCSEL Vertical Cavity Surface Emitting Lasers
• DFB Distributed Feedback
• Currently the most common form of signal carrier source used in telecommunication applications are edge emitting index coupled DFB laser sources, which have good performance in terms of modulation speed, output power, stability, noise and side mode suppression ratio (SMSR). In this sense SMSR refers to the property of DFB lasers to have two low threshold longitudinal modes having different wavelengths at which lasing can occur, of which one is typically desired and the other is not.
• SMSR noise and side mode suppression ratio
• SMSR comprises a measure of the degree to which the undesired mode is suppressed, thus causing more power to be diverted into the preferred mode, while also having the effect of reducing cross-talk from the undesired mode emitting power at the wavelength of another DWDM channel.
• communication wavelengths can be readily produced.
• edge emitting lasers as signal sources. The major issue is the bulk and cost of packaging the laser due to the requirement in most cases of including an optical isolator and expensive aspheric lenses to couple the light into a single mode fiber.
• edge emitting lasers can only be properly tested once the wafer has been cleaved into bars and the edges anti-reflectibn coated.
• a surface-emitting DFB laser suitable for use as a communications signal source consists of an active gain layer sandwiched between optical confinement layers having a lateral optical confinement structure such that there is a single transverse mode.
• the use of a second-order index grating in edge-emitting DFB lasers was proposed to lift the degeneracy problem of the spectrum of a symmetric first order DFB laser.
• the two counter-propagating modes can interfere constructively and destructively to produce two primary potential lasing modes at the edges of the stop band.
• the stop band is defined as the region between these two primary modes where no other lasing modes can occur.
• these two modes In a first order structure, these two modes have equal modal gain and are therefore equally likely to lase (assuming the laser is symmetric at the ends of the cavity).
• For a second order structure these two modes experience different radiation loss and therefore there is now a net gain discrimination mechanism at play.
• the mode with destructive interference of optical amplitudes within the cavity has less radiation loss and hence a lower threshold gain in comparison with the second mode.
• This approach for avoiding the degeneracy problem in symmetric first- order DFB lasers is preferable to the more usual method, which is done by breaking the symmetry of the laser by anti-reflection (AR) coating one facet and high-reflection (HR) coating the other.
• AR anti-reflection
• HR high-reflection
• the radiation loss mode selection mechanism in second-order DFB lasers described above favors a lasing mode having a poor surface-emitting near-field profile for coupling into single mode fibers.
• the favored mode which by definition has less radiation loss, also emits correspondingly less power from the surface. Therefore, simply using a second-order index coupled grating DFB laser is not sufficient to make a surface-emitting laser suitable for optical communications applications.
• the use of a quarter- wave phase shift region in a second order grating was proposed by Kinoshita [J.-l.
• Spatial hole burning is a non-linear effect that results from a highly non-uniform optical field within the laser cavity.
• areas where the optical field is most intense become saturated more quickly and therefore carrier concentrations in these areas become depleted relative to other areas in the laser cavity.
• This local carrier depletion leads to a local refractive index change.
• the local refractive index change leads to nonlinear effects that degrade the performance of the laser.
• the most obvious symptom is a decrease in the SMSR as secondary modes are enhanced by the effect relative to the main mode. In more extreme operating conditions, mode hopping can occur.
• Spatial hole burning comes into play differently for edge emitting and surface emitting lasers employing second-order gratings.
• the coupling coefficient is kept relatively low by design, otherwise the efficiency of emission from the edge is low.
• the low coupling coefficient in turn helps alleviate hole burning because the optical field intensity remains fairly uniform throughout the cavity.
• a surface emitting laser what is desired is a concentrated single-lobed optical field to achieve optimal coupling to a single mode fiber. While achievable through different designs, the simplest is to incorporate a quarter-wave phase shift. Optimal theoretical performance also calls for a high coupling coefficient to improve the surface emission efficiency and more tightly concentrate the field over the phase shift.
• An in-phase complex grating is one in which the real and imaginary terms in the coupling coefficient are the same sign and is normally embodied as a gain-coupled grating. It follows that an anti-phase complex grating is one in which the signs are opposite, the most common example being a loss-coupled grating.
• in-phase first order complex gratings can suppress spatial hole burning while anti-phase complex gratings intensify hole burning and deteriorate the laser performance.
