CA2006129C - Quartz lamp envelope with molybdenum foil having oxidation-resistant surface formed by ion implantation - Google Patents

Abstract

DISTRIBUTED FEEDBACK LASER FOR
FREQUENCY MODULATED COMMUNICATION SYSTEMS
Abstract To overcome the deleterious effects of the nonuniform frequency modulation response in semiconductor lasers due to current injection in direct frequency modulation applications, it has been determined that the linewidth enhancement factor a be made as large as possible. In one embodiment, distributed feedback lasers well suited for frequency modulation lightwave communication systems are designed to have an integrated feedback element such as a corrugation grating whose effective pitch is selected to cause the Bragg wavelength and, therefore, the laser operating wavelength to be longer than the wavelength at substantially the maximum gain or gain peak in the semiconductor structure without the grating. That is, the wavelength of the grating is effectively detuned toward the longer wavelength and lower energy side of the peak of the gain profile. Such detuning increases the linewidth enhancement factor in such a way that the nonuniform frequency modulation response and its effects are minimized and, in some cases, substantially eliminated.

Description

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DISTRIBUTED FEEDBACK LASER FOR
FREQUENCY MODULATED COMMUNICAIION SYSTE:MS

Technical Field This invention relates to the field of lightwave sys~ems for frequency 5 modulation in which the system employs a single frequency distributed feedback laser.
Back~round of the ~vention Future lightwave systems are expected to accommodate large numbers of transmission channels separated by small guard bands. The transmission channels 10 operating athigh data rates are planned to utilize more fully the exisdng available bandwidth of single mode optical fibers for delivery of network and other services such as entertainment television. As system planners continue to make trade-offsbetween design parameters such as coherent and non~oherent approaches, direct and heterodyne detection techniques and the like, it is increasingly apparent that 15 frequency modulation of a single frequency light source such as a distributedfeedback (DFB) laser has become an attractive approach for the lightwave transmitter design.
Frequency moduladon is often preferred over amplitude or intensity modulation at high data bit rates because chirping and current switching problems, 20 both of which arise ~rom current variations on the light source, decrease thedesirability of amplitude and intensity modulation systems. For intensity modulation, large amounts of current must be switched rapidly to the light source.
The amount of d~ive current is typically in the range of 30 - 60 mA for semiconductor lasers. As the current ~o the semiconductor laser varies, it causes a 25 small but significant amount of frequency modulation in the laser called chirp.
Chirping causes a broadening of the spectral linewidth of emitted radiation.
Obviously, such spectral spreading penalizes even the best single frequency light sources. Semiconductor lasers especially DFB lasers have been improved through better fabricadon techniques to have a lower linewidth enhancement factor and, 30 thereby, a reduced suscepdbility to chirping. Even with such improved light sources, lightwave systerns employing amplitude and intensity modulation may have substandal drawbacks when compared with frequency moduladon lightwave systems. 3~k ; :
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The appeal of frequency moduladon for lightwave communicadon systems can be related to the fact that it permits a simplified transmitter design. By directly modulating or varying the injecdon current to a semiconductor laser, it is possible to modulate the frequency of the laser. For single frequency semiconductor S lasers, the carrier density effect which shows a change of frequency with injection current, ~f/~i, is sufficiently large, generally, several hundred Mhz/rnA, to minimize residual intensity moduladon effects for the frequency excursions required by most FM systems. However, nonuniforrnity exists for the FM response of such lasers over the modulation bandwidth because of competition between temperature and carrier 10 density effects on the laser frequency.
