Multiple wavelength laser having a reduced number of wavelength controls

[0002] Related subject matter is disclosed in the concurrently filed application entitled “A BROADBAND ELECTRONIC N×N CROSS-CONNECT SWITCH USING TUNABLE LASERS by the inventors, C. R. Doerr, C. P. Dragone, I. P. Kaminow and A. M. Glass, both applications being assigned to the same Assignee.

[0003] The present invention relates to a multiple wavelength laser and, more particularly, to a multiple wavelength laser configuration characterized by N=N_aN_boutput wavelengths selected using only N_a+N_bcontrol signals.

[0004] Wavelength division multiplexing (WDM) allows high transmission capacity by allowing many wavelengths to be simultaneously transmitted in each fiber. An important device that is needed at each WDM node is a broadband cross-connect switch for providing complete connectivity between incoming and outgoing fibers. Such a cross-connect switch can be realized by combining a waveguide grating router with a set of tunable lasers. A tunable lasers can be simply realized by using an N×1 router as described in the article “Chromatic focal plane displacement in parabolic chirped waveguide grating router” by C. Doerr et al, published in IEEE Photon. Technol. Lerr., vol. 9, pp. 627-635, May 1997. However, in such an arrangement N controls are needed and such laser may be difficult to realize in integrated form for large N. A tunable laser having a reduced number of controls is described in the article entitled “Arrayed-Waveguide grating lasers and their applications to tuning-free wavelength routing” by Y. Tachikawa et al, IEE Proc. -Optoelectron., Vol 143, No. 5, pp. 322-328, October 1996. However, the article does not describe a practical way to integrate a single output tunable laser.

[0005] Therefore, there is a continuing need for an integrated single output tunable laser having a reduced number of wavelength selection controls.

[0006] In accordance with the present invention, we disclose an integrated, single output port, tunable multiple wavelength laser having N=N_aN_boutput wavelengths selected using only N_a+N_bcontrol signals. Each of N_bports can output N_aof the laser wavelengths, the port being selected by the N_bcontrol signals and the particular one of the N_awavelengths being selected by the N_acontrol signal. An N_b×1 router combines the signals from the N_boutput ports into a single output.

[0007] In another embodiment, the laser apparatus comprises a waveguide grating router including a first and second free-space regions connected by a plurality of waveguide arms having a quadratic path length variation. The input side of the first free-space region includes N_ainput waveguides each separated by a spacing “a” and each including a reflective termination which is controlled by a separate one of a first group of control signals, 1−N_a. The output side of the second free-space region includes N_boutput waveguides each separated by a spacing “b”, where “a” is equal to N_btimes “b”, each output waveguide including a reflective termination which is controlled by a separate one of a second group of control signals, 1−N_b, each reflective termination being partially transmitting and connected as a different one of the N_boutput ports of the laser. In yet another embodiment, the laser is connected to a N_b×1 router that combines the N_boutputs from the laser into a single output.

[0008] In the drawings,

[0009] FIG. 1 shows a block diagram of our multifrequency laser having N_boutput ports connected to an N_b×1 router;

[0010] FIG. 2 shows the combs of wavelengths produced at the various N_boutput ports of the laser of FIG. 1;

[0011] FIG. 3 shows a waveguide grating router consisting of a waveguide grating between two free-space regions. The input and output waveguides are spaced by a and b along the input and output boundaries of the router;

[0012] FIG. 4 shows the wavelengths of maximum transmission produced by a particular order m_oof a conventional N×N router when the input and output waveguides are equally spaced by a=b along the input and output boundaries of the two free-space regions;

[0013] FIG. 5 shows an illustration of a laser used in FIG. 1 implemented using an N_a×N_brouter combined with N_a+N_bcontrol elements;

[0014] FIG. 6ashows a more detailed implementation of the laser of FIG. 5 realized by including suitable reflections in the input and output waveguides of a waveguide grating router. Each reflection is produced by an active element whose gain or loss can be controlled electronically via a control lead. Shown in FIG. 6bare the wavelengths produced by a particular order m₀when the spacing b of the output waveguides satisfies b=a/N_b; and

[0015] FIG. 7 illustrates that arbitrary spacings for “a” and “b” in FIG. 6, results in arbitrary separation between the wavelengths of each comb and arbitrary displacement of the combs.

[0016] In the following description, identical element designations in different figures represent identical elements. Additionally in the element designations, the first digit refers to the figure in which that element is first located (e.g., 104 is first located in FIG. 1).