• a surface emitting laser structure which can provide useful amounts of output power without the detrimental spatial hole burning problems or complicated and partial solutions associated with the prior art phase shifted designs. What is also desired is a structure which has low chirp and is insensitive to back-reflection.
• the present invention relates to the theory and physics of suppression of the spatial hole burning effect in a first order quarter-wave phase shifted DFB laser. With a proper understanding of the physics, it is shown that a gain- coupled, second order, quarter-wave phase shifted grating with appropriate duty cycle constitutes a surface emitting laser having excellent optical mode and spectral properties while at the same time being virtually impervious to spatial hole burning.
• a laser design according to the present invention eliminates the necessity for the myriad ways, generally complicated, designed to alleviate hole burning.
• Experimental results from gain-coupled, phase shifted, second order grating lasers according to the present invention are also provided which demonstrate the performance of the present invention.
• An aspect of the present invention is to show that without using complicated multi-electrode injection techniques or difficult phase-shifting methods, it is possible to greatly reduce the occurrence of hole-burning-induced multimode operation of a second-order DFB laser having a quarter-wave phase shift region through judicious choice of the duty cycle.
• This possibility arises from the fact that a second-order grating is a complex coupled grating by nature and with a complex coupled grating it is possible to strongly reduce spatial hole burning effects.
• duty cycle of the grating defined as the ratio ofthe grating tooth width to the grating period, has not been considered as an important design parameter. According to the present invention this is because until now there has been a failure to fully recognize and understand the design factors which directly affect spatial hole burning. According to the present invention, within a particular range of duty cycles, the detrimental effect of spatial hole burning -which limits the operating current of the laser and therefore the output power - is naturally mitigated by making appropriate design choices. Further, according to the present invention this effect can be additively combined with a gain coupled grating design such that the laser is virtually impervious to hole burning.
• a laser design according to the present invention has the advantages of a quarter-wave phase shift (namely good single mode operation and good surface-emitting optical mode shape for fiber coupling) without incurring the typical detrimental effects due to spatial hole burning, such as mode-hopping.
• the design has inherently low chirp and is highly insensitive to back-reflected light.
• a second-order grating is inherently a complex grating, it is possible to reduce or avoid spatial hole burning by judicious choice of the duty cycle of the grating. Therefore even an index-coupled design can show improved resistance to spatial hole burning if the duty cycle of a second order grating is chosen properly.
• An object ofthe present invention is to provide a surface emitting laser structure which is both suitable for telecommunications applications and which avoids or minimizes spatial hole burning problems associated with the prior art designs.
• An object ofthe present invention is to provide a low-cost optical signal source that is capable of generating signals suitable for use in the optical broadband telecommunications signal range. Most preferably such a signal source would be in the form of a surface emitting semiconductor laser which can be fabricated using conventional semiconductor manufacturing techniques and yet which would have higher yields than current techniques.
• it is an object ofthe present invention to produce signal sources at a lower cost than as compared to the prior art techniques referred to above.
• Such a signal source would have enough power, wavelength stability and precision for broadband communications applications without encountering impractical limits due to spatial hole burning. More particularly what is needed is a laser structure where the mode shape is optimised to permit fibre coupling and yet which can be made using conventional lithographic and materials techniques in the semiconductor art.
• a surface emitting laser which includes a means to ameliorate spatial hole burning to permit practical values of output power to arise from the laser.
• Such a device would display minimal chirp to permit signal transportation and manipulation without unacceptable pulse broadening.
• the device would exhibit an insensitivity to back-reflected light, allowing the device to be operated as a communications signal source without the need for the inclusion of an optical isolator to maintain stable performance.
• a semiconductor laser signal source having a signal output that is easily and efficiently coupled to a single mode optical fibre.
• Such a device would also preferably be fabricated as an array on a single wafer-based structure and may be integrally and simultaneously formed or fabricated with adjacent structures such as signal absorbing adjoining regions and photodetector devices.
• a further feature of the present invention relates to efficiencies in manufacturing.
• a forty source array fabricated at a yield of 98% per source will produce an array fabrication yield of only 45%.
• improved fabrication yields are important to cost efficient array fabrication.
• each laser source of the array can be fabricated to operate at the same or to different wavelengths and most preferably to wavelengths within the telecommunications signal bands. Further, such a device could have a built in detector that, in conjunction with an external feedback circuit, could be used for signal monitoring and maintenance.
