Patent Application: US-90918804-A

Abstract:
a monolithically - integrated semiconductor optical transmitter that can index tune to any transmission wavelength in a given range , wherein the range is larger than that achievable by the maximum refractive index tuning allowed by the semiconductor material itself . in practice , this tuning range is & gt ; 15 nm . the transmitter includes a mach - zehnder modulator monolithically integrated with a widely tunable laser and a semiconductor optical amplifier . by using an interferometric modulation , the transmitter can dynamically control the chirp in the resulting modulated signal over the wide tuning range of the laser .

Description:
in the following description of the preferred embodiment , reference is made to the accompanying drawings which form a part hereof , and in which is shown by way of illustration a specific embodiment in which the invention may be practiced . it is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention . fig1 and 2 are views that schematically illustrate a monolithically - integrated , semiconductor optical transmitter device 100 according to one embodiment of the present invention . fig2 is a cross - sectional side view of the device 100 , and fig1 is a top view of the device 100 . the device 100 is comprised of a common substrate 102 ( which may comprise inp ); at least one epitaxial structure 104 ( which may comprise varying layers of inp , ingaasp , ingaas , ingaasp , etc .) formed on the common substrate 102 ; a widely - tunable sampled grating distributed bragg reflector ( sgdbr ) laser resonator 106 , formed on the common substrate in the epitaxial structure 104 , for producing a light beam ; and a semiconductor mach - zehnder ( mz ) modulator 108 , formed on the common substrate 102 in the same or different epitaxial structure 104 as the laser 106 , for modulating the light beam , wherein the mz modulator 108 is positioned external to the laser 106 , but along a common waveguide 110 with the laser . preferably , a wavelength tuning range of the laser 106 is wider than what is achievable through an index change and a chirp of the modulated light beam is dynamically controlled by the mz modulator 108 over the wider wavelength tuning range of the laser 106 . the laser 106 preferably is comprised of a front mirror or reflector 112 , a back mirror or reflector 114 , a gain section 116 positioned between the front and back mirrors 112 , 114 or incorporated within the mirrors 112 , 114 , and a phase section 118 , all of which are situated along the common waveguide 110 . by applying an appropriate combination of currents to 112 , 114 , 116 and 118 , a light beam is produced by the laser 106 , wherein any frequency of the light beam within the designed tuning range can be emitted from the laser 106 . in this embodiment , the wavelength of the light beam is tunable over a wider wavelength range than is achievable by index tuning of any one section 112 , 114 , 116 and 118 , and the wider wavelength range is represented by δλ / λ & gt ; δn / n , wherein λ represents the wavelength of the light beam , δλ represents the change ( or delta ) in the wavelength of the light beam , n represents the index tuning of the laser 106 , and δn represents the change ( or delta ) in the index tuning of the laser 106 . to simplify operation and decouple power control from the wavelength tuning , a semiconductor optical amplifier ( soa ) 120 is situated after the laser 106 and before the mz modulator 108 , wherein the soa 120 amplifies the light beam produced by the laser 106 . the soa 120 is formed on the common substrate 102 in the same or different epitaxial structure 104 as the laser 106 and / or mz modulator 108 . the device 100 also includes a back facet monitor 122 positioned adjacent the back mirror 114 and a front tap 124 positioned between the soa 120 and mz modulator 108 . other embodiments of widely tunable lasers are known to those skilled in the art [ 19 , 20 ] and , in general , they can be classified as having more than one independently controlled section wherein the output wavelength of the laser is tunable over a wider wavelength range than is achievable by index tuning in any one section , and the wider wavelength range is represented by δλ / λ & gt ; δn / n . preferably , the mz modulator 108 ( also known as an mz interferometer or mzi ) is comprised of a first 1 × 2 ( or n × 2 ) multimode interference ( mmi ) coupler 126 that splits ( equally or unequally ) the light generated from the laser 106 and amplified by the soa 120 into first and second components of equal or unequal magnitude that are directed by first and second interferometric arms 128 of an optical waveguide , respectively , to two inputs of a second 2 × 2 ( or 2 × n ) mmi coupler 130 that combines ( equally or unequally ) the first and second components interferometrically , thereby directing the combined components to one of the output waveguides of the coupler 130 , wherein the optical path length difference of the arms 128 determines into which