Nonuniform FM response is viewed with respect to thermal cutoff of the single frequency laser. Below the thermal cutoff frequency, the FM response is extremely large in magnitude on the order of Ghz/mA whereas it is opposite in phase to the FM response above the thermal cutoff frequency~ Far above the thermal cutoff 15 frequency, the FM response approaches several hundred MhzlmA while gradually reversing phase with respect to that below the thermal cutoff frequency. As a result, lower frequency components of a modulated optical signal undergo severe waveforrn distortion due primarily to temperature or thermal modulation effects on the active region of the laser.
Frequency modulation based lightwave communication systems using lasers whose FM response is nonuniform suffer degradation. In an M-ary FSK
system, nonuniform FM response causes drift of a transmitted frequency representing one of M levels per symbol whenever the laser remains at that frequency for a time which is significant as compared to a thermal dme constant for 25 the laser. As thc frequency drifts, crosstalk increases resulting in degraded bit error rate perfaqmance and, uldmately, causes complete failure of the affected link for the lightwavc system.
These problems can be ameliorated to some degree by limiting the length of non-alternadng data patterns to effecdvely eliminate the low frequency30 components of the data sequence. There are other approaches commonly employed for working with the nonuniform FM response of the laser which employ a moduladon format or data encoding scheme to also avoid the low frequency modulation region. In one example, Manchester coding is employed with its concomitant penalty of increased system bandwidth requirements. Addidonally, 35 problems such as power consumpdon and device complexity preclude the use of most encoding and moduladon techniques. Acdve and passive e~ualizadon networks have been combined with DFB lasers to overcome distortion induced by the nonuniform FM response of the DFB laser. In theory, these networks compensate the nonuniform FM response of the DFB laser by using combined pre-distortion, post-distortion and feedback control methods to realize a somewhat 5 uniform FM response. Both active and passive equalization techniques generallyresult in reladvely small FM response and, therefore, increased drive current requirements. While the combination appears to have a uniform FM response, it isimportant to realize that the DFB laser itself exhibits a totaUy nonuniform FM
response.
Phase-tunable DFB lasers have also been proposed to overcome the nonuniform FM response problem. These devices are generally fabricated to include two distinct regions: a DFB region for operating as a standard DFB laser and a modulation region without a grating separately contacted for modulating the DFB
laser signal. ln this way, canier density effects are artificially controlled through 15 electrode partitioning to achieve quasi-uniform FM response and chirp suppression.
Quasi-uniform FM response for two-electrode DFB lasers is reported up to severalhundred megahertz. However, DFB regions employed in these devices exhibit unwanted nonunifolm FM response and are primarily designed to have inherently low linewidth enhancement factors for chirp suppression. Moreover, design and 20 fabrication complexiq together with operational speed limitadons caused by the multi-section structure diminish its desirabiliqy for use in future lightwave systems.
While the alternatives described above have been proposed and demonstrated for dealing with the nonuniform FM response of directly modulated lasers, in particul~r, DFB lasers, it has been noted recendy that '`[t]he potentially 25 most rewarding soludon is to construct a laser having an inherendy uniform FMresponsc." J. Qf Li~htwave Tech., Vol. 7, No. 1, pp. 11-23 (January 1989). As noted in the descripdons above, each laser element still exhibits an inherent nonuniform FM response. Upon realizing this fact, the authors of the above-cited article lament as follows, "[u]nfortunately, dhe goal of obtaining single mode operation, high output 30 power, narrow linewidth, long life, along with a uniform FM response, in a wide selection of commercial devices at various wavelengths, is still elusive."
Summary of the Invention Single mode operation and uniform FM response are achieved in a frequency modulation ~ansmitter for a lightwave system in accordance with the 35 principles of the present invention by frequency modulating a distributed feedback laser having a modal or effective index of refraction (n) which comprises a gain .,. . .. ~., .