• Figure 1 is a side view of one embodiment of a surface emitting semiconductor laser according to the present invention having a quarter-wave phase shifted second order grating formed in a gain medium;
• Figure 2 is an end view of the embodiment of Figure 1 ;
• Figure 3 is a plot mode spectra from various lasing structures;
• Figure 4a is a plot of mode spectra for duty cycle of greater than 50%
• Figure 4b is a plot of mode spectra for duty cycle of less than 50%
• Figure 6 is a plot of a mode spectrum for a gain-coupled grating where
• Figure 8 is a plot of a mode spectrum for an index-coupled grating
• Figure 9 is a plot of a mode spectrum for a loss-coupled grating
• Figure 10 is a plot of a mode spectrum for a gain-coupled grating
• Figure 13 is a plot of power versus injection current for a laser according to the present invention.
• Figure 14 is a plot of a spectrum for a laser according to the present invention for a current just above threshold current.
• Figure 15 is a plot of a spectrum for a laser according to the present invention for a current far above threshold current.
• Figure 1 is a side view of one embodiment of a surface emitting semiconductor laser structure 10 according to the present invention, while Figure 2 is an end view of the same structure.
• the laser structure 10 is comprised of a number of layers built up one upon the other using, for example, standard semiconductor fabrication techniques. It will be appreciated that the use of such known semiconductor fabrication techniques for the present invention means that the present invention may be fabricated efficiently in large numbers without any new manufacturing techniques being required.
• a p- region of a semiconductor is a region doped with electron acceptors in which holes (vacancies in the valence band) are the dominant current carriers.
• An n- region is a region of a semiconductor doped so that it has an excess of electrons as current carriers.
• An output signal means any optical signal which is produced by the semiconductor laser of the present invention.
• the mode volume means the volume in which the bulk ofthe optical mode exists, namely, where there is significant light (signal) intensity. For example, the mode volume could be taken as the boundary enclosing 80% ofthe optical mode energy.
• a distributed diffraction grating is one in which the grating is associated with the active gain length or absorbing length of the lasing cavity so that feedback from the grating causes interference effects that allow oscillation or lasing only at certain wavelengths, which the interference reinforces.
• the diffraction grating ofthe present invention is comprised of grating or grid elements, which create alternating optical properties, most preferably alternating gain and/or refractive index effects.
• Two adjacent grating elements define a grating period.
• the alternating gain effects are such that a difference in gain arises in respect of the adjacent grating elements with one being a relatively high gain effect and the next one being a relatively low gain effect.
• the present invention comprehends that the relatively low gain effect may be a small but positive gain value or may be no actual gain.
• the present invention comprehends any absolute values of gain effect in respect of the grating elements, provided the relative difference in gain effect and index is enough between the adjacent grating elements to set up the interference effects of lasing at only certain wavelengths.
• the present invention comprehends any form of grating that can establish the alternating gain effects described above, including gain coupled gratings in the active region.
• the overall effect of a diffraction grating according to the present invention may be defined as being to limit laser oscillation to one of two longitudinal modes which may be referred to as a single-mode output signal.
• various techniques are employed to further design the laser such that the mode profile is capable of being effectively coupled to a fibre.
• the two outside layers 12 and 14 of the laser structure 10 are electrodes.
• the purpose ofthe electrodes is to be able to inject current into the laser structure 10.
• electrode 12 includes an opening 16.
• the opening 16 permits the optical output signal to pass outward
• the present invention comprehends the use of a continuous electrode, providing the same is made transparent, at least in part, so as to permit the signal generated to pass out of the laser structure 10.
• Simple metal electrodes, having an opening 16 have been found to provide reasonable results and are preferred due to ease of fabrication and low cost.
• the window opening for the light output can be situated in the electrode 14 (n-side opening). In the latter case, it is also comprehended that removal of part ofthe substrate is conceivable within the spirit of this invention to allow for better access to the optical output.
• Adjacent to the electrode 14 is an n+ InP substrate, or wafer 17. Adjacent to the substrate 17 is a buffer layer 18 which is preferably comprised of n-lnP.
• the next layer is a confinement layer 20 formed from n-lnGaAsP.
• the generic composition of this and other quaternary layers is of the form ln x Ga- ⁇ . x ASyP-i-y while ternary layers have the generic composition ln ⁇ -x Ga x As.