[0017] One technique for reducing the number of wavelength selection controls of a multiple wavelength laser, for instance by a factor P, is by including temperature tuning of the lasers. Thus by dividing the set of N wavelengths into Q groups, each consisting of a comb of P wavelengths, one obtains the set of wavelengths

[0018] λ_i,r, with i=1, . . . , P and r=1, . . . , Q

[0019] where λ_i,ris the r-th wavelength of the i-th group. Now P controls are needed to change i, and the value of r can be changed by temperature tuning. Therefore, each laser can be realized by using a P×1 router with P controls. Each laser is capable, at any particular temperature, of P equally spaced wavelengths, each belonging to a particular comb. Moreover, for any control choice, any of the Q wavelengths for r=1, . . . , Q can be produced by temperature tuning. In this manner P×Q=N different wavelengths are produced by each laser 101. However, temperature tuning is typically slow and only small values of Q can be realized realistically.

[0020] In accordance with the present invention, we describe our technique for reducing the number of controls for a multiple wavelength laser by including several output ports in each laser. Shown in FIG. 1 is a block diagram of our multifrequency laser 101 having N_boutput ports 102 connected to an N_b×1 router 103. Our multiple wavelength laser can produce any one of N_adifferent wavelengths available at each of the N_boutput ports. Thus a total of N=N_aN_bdifferent output wavelengths may be selected using only N_a+N_bcontrol signals. The N_acontrols are used to select a wavelength from the N_adifferent wavelengths available at each of N_boutput ports (e.g., 201 of FIG. 2) and the N_bcontrols are used to select the port (e.g., 201 of FIG. 2) which is to output the wavelength selected by the N_acontrols. For some applications it may be desirable to have the wavelengths from the a multiple wavelength laser available from one port. In such an application, an N_b×1 wavelength router 103 is connected to combine the outputs from the N_boutput ports 102 of laser 101 into one signal that is outputted at a single output port 104.

[0021] Our multifrequency laser 101 and router 103 arrangement of FIG. 1 can be readily integrated using known technology to provide a large number N=N_aN_bof wavelengths.

[0022] Shown in FIG. 2 are the N_bcombs 102 of N_awavelengths 201 produced at each of the various N_boutput ports 102 of the laser of FIG. 1. Notice that the spacing w_aof the wavelengths of a particular comb is the same for all combs. Therefore, it is possible to transfer each of the N_awavelength combs 201 to one output port, 104 of FIG. 1, by combining the laser 101 with a wavelength router 103. The wavelength router 103 has wavelength spacing w_band a free-spectral range w_a. The wavelength router 103 is designed so that it transfers the first wavelength 202 of each comb 201 to its output port. Thus, since the router wavelength behavior is essentially periodic with period w_a, each subsequent wavelength 203-205 of each comb 201 is transferred to the output port. Note that the router wavelengths of maximum transmission are determined by the locations of the various input waveguides along the boundary of the router free-space region. Therefore, by properly choosing these locations, any specified values of the first wavelength of the various combs can be chosen. In particular these wavelengths need not be equally spaced.

[0023] The router 103 in FIG. 1 can be realized in integrated form as shown in FIG. 3 by using an imaging arrangement of waveguides (arms) or grating 301 having a constant path-length difference. The grating 301 is placed between two dielectric slabs forming two planar free-space regions 302 and 303 on a substrate. The input 304 and output 305 waveguides are connected to the input and output boundaries of the two free-space regions 302 and 303, respectively. In a conventional router each transmission coefficient between two particular ports has essentially periodic behavior characterized by equally spaced peaks, and each peak is produced by a particular order m of the grating. For instance, the wavelengths of maximum transmission from the i-th input waveguide, x_i, to the output r-th waveguide, x_r, are specified by the relation $\begin{array}{cc}\frac{x_i-x_r}{R}=\frac{m\lambda -m_0{\lambda }_0}{S}& \left(1\right)\end{array}$

[0024] where x_i, x_rare the coordinates specifying the locations of the two waveguides, m is the order of the grating, S is the spacing of the receiving and transmitting ends of the arms, m₀is a particular value of m, λ₀is a particular design wavelength, R is the radial distance of the input waveguides from the central arm of the grating, and the same distance R is assumed between central arm and the output waveguides. We initially let the spacing a of the input waveguides be equal to the spacing b of the output waveguides and choose $\begin{array}{cc}a=b=\frac{1}{N}\frac{R{\lambda }_0}{S}& \left(2\right)\end{array}$

[0025] FIG. 4 shows the wavelengths of maximum transmission produced by a particular order m_oof a conventional N×N router, e.g., FIG. 3, when the input and output waveguides are equally spaced by a=b along the input and output boundaries of the two free-space regions.

[0026] Then in Eq (2) letting

[0027] x_a=ia, x_b=rb,  (3)

[0028] we find from (1) that the wavelengths of maximum transmission from input port i to output port r for a particular order m=m₀are approximately given by

[0029] λ_i,r≅λ₀+(i−r)w,  (4)

[0030] where w denotes the wavelength spacing. More generally, by choosing different input and output spacings one obtains

[0031] λ_i,r=λ₀+(i−i₀)w_a−(r−r₀)w_b  (5)

[0032] where i₀, r₀are two particular values of i,r, corresponding to the wavelength λ₀=λ_irand w_a, w_bare determined by the input and output spacings a, b,

[0033] w_a/w_b=a/b.