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medium having a characteristic wavelength and a feedback structure such as a gradng coupled to the gain medium, wherein the feedback structure controls the laser to emit lightwave signals at a Bragg wavelength ~B which is greater than the characteristic wavelength. In the resuldng lightwave transmitter of frequency 5 modulated signals, the distributed feedback laser operates with an increased carrier density effect and, thereby, a higher linewidth enhancement factor than that at substantially the gain peak wavelength. As a result, the laser has a uniform FM
response while maintaining single mode operadon.
~n one embodiment of the invention, the integrated feedback structure in 10 the distributed feedback laser includes a corrugation grating wherein the grating exhibits an effective gradng period ~eff related to the Bragg wavelength as ~B = 2~eff/M for M being an integer greater than or equal to one and idendfying the order of the grating. According to the principles of the present invention, the Bragg wavelength ~B iS selected to be greater than the characteristic wavelength of 15 the gain medium.
Brief Description of the Drawin~
A more complete understanding of the invention may be obtained by reading the following descripdon of specific illustradve embodim~nts of the invendon in conjunction with the appended drawing in which:
FIG. 1 is a simplified schemadc diagram of a frequency moduladon lightwave communicadon system;
FIG. 2 is a perspecdve cross-secdonal and cutaway view of a distributed feedback semiconductor laser for use in the lightwave system of FIG. 1 in accordance with the principles of the invention;
FIG. 3 is a cross-secdonal view of an altemadve embodiment of the laser from FIG. 2 viewed through secdon line X-X; and FIG. 4 is a plot of the linewidth enhancement factor and the gain envelope as a functdon of wavelength.
Detailed DescriPtion FIG. 1 shows a simplified schemadc diagram of lightwave communicadon system 10 employing frequency moduladon at a transmitter location.
Lightwave communicadon system 10 includes a transmitter for generadng and supplying frequency modulated signal 14 to transmission medium 15, transmission medium 15 for suppordng propagadon of lightwave signals from a local locadon to a 35 remote location, and receiver 17 for obtaining light vave signal 16 from transmission medium 15. Remote is intended to mean any location away from the transrnitter il24 either in a microscopic sense such as being co-located on the same semiconductorchip or in a macroscopic sense such as being geographically separated.
The transrnitter comprises modulator 11 connected via path 13 to distributed feedback ~DFB) laser 12. Modulator 1 l provides frequency modulationS of DFB laser 12 so that lightwave signal 14 is generated as a frequency modulated signal. As contemplated, modulator 11 may be electricaUy connecud to DFB
laser 12 for direct modulation by varying tne current applied to the laser.
Alternatively, modulator 11 may be opdcally connected to DFB laser 12 as an in-line element for frequency modulating lightwave signals generated by DFB laser 12.
Frequency moduladon is understood to include all forms of frequency modulation whet'ner analog or digital. Hence, use of specific terms such as FM
(frequency modulation) or FSK (frequency-shift-keying) is intended to help the reader understand t'ne principles of an embodiment of the invendon without beinglimiting to the scope of this invention. Moreover, the term FSK is understood to15 include variations such as binary FSK and M-ary FSK. Finally, it is contemplated that other moduladon techniques such as intensity moduladon, either continuous (AM or IM) or discrete (M-ary ASK, M=2,3,...), and phase moduladon, either contdnuous (PM) or discrete (M-ary PSK, M=2,3,...), may be used in conjunction with frequency modulation without departing from the spirit and scope of the 20 principles of the present invendon.
Transmission medium 15 provides a propagation path for lightwave signals between the lightwave transmitter and the lightwave receiver. In general, transmission mediurn 15 is understood to include dielectric waveguides such as opdcal fiber, semiconductor waveguides, metal-indiffused lithium niobate or lithium 25 tantalate waveguide elements, and the like. Of course, other elements such ascombiners, couplers, star distribudon networks, switching elements, opdcal amplifiers, signal regenerators, reconditioners, and repeaters, and the like may be present within the transmission medium 15 without any loss of generality or applicability for the principles of the present invention. In its simplest embodiment, 30 transmission medium 15 supports opdcal propagadon of an input signal, ligh~wave signal 14, until an output signal, lightwave signal 16, is uldmately delivered to the receiver at the remote end of the transmission medium.
Receiver 17 accepts lightwave signal 16 from the transmission medium.
Based upon the system architecture and the actual funcdon of the receiver, 35 receiver 17 operates on received lightwave signal 16 in a prescribed manner. For example, the receiver may provide coherent detecdon via homodyne or heterodyne ;~0~12~