• the next layer is an active layer 22 made up of alternating thin layers of active quantum wells and barriers, both comprised of InGaAsP or InGaAs.
• InGaAsP or InGaAs is a preferred semiconductor because these semiconductors, within certain ranges of composition, are capable of exhibiting optical gain at wavelengths in the range of 1200 nm to 1700 nm or higher, which comprehends the broadband optical spectra ofthe 1300 nm band (1270 -1320 nm), the S-band (1470 - 1530 nm), the C-band (1525nm to 1565 nm), and the L-band (1568 to 1610 nm).
• Other semiconductor materials for example GalnNAs, InGaAIAs are also comprehended by the present invention, provided the output signal generated falls within the broadband range.
• Another relevant wavelength range of telecommunications importance for which devices following this invention could be designed using appropriate material compositions is the region from 910 to 990 nm, which corresponds to the most
• the next layer above the active layer 22 is a p-lnGaAsP confinement layer 34.
• a diffraction grating 24 is formed in the active layer 22 and confinement layer 34.
• the grating 24 is comprised of alternating high gain portions 27 and low gain portions 28.
• the grating 24 is a regular grating, namely has a constant period across the grating, and is sized, shaped and positioned in the laser 10 to comprise a distributed diffraction grating as explained above.
• the period ofthe grating 24 is defined by the sum of a length 32 of one high gain portion 27 and a length 30 of the adjacent low gain portion 28.
• the low gain portion 28 exhibits low or no gain as compared to the high gain portion 27 as in this region most or all of the active structure has been removed.
• the grating 24 is a second order grating, namely, a grating having a period equal to the guide wavelength within the cavity which results in output signals in the form of surface emission.
• phase shifting Located centrally in the grating 24 is a means for phase shifting, which comprises a slightly wider high gain "tooth" 26.
• This tooth 26 is sized and shaped to deliver a phase shift of one quarter of a wavelength.
• the present invention comprehends other forms of phase shift elements as will be understood by those skilled in the art. What is needed is to provide enough of a phase shift to the grating to alter the near field intensity profile to change the dominant mode from a dual peaked configuration to a single peaked configuration where the peak is generally located over the phase shift.
• Such a mode profile can be more efficiently coupled to a single-mode fibre than the dual lobed profile.
• the mode profile is altered to improve coupling efficiency, the amount of the phase shift, and the manner of affecting the phase shift can be varied without departing from the spirit of the present invention.
• phase shifts may be employed yielding an overall quarter wave shift, e.g. two ⁇ /8, or two 3 ⁇ /8 or other combinations are comprehended.
• an overall quarter wave shift e.g. two ⁇ /8, or two 3 ⁇ /8 or other combinations are comprehended.
• a continuously chirped grating or a modulated pitch grating are also comprehended although these are more difficult to fabricate.
• the next layer above the active layer 22 and confinement layer 34 is a layer of InP to bury and in-fill the grating 35.
• a layer of InP to bury and in-fill the grating 35.
• a p-lnP buffer region 36 Located above the grating burying layer 35 is a p-lnP buffer region 36.
• a p-lnP cladding layer 40 Located above layer 36 is a p-lnP cladding layer 40, which is in turn surmounted by a p ++ -lnGaAs cap layer 42.
• a semiconductor laser built with the layers configured as described above can be tuned to produce an output signal of a predetermined wavelength as the distributed feedback from the diffraction grating written in the active layer renders the laser a single mode laser.
• the precise wavelength ofthe output signal will be a function of a number of variables, which are in turn interrelated and related to other variables of the laser structure in a complex way.
• variables affecting the output signal wavelength include the period of the grating, the index of refraction ofthe active, confinement, and cladding layers (some of which in turn typically change with temperature as well as injection current), the composition of the active regions (which affects the layer strain, gain wavelength, and index), and the thickness of the various layers that are described above.
• Another important variable is the amount of current injected into the structure
• a laser structure can be built which has an output with a predetermined and highly specific output wavelength.
• Such a laser is useful in the communications industry where signal sources for the individual channels or signal components which make up the DWDM spectrum are desired.
• the present invention comprehends various combinations of layer thickness, gain period, injection current and the like, which in combination yield an output signal having a power, wavelength and bandwidth suitable for telecommunications applications.
• merely obtaining the desired wavelength and bandwidth is not enough.