output of the second mmi coupler 130 to direct the combined components . both arms 128 comprise curved waveguides , and each of the arms 128 contain a first electrode 132 for applying an electric field to modulate the light beam , and at least one of the arms 128 contains a second electrode 134 for applying a current to adjust a phase of the light beam . specifically , the electrodes 132 accept a modulation voltage to adjust a relative optical phase length of the arm 128 at high speed through an electro - optic effect , while the electrodes 134 permit a free selection of a differential phase shift between the interferometric arms 128 with minimal attenuation . the first mmi coupler 126 successively splits the light beam into separate paths for the arms 128 and the second mmi coupler 130 then constructively or destructively combines the light beams from the arms , depending on their modulated phase difference , into an output . the mmi couplers 126 and 130 are designed to prevent reflection of light beam back into the laser 106 cavity by ensuring that the input / output faces of the mmi couplers 126 and 130 form an obtuse angle with the sides of the input / output waveguides 110 . further , the second mmi coupler 130 has two outputs such that a residual reflectivity is the same when the light beam is directed toward either of the outputs , which ensures that the laser 106 is not perturbed differently as the mz modulator 108 switches the light between the paths under modulation . following the 2 × 2 mmi coupler 130 , two output couplers 136 are formed on the common substrate 102 in the same or different epitaxial structure 104 , wherein at least one of the output couplers 136 is positioned and configured to receive the light beam output from the mz modulator 108 , and couple the light beam output from the mz modulator 108 to a following optical assembly ( not shown ). these output couplers 136 reduce back reflections to the mz modulator 108 . in addition , the output couplers 136 may be used to transform a shape of an optical mode of the light beam at the output of the mz modulator 108 to a substantially circular pattern to produce a symmetric farfield pattern , as opposed to a conventional elliptical pattern typical of semiconductor waveguides . in general , the farfield should be modified to match the requirements of the optical assembly used to couple the light beam into an optical fiber and is not necessarily limited to circular patterns . fig3 is a top view that schematically illustrates the output couplers 136 according to an embodiment of the present invention . when the mz modulator 108 is in an “ on ” state , the light beam exits through the lower or first of the output couplers 136 , and when the mz modulator 108 is in an “ off ” state , the light beam exits through the upper or second of the output couplers 136 . both of the output couplers 136 are curved to prevent the light beam exiting from a facet of the device 100 in a direction that is perpendicular to the facet . in addition , the paths of the output couplers 136 preferably are at an angle relative to each other . consequently , the respective light beams generated during the “ on ” state and “ off ” state of the mz modulator 102 propagate at an angle φ relative to each other that is greater than 20 degrees from each other after exiting the device 100 . the device 100 also includes an electrode 138 that monitors an optical power of the light beam output from the mz modulator 108 , through the collection of photocurrent , wherein the electrode is positioned to receive the light beam from the second output coupler 136 . the electrode 138 can be positioned to receive the light beam before it reaches the facet , as shown in fig1 , 2 and 3 , or it can be positioned to receive the light beam reflecting from the facet , as depicted in fig4 . in fig4 , the angle between the direction of propagation along the second output coupler 136 and the facet of the device may be made greater than a critical angle to induce a total internal reflection of the light beam . the prior art has disclosed an arrangement absorb the light from the output of the modulator [ 18 ]; however , the present invention is intended to convert all of the modulated light to photocurrent and is not suitable for applications where the light output of a mz modulator will be coupled to an optical assembly separate from the integrated device . furthermore , the prior art uses a 2 × 1 combiner , necessitating a complex scheme for coupling the light to the tap detector . the use of a 2 × 2 mmi coupler 130 in the present invention allows for the substantially simpler embodiments . the high efficient nature of current induced phase tuning can be used to create additional enhancements to the monolithically integrated tunable transmitter . the prior art has discussed that the chirp of semiconductor modulators can be adjusted by changing the splitting ratio of the splitters used in an mz modulator [ 11 ] or by modifying the rf drive applied to a dual drive mzi [ 12 , 13 ]. the