reception of lightwave signal 16. The need for local oscillators at the receiver may be eliminated by including in M-1 bandpass optical fil~ers such as Fabry-Perot filters tuned to M-l diffeIent wavelengths included in lightwave signal 14, an M-ary FSKsignaL In the latter configuration, the M-ary FSK signal is detected and output as an 5 M-ary ASK signal.
It is understood by those skilled in the art that lightwave system 10 may be included without any loss of generality in a larger lightwave system such as a wavelength division multiplexed (WDM) system or the like.
Embodiments of the modulator, transmission medium and receiver 10 described above are well known to those skilled in the art. Accordingly, further discussion will provide a more detailed description of the transmitter and, particularly,-DFB laser 12. For background on DFB lasels, the teachings of U. S. Patent 3,760,292 are expressly incorporated herein by reference.
FIG. 2 shows a perspective cross-section and cutaway view of an 15 exemplary distributed feedback semiconductor laser for use as DFB laser 12 inlightwave system 10 in accordance with the principles of the invention. The DFB
laser shown in FIG. 2 is a buried heterostructure having a reversed-bias p-n blocking region. Other structures such as buried ridge, crescent or V-groove, double channel planar buried heterostructure, semi-insulating blocking region planar buried 20 heterostructuIe and the like are contemplated for use as embodiments of DFB laser 12.
Semiconductor structures such as the one shown in FIG. 2 are grown using epitaxial growth techniques such as liquid phase epitaxy, molecular beam epitaxy, chemical beam epitaxy and vapor phase epitaxy. These techniques are 25 described in the literature and are well known to those sldlled in the art. See, for example, H. C. Casey et al., Heteros~ucture ~ Vols. A and B, Academic Press (1978). Also, see U. S. Patent 4,023,993 for a description of a method for making a distributed feedback laser.
As sho vn in FIG. 2, the DFB laser includes an n-type Sn:InP
30 substrate 23 on which the reversed-bias p-n blocking region and the buried heterostructure are grown. Contact layers 24 and 25 are shown as broad area metallic contacts deposited on opposite sides of the DFB laser for biasing and curTent injection. Standard ohmic contact fabricadon techniques such as multi-layer evaporation of metal films, alloy evaporation, sputtering and annealing may be 35 employed to realize the ohmic contacts for the particular DFB laser. In the laser shown in FIG. 2, contact 24 is a standard Au-Zn contact whereas contact 25 is an ~ .

2C)()~)12 evaporated Au-Ge-Ni contact.
Using standard epitaxial growth techniques, a heterostructure is grown on substrate 23 in the following order: an addidonal n-type Sn:lnP buffer layer (not shown) approximately 511m thick; an undoped quaternary (InxGal xAsyPl y) active 5 layer 26 approximately O. l S ~m thick and having suitable mole fracdons x and y to produce a characterisdc wavelength ~p substandally at the peak of the gain profile curve as desired --- in this example, the characteristic wavelength is selected to be approximately l.Sl ~m; a p-type guide layer 27 comprising Zn:InxGal xAsyPl y approximately O.lS llm thick and having suitable mole fracdons x, y for 10 approximately 1.3 ~m; a p-type Zn:InP cladding layer 28 approximately 3 ~,lm thick;
and p-type quaternary cap layer 29 approximately 0.7 llm thick. Standard stripe masking using photolithography and etching techniques (for example, bromine methanol etch) are employed to produce the heterostructure mesa.
After the heterostructure mesa is formed, successive growth steps for p-15 blocking layer 22 and n-blocking layer 21 are performed over the substrate 25.
Blocking layer 22 comprises Zn:InP approximately 0.5 llm thick and blocking layer 21 comprises Sn:InP to a thickness sufficient ~o substandally planarize the endre semiconductor structure for contacdng.
It is understood that dopant concentradons of approximately lOl7 to 20 lOl8 cm~3 are suitable for the Sn and Zn dopants in the layers of the DFB laser described above. After &al preparadon, the laser is cleaved to produce at least two end facets in planes perpendicular to a directdon of light propagatdon supported in the heterostructure. Since the laser shown has a corrugadon grating as the integrated feedback structure between the facets, it is generally acceptable practice to apply 25 and-reflecdon coatings to the at least two end facets to reduce end facet reflections to a minimurn.
Also shown in FIGS. 2 and 3, the integrated feedback structure of the DFB laser includes a corrugadon gradng 3l which is formed in guide layer 27 on the side opposite the interface with acdve layer 26. Shape, depth and pitch or period of 30 the gradng are variable and depend on the gratdng placement together with the result desired therefrom.
In principle, the integrated feedback structure of the DFB laser includes spadally periodic perturbadons in the transmission characterisdcs of the laser waveguide formed substandally condnuously along the direcdon of lightwave 35 propagadon in the laser waveguide and substandally transverse to the propagation direcdon of optical energy in the waveguide. Spadally periodic perturbatdons of the 200Gl~