• a more difficult problem solved by the present invention is to produce the specific wavelength desired from a second order grating (and thus, as a surface emission) in such a manner that it can be controlled for efficient coupling, for example, to an optical fibre.
• the spatial characteristics of the output signal have a big effect on the coupling efficiency, with the ideal shape being a single mode, single-lobed Gaussian.
• the two primary modes include a divergent dual-lobed mode, and a single-lobed mode.
• the former is very difficult to couple to a single mode fibre as is necessary for most telecommunications applications because the fibre has a single Gaussian mode.
• duty cycle means the fraction of the length of one grating period that exhibits high gain as compared to the grating period.
• the duty cycle may be defined as the portion of the period of the grating 24 that exhibits high gain. This parameter of duty cycle is controlled in gain coupled lasers, such as illustrated in Figure 1 , by etching away portions of the active layers, with the remaining active layer portion being the duty cycle.
• the present invention comprehends that the desired lasing mode is single lobed and approximates a Gaussian profile. In this way the lasing mode can be more easily coupled to a fibre, since the profile of the power or signal intensity facilitates coupling the output signal to a fibre.
• the phase shifted second order active-coupled grating has three modes that can lase, with two modes having a higher gain threshold and less coupling efficiency to a single mode fiber in comparison with the dominant mode which is a single lobed mode and having the lowest gain threshold.
• the dominant mode has a peak at the position ofthe phase shift, which according to the present invention is placed at the midpoint of the laser structure for optimal coupling into a fibre.
• FIG 2 a side-view of the laser structure of Figure 1 is shown.
• the electrodes 12 and 14 permit the application of a voltage across the semiconductor laser structure 10 to encourage lasing as described above.
• the buried heterostructure formed by the waveguide encapsulated by blocking layers 38 serves to confine the optical mode laterally to within the region through which current is being injected.
• a dielectric layer 44 is provided between the electrode 12 and the cap layer 42 except for a small region above the buried heterostructure. This dielectric layer configuration limits current injection to positions close to the buried heterostructure in a known manner. While a buried heterostructure is shown in this embodiment it is comprehended that a similar structure could be fabricated using a ridge waveguide design to confine the carriers and optical field laterally.
• the optical field is strongly peaked in the centre of the cavity over the phase shift. Therefore, in this region the rate of stimulated emission (i.e. stimulated carrier recombination) is highest. Increasing the injection current, and hence stimulating more emission, depletes the carriers at the center of the cavity in the high field region. Due to the plasma effect (where the refractive index increases with a decrease in carrier density) the refractive index in the high field region increases, making the refractive index within the cavity highly non-uniform. This refractive index change modifies the phase of the optical field (effectively making the central quarter-wave phase shift larger) such that the mode at the shorter wavelength side of the stop band competes with the main mode at the center of the stop band.
• FIG 3 The main mode and the two dominant side modes of a quarter-wave phase-shifted laser are shown in Figure 3 by trace A.
• Figure 3 in addition to the mode spectrum of a quarter-wave phase shifted grating shown at A there is an intrinsic mode spectra of a symmetric index-coupled grating at B, a symmetric index-coupled grating with spatial hole burning effects included at C, a symmetric in-phase (gain-coupled) grating at D, and a symmetric anti-phase (loss-coupled) grating at E. Note that no phase shift region is incorporated in DFB lasers with the spectra shown in Figs 3 B-E.
• intrinsic cavity we mean a cavity obtained by removing the quarter-wave phase shift from the grating.
• the mode spectrum of the intrinsic cavity plays an important role in the corresponding quarter-wave phase shifted laser.
• the dominant mode ofthe corresponding intrinsic cavity should be on the side of the stop band such that make a balance with the mode competing with the main mode due to the spatial hole burning. In other words, the dominant mode of the corresponding intrinsic cavity should be on the longer wavelength
• anti-phase (loss- coupled) and index-coupled gratings in a quarter-wave phase shifted design intensify the spatial hole burning effect since the dominant mode of the intrinsic cavity is located at the shorter wavelength side of the stop band, thus deteriorating the corresponding quarter-wave phase shifted laser performance.
• the present invention comprehends the following results.
• the corresponding intrinsic cavity supports the mode at the longer side of the stop band. Therefore, there will be some suppression of spatial hole burning in the corresponding quarter-wave phase shifted grating.