prior art has also disclosed the use of control electrodes to modify the chirp of the modulated waveform by applying dc voltage to the control electrodes contained in the arms [ 8 ], but this approach has a limited ability to change the chirp and can adversely effect the modulator extinction ratio due to imparting a loss imbalance in the arms . additional prior art has disclosed the addition of an electrode to modify the splitting ratio to increase the extinction ratio of the modulator [ 16 ], but has not discussed modifying the splitting ratio of the combiner or discussed the possibility of tuning the chirp . fig5 illustrates another embodiment of the present invention wherein one or more additional short mz modulators 140 , which act as variable splitters or combiners , are positioned before and / or after the mz modulator 108 . the additional short mz modulators 140 , which are formed on the common substrate 102 in the epitaxial structure 104 , modify chirp properties of the mz modulator 108 . the splitting and / or combining ratio of the additional short mz modulators 140 can be modified by injecting current into their electrodes 134 to dynamically control the chirp properties of the mz modulator 108 without adjusting the modulation voltage applied to the electrodes 132 in either arm 128 . the use of electrodes 134 as current - induced phase shifters in the splitting / combining mz modulators 140 makes this practical , as they do not significantly lengthen the device 100 (& lt ; 500 um ) or add substantial insertion loss (& lt ; 2 db ). further , the insertion loss would remain relatively constant for a range of splitting ratios around the nominal unbiased value due to the low increase in loss incurred by using current induced index change . voltage - based electrodes 132 performing this function would be impractical due to the high additional loss (& gt ; 6 db ), large size (& gt ; 1 mm ) and large variation in insertion loss as the splitting ratio changes . fig6 and 7 illustrate another embodiment of the present invention , wherein the device 100 power is adjusted over a large dynamic range . this is desirable feature can be obtained by adding one or more additional short mz modulators 142 , which are formed on the common substrate 102 in the epitaxial structure 104 , to the monolithically integrated device 100 . in this embodiment , each of the additional short mz modulators 142 functions as a variable optical attenuator ( voa ). fig6 illustrates an embodiment wherein the additional short mz modulator 142 is positioned before the mz modulator 108 , while fig7 illustrates an embodiment wherein the additional short mz modulator 142 is positioned after the mz modulator 108 . the prior art has disclosed the addition of an absorptive voa prior to a modulator [ 15 ]; however , there are several deficiencies with the prior art approach . first , the power dissipation of this approach scales dramatically with the input power to be attenuated and the degree of attenuation ( 5 - 10 times the input optical power ) necessitating designs that are multi - section to avoid catastrophic damage due to heating . these multi - section designs add substantial length to the modulator ( over a factor of 2 as compared to a modulator alone ). fig6 and 7 , on the other hand , illustrate configurations where the voa is created using a short interferometric optical attenuator , i . e ., the additional short mz modulator 142 , controlled by current injection via electrodes 134 . the advantage of this embodiment is that it adds only & lt ; 500 um to the length ( less than 30 % increase ) and the power dissipation is limited to & lt ; 20 mw regardless of the optical input power . zucker , j . e . ; monolithically integrated laser / mach - zehnder modulators using quantum wells , lasers and electro - optics society annual meeting , 1993 . leos &# 39 ; 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carr , n . ; asplin , j . c . ; kearley , m . q . ; hunt , w . j . ; brambley , d . r . ; rawsthome , j . r . ; a novel broadband dbr laser for dwdm networks with simplified quasi - digital wavelength selection , optical fiber communication conference and exhibit , 2002 . ofc 2002 , 17 - 22 mar . 2002 , page ( s ): 541 - 543 . u . s . pat . no . 6 , 516 , 017 , issued feb . 4 , 2003 , to matsumoto , and entitled multiwavelength semiconductor laser device with single modulator and drive method therefor . u . s . pat . no . 4 , 896 , 325 , issued jan . 23 , 1990 , to coldren , and entitled multi - section tunable laser with differing multi - element mirrors . this concludes the description of the preferred embodiment of the present invention . the foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description . it is not intended to be exhaustive or to limit the invention to the precise form disclosed . many modifications and variations are possible in light of the above teaching . it is intended that the scope of the invention be limited not by this detailed description , but rather by the claims appended hereto .