transmission characterisdc of the waveguide may take the form of variadons in gain, index of refracdon, propagadon constant, or other parameter of the waveguide medium for the laser.
In accordance with the principles of this invention, the period of the S gradng effecdve over the guiding region of the laser is given as an effective period, ~eff > ~pM/2n, where ~p is the characteristdc wavelength substantially at the gain peak or gain maximum for the semiconductor structure, M is the order of the grating expressed as an integer greater than or equal to one, and n is the modal or effective index of refractdors for the waveguide mode of the semiconductor laser. It is 10 contemplated that, while transverse positioning of the gratdng lines is desired, an , angular displacement (twist) of the grating lines may occur so that the grating lines lie substantially transverse to the direcdon of lightwave propagation for the DFB
laser.
It is contemplated that first (M=1) or higher order (M=2,3,...) integrated 15 feedback structures such as corrugation gradngs may be udlized. Such gradngs may be fabricated using standard electron beam, photolithographic and/or holographicpatterning techniques with the necessary subsequent wet or dry etching steps. The gradng shape may be sinusoidal as shown in FIGs. 2 and 3 or triangular, rectangular, trapezoidal, semi-circular or some other known complex functdon. For various 20 gradng profiles and fabricadon techniques, see Elect. ~ ~ Vol. 19, No. 25/26, pp.
107~7 (1983).
Positior~ing of the grating with respect to the actiYe layer can be varied so that the grating rnay be on the substrate below the acdve layer, or on the active layer, or on sorne other layer near the active layer. Of course, grating coupling 25 strength must be considered when selecting a grating position because the grating coupling strength is determined by the grating position vis-a-vis the waveguide mode, the gradng or corrugadon depth measured from peak to trough, and the difference between refracdve indices for the materials bounding the corrugation or grating.
As one addidonal modification of the gradng structure, it is well known that ~J4 shift regions may be included within the gradng. These ~J4 shift regions are known to provide additional frequency stability for the DFB laser. One exemplary ~/4 shift is shown as region 30 in FIG. 3. Such regions need not be centrally located in the gradng structure. Other types of shift regions are contemplated for use herein 35 such as step-index of refracdon changes in a guide layer or a linearly increasing thickness of a guiding layer or the like as disclosed in U. S. Patents 4,096,446, 20061~
.9 4,648,096, 4,665,528, 4,701,930.
In the exemplary embodiment shown in F~Gs. 2 and 3, a first order grating i5 shown with an effective period ~,lr which satisfies the criterion described above for 5 detuning the grating to be at a wavelength which is longer than the gain peak or gain maximum wavelength as described above mathemat;cally. The corrugation grating shown in the FIGs. has a pitch of approximately 2384 A and a depth of approximately 800 This grating was chosen to achieve detuning of approximately 400 ~ from a gain peak wavelength of approximately 1.51 ~Lm (lp) to an operating wavelength of 1.55 ,um (1B)-In order to accomplish this detuning, it is necessary to select the amount of wavelength detuning desired. Using standard calculation techniques which are well known to those skilled in the art, the modal index of refraction of the laser seructure is determined using the compositions and layer dimensions for the DFB laser. Index values are obtained from IEEE J of Quant. Elect.. QE-21, pp. 1887 et seq. (1985~.
Detuning the grating period to be such that the Bragg wavelength is longer than the wavelength of the gain peak for the semiconductor material causes the resulting DFB
laser to have an unusually large linewidth enhancement factor, ~. For general discussion of measurement of the linewidth enhancement factor, see the following articles: IEEE J.
of Quant. Elect.~ QE-18, pp. 259 et seq. (1982); Appl. Phvs. Lett.. 42(8), pp. 631 et seq.
(1983); Elect. Lett.~ 23, pp. 393-4 (1987); Elect. Lett.. 22, pp. 580-1 (1986). As a result of proper detuning in accordance with the principles of the invention, the resulting DFB
laser provides a large carrier-mediated FM response for reducing current drive requirements and also for flattening the F~ response to be substantially uniform.
FIG. 4 shows a combined plot of linewidth enhancement factor versus wavelength (curves 42 and 43) and gain versus wavelength (curve 41). The active layer was designed to be quaternary m-v semiconductor material, LnGaAsP, with mole fractions x=0.74 and y=0.6 so that Ap is slightly less than 1.3 ~m. The linewidth enhancement factor is shown to increase with increasing wavelength for a buried heterostructure DFB laser in curve 42 and a multiple quantum well DFB laser in curve 43. That is, each DFB laser exhibits more chirp with increasing wavelength. Shaded region 44 depicts those wavelengths to which the integrated feedback stNcture such as a Bragg grating may be tuned for the DFB laser so that A

