Wavelength switched laser source

A laser source in accordance with the invention includes a broadband laser generating optical output signals including a plurality of predetermined wavelength components. A bi-directional optical switch having a first port and a plurality of second ports is responsive to control signals for establishing an optical coupling between said first port and a selected one of the plurality of second ports. A circulator having a first port is coupled to the broadband laser, a circulator second port is coupled to the bi-directional optical switch first port and a circulator third port provides output optical signals. A plurality of wavelength selective reflectors is provided. Each wavelength selective reflector is coupled to a corresponding one the optical switch second ports, each of the wavelength selective reflectors reflects optical signals at a predetermined one optical wavelength selected from a plurality of predetermined optical wavelength. Apparatus is provided for generating the control signals. The control signal generating apparatus comprises a micro controller in one embodiment of the invention.

BACKGROUND OF THE INVENTION
 This invention relates to optical communications systems, in general, and
 to a laser source for use in such communications systems, in particular.
 It is desirable to provide a laser source that can provide an optical
 output that is switchable to a plurality of different wavelengths. It is
 particularly desirable that such a laser source be capable of switching
 from one wavelength to another at a rapid rate.
 SUMMARY OF THE INVENTION
 A laser source in accordance with the invention includes a broadband laser
 generating optical output signals including a plurality of predetermined
 wavelength components. A bi-directional optical switch having a first port
 and a plurality, N, of second ports, is responsive to control signals for
 establishing an optical coupling between said first port and a selected
 one of the plurality of second ports. A circulator having a first port is
 coupled to the broadband laser, a second port is coupled to the
 bi-directional optical switch first port and a third port for providing
 output optical signals. A plurality of wavelength selective reflectors is
 provided. Each wavelength selective reflector is coupled to a
 corresponding one the optical switch second ports, each of the wavelength
 selective reflectors reflects optical signals at a predetermined one
 optical wavelength selected from a plurality of predetermined optical
 wavelength. Apparatus is provided for generating the control signals. The
 control signal generating apparatus comprises a micro controller in one
 embodiment of the invention.
 In one embodiment in accordance with the invention, bias circuitry is
 coupled to the bi-directional optical switch and to the micro controller.
 In an embodiment in accordance with the invention an optical switch is
 formed on a first substrate of electro-optic material. The substrate
 comprises LiNbO.sub.3.
 A second substrate carrying said plurality of wavelength selective
 reflectors. In one embodiment, the second substrate comprises silicon and
 is bonded to said first substrate.
 Each reflective filter comprises a Bragg grating that in one embodiment of
 the invention is a fiber Bragg grating.

DETAILED DESCRIPTION
 FIG. 1 illustrates the general configuration of a wavelength switchable
 laser in accordance with the principles of the invention. Optical signals
 from a broadband laser source 1000 are applied to an input port 101 of a
 three port optical circulator 100. Broadband laser source 1000 has
 wavelength components at the wavelengths of interest and may be any
 broadband laser source of a type known in the art. Optical circulator 100
 has a second port 103 coupled to optical switch 110. A third port 105
 serves as an output port. Circulator 100 may be any one of a number of
 known circulators. An isolator may be inserted into the optical path
 coupling the source of optical signals to port 101 to make port 101
 unidirectional. Similarly. an optical isolator may be inserted into the
 optical path coupled to port 105 so that optical signals flow
 unidirectionally out from port 105. Port 103 is a bi-directional port that
 receives broadband optical signals from port 101 and couples a selected
 optical signal wavelength component received at port 103 to port 105. The
 polarity of circulator 100 is indicated by directional arrow 102. The flow
 of input optical signals to switch 120 is shown by arrows 104, 106. The
 flow of wavelength selected optical output signals from optical switch 120
 to port 103 and out from port 105 is shown by arrows 108, 110. Optical
 switch 120 is operable to couple port 121 to any one of a plurality, n, of
 ports 123. Each of the plurality of ports 123 has coupled thereto a
 corresponding one of a plurality of reflective wavelength filters 125.