• Figure 4 shows mode spectra as follows: For a duty cycle > 50 % for index (A), gain (B) and loss (G) coupled gratings and for a duty cycle ⁇ 50% for index (D), gain (E) and loss (F) coupled gratings.
• the laser cavity may not have sufficient gain to lase at room temperature. Even at high levels of gain or with a longer cavity, the coupling coefficient due to the gain perturbation and the coupling coefficient due to the radiation field tend to cancel each other and the grating may even become anti-phase, which is harmful as far as spatial hole burning is concerned.
• the use of a quarter-wave phase shifted grating etched into the active region (gain- coupled) and with a duty cycle larger than 50% is preferred.
• the effect of the in-phase or anti-phase grating on the spatial hole burning of a quarter-wave phase shifted laser is calculated using numerical examples.
• Another important advantage ofthe second order surface emitting DFB laser design is that because of the nature of the coupling of the radiation out of the cavity, reflections within the optical path can not result in the creation of an external cavity, which would compete with and destabilize the internal cavity. The result is a laser much more robust to back-reflections than all traditional designs, including edge-emitting DFB, external cavity, and VCSEL lasers. This feature is particularly important in telecommunications applications over intermediate and longer distances (typically over 40 km) where optical isolators are routinely employed to prevent the performance degradation associated with back-reflected light.
• the preferred material systems are InGaAsP/lnP and AllnGaAs/lnP since they are the current primary material systems for producing laser wavelengths in the range of 1.25 to 1.65 ⁇ m.
• newer material systems based on nitrides are under development and would also be suitable for telecommunications application.
• the preferred embodiment employs an appropriate multi-quantum well structure of 5 to 10 quantum wells for providing gain in the desired wavelength band.
• the DFB grating is etched preferably using a dry-etch process to produce a square-shaped grating with a duty cycle (defined as the fractional length not etched in the grating formation) of greater than 50% and less than 90% and having an optimal range of 60-67%. This produces a balance between providing a strong coupling coefficient for high feedback and field concentration along with a high radiative coupling coefficient. Note that if the duty cycle drops to 50%, the radiative coupling is high but the coupling coefficient drops to 0.
• the coupling coefficient increases to a maximum at 75% duty cycle and then decreases to 0 at 100%, while the radiative coupling monotonically decreases to 0 at 100% duty cycle.
• the optimum range is below 75% in the 64% range where the coupling is relatively strong for feedback and a localized optical mode while at the same time the radiative coupling has not decreased too strongly.
• the depth of the grating is chosen such that the normalized coupling coefficient KL is between 3 and 7, and is preferably between 4.5 and 5.5.
• the grating also performs admirably though not as efficiently if it is wet- etched, which typically produces a triangular (or possibly trapezoidal) shaped grating.
• the duty cycle here defined as the fractional length not
• the device can be constructed using either a typical ridge waveguide (RWG) structure or a buried heterojunction (BH) structure. While the former is easier to fabricate, the junction is more difficult to thermally control, making performance in an uncooled application degraded. It is also worthy of note that for a RWG structure, the surface emission is best taken from the n-side, or substrate, ofthe device since opening a sufficiently long hole over the electrode injecting current into the ridge degrades the performance. In contrast, we have demonstrated that current injection can be well maintained even with openings as long as 250 ⁇ m in a BH structure, allowing light to be taken from the p-side top surface. From an optical perspective, both cases are easily workable.
• RWG ridge waveguide
• BH buried heterojunction
• a BH structure is preferred. Further, in fabricate the BH structure, it is preferred that the current blocking structure be formed using semi-insulating material rather than a reverse-biased p-n junction. The former case allows enhanced thermal management to be employed while reducing the parasitic capacitance that leads to degradation in high-speed applications.
• the present invention comprehends a method of manufacturing where there is no need to cleave the individual elements from the wafer, nor is there any need to complete the end finishing or packaging of the laser structure before even beginning to test the laser structures for functionality.
• the electrodes 12 and 14 are formed into the structure 10 as the structure is built and still in a wafer form.
• Each of the structures 10 can be electrically isolated from adjacent structures when on wafer, by appropriate patterning and deposition of electrodes on the wafer, leaving high resistance areas in the adjoining regions between gratings as noted above. Therefore, electrical properties of each ofthe structures can be tested on wafer, before any

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