the operadng wavelength of the laser (~B) iS greater than the gain peak wavelength for achieving large linewidth enhancement and excellent direct current frequencymoduladon operadon.
In another example from experimental pracdce, a DFB laser having a S properly designed gradng and waveguide structure was frequency modulated directly using NRZ data sequences with a peak-to-peak current drive of 4 mA. The residualintensity moduladon was less than 7% and there was no apparent degradadon due tononuniform FM response which indicates that the invendve laser structure overcomes the problems of the prior art by substandally eliminadng nonuniform FMlO response. Degradadon, if any, would have been noticed because the pseudorandom sequence has a length 223-l at a data rate of 2Gbps giving rise to spectral components below 1 KHz which is well below the thermal cutoff frequency --- a regime identified with classic nonuniform FM response.
It is understood that, while the material system InGaAsP/InP is 15 described above for fabricadng the distribused feedback laser, other materialcombinadons may be selected from other semiconductor Group m-v systems such as GaAs/AlGaAs, InGaAs/InAlAs, InGaAs/InGaAlAs, GaAsSb/GaAlAsSb and GaAs/AlAs. In these sen~conductor systems, the layers may be latdce-matched to suitable GaAs or InP substrates. Mismatching is also contemplated wherein strained 20 layers are grown over the substrate materia1. Finally, extension of the device structures is also contemplated to semiconductor compounds in Group II-VL

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Claims (4)

1. A lightwave transmitter including a distributed feedback laser and means for frequency modulating said laser, said laser comprising a semiconductorheterostructure including a gain medium having a characteristic wavelength .lambda.p and a modal refractive index ? and an integrated feedback means coupled optically to said gain medium, said integrated feedback means including spatially periodic perturbations of a transmission characteristic of said semiconductor heterostructure and having an effective period Aeff, said transmitter CHARACTERIZED IN THAT, said effective grating period is related to said characteristic wavelength as Aeff>.lambda.pM/2?, where M is an integer greater than or equal to 1 for characterizing an order of the integrated feedback means.

2. A lightwave transmitter including a distributed feedback laser and means for frequency modulating said laser, said laser comprising a waveguide andgain medium included therein wherein said waveguide has a characteristic wavelength .lambda.p,said laser further comprising an integrated feedback means coupled optically to said gain medium, said integrated feedback means including spatially periodic perturbations of a transmission characteristic of said waveguide and having an effective period for causing the laser to operate at a Bragg wavelength .lambda.B, said transmitter CHARACTERIZED IN THAT, said Bragg wavelength is greater than said characteristic wavelength.

3. A lightwave communication system comprising a lightwave transmitter, a lightwave receiver and a transmission medium optically coupled jointly to said lightwave transmitter and said lightwave receiver for supportinglightwave signal propagation therebetween, said lightwave transmitter including a distributed feedback laser and means for frequency modulating said laser, said laser comprising a semiconductor heterostructure including a gain medium having a characteristic wavelength .lambda.p and a modal refractive index ? and an integrated feedback means coupled optically to said gain medium, said integrated feedback means including spatially periodic perturbations of a transmission characteristic of said semiconductor heterostructure and having an effective period Aeff, said transmitter CHARACTERIZED IN THAT, said effective grating period is related to said characteristic wavelength as Aeff>.lambda.pM/2?, where M is an integer greater than or equal to 1 for characterizing an order of the integrated feedback means.

4. A lightwave communication system comprising a lightwave transmitter, a lightwave receiver and a transmission medium optically coupled jointly to said lightwave transmitter and said lightwave receiver for supportinglightwave signal propagation therebetween, said lightwave transmitter including a distributed feedback laser and means for frequency modulating said laser, said laser comprising a waveguide and gain medium included therein wherein said waveguide has a characteristic wavelength .lambda.p,said laser further comprising an integrated feedback means coupled optically to said gain medium, said integrated feedback means including spatially periodic perturbations of a transmission characteristic of said waveguide and having an effective period for causing the laser to operate at a Bragg wavelength .lambda.B, said transmitter CHARACTERIZED IN THAT, said Bragg wavelength is greater than said characteristic wavelength.