 Each reflective wavelength filter is a narrow filter and in the
 illustrative embodiment may be either a fiber Bragg grating or a
 dielectric interference filter. Both fiber Bragg gratings and dielectric
 interference filters are known in the art. Each wavelength filter is
 selected to reflect optical signals that are only at a specific centerline
 wavelength designated as .lambda.1-.lambda.n. The number of filters 125
 utilized is dependant upon the specific application and the incremental
 wavelength difference between adjacent selected wavelengths. Stated
 another way, the number of filters is determined by the wavelength range
 over which tuning is to occur and the incremental wavelength, or
 wavelength granularity between selections. Optical switch 120 receives
 wavelength selection signals and couples port 121 to a selected one of
 ports 123 based upon the selection signals. The selected one of ports 123
 is made based upon the desired wavelength of optical signals desired. Each
 of the narrow filters 125 reflects optical signals only at the particular
 center wavelength of the filter and passes or in effect absorbs all other
 optical signals. Input optical signals received at circulator 100 port 101
 are coupled to port 103 and coupled to port 121 of switch 120. Switch 102
 couples the optical signals to a selected one of filters 125. The selected
 filter 125 is determined by wavelength select signals received by switch
 120.
 The selected filter 125 reflects only optical signals at the selected
 wavelength back to port 121 and thence to circulator 100 port 103. The
 selected wavelength optical signals are coupled out of circulator 100 at
 port 105. In a first embodiment of the invention, 1.times.N optical switch
 120 is an electro-mechanical switch of a type well known in the art or a
 thermal-optic switch also of a type known in the art. In a second
 embodiment of the invention, 1.times.N optical switch 120 is an integrated
 optic waveguide switch formed on a LiNbO.sub.3 substrate or a substrate of
 other electro-optic material. This embodiment has the advantages of a high
 wavelength channel count, fast switch speed and small size.
 In a second embodiment of a wavelength selectable laser source in
 accordance with the invention shown in FIG.2, 1.times.N optical switch 120
 is again formed on a LiNbO.sub.3 substrate 220 or a substrate of other
 electro-optic material. Particular details of the 1.times.N switch
 structure are not shown on the structure of FIG. 2, however, in this
 particularly advantageous embodiment of the invention, the plurality of
 filters 125 is arranged as a fiber Bragg grating array 225 of filters. A
 plurality, n, of fiber Bragg gratings 225 are provided on a separate
 substrate 230 that is affixed to substrate 220. More specifically, a
 plurality, n, of fiber Bragg gratings 225 are bonded to grooves or
 channels formed on the surface of a substrate 230. In the specific
 embodiment shown, substrate 230 is selected to be a silicon substrate. The
 end surface 232 of substrate 230 that is adjacent to substrate 220 is
 polished. End surface 232 is bonded to surface 222 of 1.times.N optical
 switch substrate 220. Bonding of substrate 220 to substrate 230 may be by
 any one of several known arrangements for bonding substrates together.
 FIGS. 3 and 4 show a fiber Bragg grating array 225 with 8 fiber Bragg
 grating filters .lambda.1-.lambda.8. Each of the fiber Bragg grating
 filters .lambda.1-.lambda.8 is a separate fiber segment 301-308 having a
 Bragg grating 321-328 formed thereon. Each fiber segment is a
 photosensitive fiber onto which a Bragg grating is formed by using
 ultraviolet light in conjunction with a different period phase mask for
 each different filter center wavelength. The forming of Bragg gratings on
 fibers utilizing such a technique is known in the art. Silicon substrate
 230 has a plurality of grooves 401-408 formed on a top surface 412. Each
 of the grooves 401-408 is shown as a "v" groove, but may be of different
 cross sectional shape, and rather than being shaped as a "groove" may be a
 channel. By use of the term "channel", it will be understood that various
 cross-sectional grooves is included. In the embodiment shown, the grooves
 or channels may be formed by use of a saw, or by etching or any other
 process that will permit controlled depth formation of channels. For
 example, the v-grooves may be formed by providing an oxide masking layer
 on the silicon substrate, utilizing a photolithography process to define
 each of the grooves, and applying an etchant to form the grooves 401-408.