[0034] Laser Design

[0035] With reference to FIG. 5, consider a laser 500 consisting of an N_a×N_brouter 501 combined with N_a+N_bactive elements. The router 501 can be realized using the waveguide grating router 601 as shown in FIG. 6aand, by choosing N_a=N_b, the laser can be designed so that the same N_awavelengths can be produced by each of the output ports. Here we are primarily interested in the general case N_a≠N_band assume for simplicity that all wavelengths of interest are produced by a particular order m₀of the grating.

[0036] Each element of N_aand N_bin FIG. 6acan be viewed as a reflective termination connected to a particular port of the router 601. Each element of N_aand N_bconsists of a reflector combined with a waveguide section having variable gain (or loss) that can be controlled electrically, via control leads C₁—C_Naand C′₁—C′_Nb. Note that while both sets of the N_aand N_belements can provide gain usually only one of the element sets would provide the gain and typically that one would be the N_belement set. By activating two particular elements, via their control leads, on opposite sides of the router, a cavity including the router 601 is formed and, by providing sufficient gain, lasing will generally occur at the strongest resonance, typically the one of highest gain. Notice, for the application considered here each laser is only required to produce one wavelength at the time. Therefore in each case, in order to produce a particular wavelength, only two elements (one of N_aand one of N_b) are activated while all other elements are set at maximum loss.

[0037] In a conventional router specified by Eq. (2), the waveguides are equally spaced with the same input and output spacing and the same comb of wavelengths are produced from all ports as discussed earlier. Here, however, it is generally convenient to produce from the various output ports of the laser different combs as illustrated by 610 in FIG. 6b. Shown by FIG. 6bare the wavelengths produced by a particular order m₀when the spacing b of the output waveguides satisfies b=a/N_b.

[0038] For a conventional router, each transmission coefficient between two particular ports has essentially periodic behavior characterized by equally spaced peaks. Here, instead, it is desirable to design the router as in the previous referenced Doerr publication so that one of the above peaks has amplitude appreciably larger than the others. This condition can be satisfied by designing the grating 602 with a quadratic path length variation included in the arms so as to cause the grating focal length to exhibit a strong variation as a function of the wavelength. One can insure by this technique that the peak of largest amplitude is always the one corresponding to a particular order m₀of the grating. Here we assume this condition and let the locations of the input and output waveguides along boundaries of the router free-space regions 603 and 604 be equally spaced by a and b, respectively, as shown in FIG. 6a. Then, the wavelength λ_i,rproduced for m=m₀by activating two particular active elements i,r can be written as Eq. (5), repeated below.

[0039] λ_i,r=λ₀+(i−i₀)w_a−(r−r₀)w_b  (5)

[0040] where i₀, r₀are two particular values of i,r, corresponding to the wavelength λ₀=λ_irand w_a, w_bare determined by the input and output spacings a, b.

[0041] w_a/w_b=a/b.

[0042] As a result, by choosing

[0043] a/b=N_b.

[0044] we obtain the set of equally spaced wavelengths, consisting of N_binterleaved combs, each produced by a particular port. As shown in FIG. 6b, each comb consists of N_awavelengths spaced by w_a, and each comb is displaced from the other combs by multiples of w_b=w_a/N_b. The i-th wavelength λ_i,rof the r-th comb is produced by activating the elements A_iand B_r(see FIG. 5). Here we assume that B_ris partially transmitting, so that power at that wavelength can be partially transmitted to a suitable output waveguide. Notice the above laser implementation concept illustrated in FIGS. 5, 6 effectively reduces the number of laser controls by a factor (N_a+N_b)/N_aN_band this concept can be combined with the temperature tuning concept to further reduce the number of controls or, equivalently, to increase the number of wavelength channels N for a given number of controls.

[0045] With joint reference to FIGS. 6aand b, so far we have assumed that the laser has uniformly spaced combs 610 , each comb having uniformly spaced wavelengths w_a, in which the spacing “a” of the input waveguides and the spacing “b” of the output waveguides are uniform. More generally with joint reference to FIGS. 6 and 7, by choosing arbitrary waveguide spacings for “a” in FIG. 6, the separation between the wavelengths 701 of each comb can be made not uniform or arbitrary. As before, one of the control leads N_ais used to select one wavelength of a comb. Similarly, by selecting arbitrary spacings for “b” in FIG. 6, the different combs are then not uniformly displaced 702 with respect to each other. Again, one of the control leads N_bis used to select one of the combs or outputs 702.

[0046] What has been described is merely illustrative of the application of the principles of the present invention. Other methods and arrangements can be implemented by those skilled in the art without departing from the spirit and scope of the present invention.