 After the grooves 401-408 are formed, the fiber segments 301-308 are
 placed in the grooves 401-408 with fixed spacing and are bonded in
 position with epoxy. The end surfaces 232, 333 of substrate 230 as well as
 the corresponding end faces of fiber segments 301-308 are coplanar and
 polished to optical quality. The corresponding end surface 222 of
 substrate 220 is likewise polished to optical quality. The fiber Bragg
 grating array 225 is aligned with the 1.times.N switch substrate 220 and
 bonded thereto. The bonding may with epoxy or any other method of bonding
 that provides good optical coupling.
 Turning now to FIG. 5, the wavelength selectable laser source of FIG. 2 is
 shown with broadband laser source 1000 coupled to 1.times.N optical switch
 120 via circulator 100 as in FIGS. 1 and 2. Optical switch 120 is shown in
 greater functional detail. 1.times.N optical switch 125 is formed from a
 tree of 1.times.2 optical switches 501-507 and waveguides 521-535.
 Switches 501-507 are selectively operated by a microprocessor or micro
 controller 550 that responds to wavelength signals indicating a desired
 optical wavelength and determines which optical switches 501-507 to
 operate to couple optical signals to the corresponding one fiber Bragg
 grating 125 of array 225.
 FIG. 6 illustrates a 1.times.2 switch 501 that is appropriate for use in
 the 1.times.N switch arrangement 220 of the invention. Switch 501 is a
 bi-directional, polarization independent 1.times.2 switch design. It
 includes a waveguide that forms a "y" having first, second and third
 waveguide legs 521, 522, 529. The waveguides 521, 522, 529 are formed on a
 substrate utilizing known fabrication methods for forming optical
 waveguides on electro optic substrates such as LiNbO.sub.3. Switch 501
 further includes three electrodes 601, 602, 603 that are used to determine
 the optical path through switch 501. The application of bias voltage V to
 electrodes 601, 602, 603 determines whether waveguide portion 521 is
 coupled to waveguide portion 522 or 529. The high voltage switch 501 can
 switch both TE and TM mode signals. Switch 501 has an on-off ratio of
 greater than 20 dB. In a reflective design, a double pass produces 40 dB
 of isolation. With this building block switch structure other sized
 switches may be provided.
 Although switch 501 is shown in detail in FIG. 6, each of the switches
 501-507 is of the same construction and all are fabricated on a single
 substrate 220 in the illustrative embodiment. The waveguides 521-535 are
 formed utilizing any of the known techniques for formation of waveguides
 in electro-optic substrates.
 FIG. 7 illustrates another embodiment of the invention in which the
 reflective filters 525-535 are formed on the same substrate 720 as the
 1.times.N switch. The substrate is LiNbO.sub.3 or another electro optic
 material. Each filter 725 is formed on a waveguide 525-528, 532-535 formed
 on substrate 720. Each waveguide has a photosensitive region onto which a
 Bragg grating is formed. Operation of the structure of FIG. 7 is the same
 as that of FIG. 5.
 It should be apparent to those skilled in the art that although the
 structures shown in the drawing figures illustrate only a 1.times.8 switch
 and 8 wavelengths, the number of wavelengths and the size of the 1.times.N
 switch is a matter of design selection to provide the desired number of
 selectable wavelengths. For example, 1.times.16 and 1.times.32 switches
 can be built such that the laser source has 16 and 32 selectable
 wavelengths. If it is desired to accommodate a larger number of
 wavelengths, cascading several stages can accommodate more wavelengths.
 For example, to accommodate 128 wavelengths, a 1.times.4 switch can be
 cascaded with four 1.times.32 switches.
 Various other changes and modifications may be made to the illustrative
 embodiments of the invention without departing from the spirit or scope of
 the invention. It is intended that the invention not be limited to the
 embodiments shown, but that the invention be limited in scope only by the
 claims appended hereto.