CA1313430C - Photonic switch architecture utilizing code and wavelength multiplexing - Google Patents

Abstract

A PHOTONIC SWITCH ARCHITECTURE
UTILIZING CODE AND WAVELENGTH MULTIPLEXING
Abstract An optical multiplexer and demultiplexer using combined code division and wavelength division multiplexing. The multiplexer comprises a plurality of code division multiplexers, each responsive to a plurality of input signals and a plurality of orthogonal code sequences, and a wave division multiplexer for generating an output signal representing the signals of each code division multiplexer carried on a different wavelength. Each code division multiplexer comprises a plurality of lithium niobate phase shifters, each comprising an indiffused titanium wave guide and a pair of electrodes for the accepting one of the orthogonal code sequences, to modulate the lightwave being passed through the wave guide.
The demultiplexer for extracting a selected input signal, comprises a wavelengthselector for selecting the wavelength carrying the selected input signal, a phase shifter for modulating the selected wavelength signal by the code sequence of the selected individual input signal, and a PIN photodiode for combining the multiplexed signal with the output of the phase shifter to extract the selected input signal. Advantageously, many signals can be multiplexed onto one path and selected separately through the combination of the code division and wavelength division multiplexing and demultiplexing.

Description

A P}~OTONIC SWll'CH ARCHlTECTURE
UTILIZING CQDE AND WAlVELENGTH MULTIPLEX~IG

Technical Fieldl This invention relates tO arrangements for multiplexing and S demultiplexing electrical and optical signals.
Problem Optical systems are increasingly being used for transmitdng large quantities o~ information represen~ing voice, video, and data The demands for bandwidth on such optical systems keep growing as these systems are used, for 10 example, for transmitting a large number of channels for cable television systems.
In the future, such systems may be further burdened as high definition television signals become more prevalent and as the number of chaDnels to be provided continues to grow. Such arrangements put an increasing burden on the amount of inforrnation which must be transmitted over the optical fiber.
Optical transmission systems, in contrast to coaxial cable systems, almost invariably use digital signaling to transmit information. Among the multiplexing schemes which can be used to multiplex a large number of digital signals onto a single lightwave camer system is code division multiplexing. Codedivision multiplexing is a process whereby a number of input signals are each 20 orthogonalized before being added together, so that each signal can later be extracted individually, uncorrupted by the presence of the other signals. Extracdon is usually accomplished thIough a correlation process. l'he coding is usually accomplished through a special code sequence which multiplies the input signal or phase modulates the carrier of the input signal.
Another arrangement for multiplexing a number of different signals onto an optic transmission system is to use wavelength division multiplexing whereby each group of digital signals modulates a different wavelength of light and all the waveleng~s are carried simultaneously on the optic fiber.
However, even with the best available opdc ~ansmission moduladon 30 systems, there still is a need for additional bandwidth and further there is not a satisfactory economical arrangement for extracting a specific signal from an optic ~ansmission system which is carrying a large number of different signals. The latter function is required, for example, for a receiver of cable television signals.

-` 1313~30 Solution The above problems are solved and an advance is made in the state of the art in accordance with principles of my invention wherein, illustratively, acombination of code division and wavelength (frequency) division signals is carried 5 by an optic transm~ssion or switching system and wherein, advantageously, a merged decoder is used for extracting the signal carried by any code of any wavelength of the optic system. The decoder comprises a group of optical modulators each tunedto a different wavelength; only one of these optical modulators corresponding to the desired wavelength is made active. The output of that optical modulator is combined lû in a phase shifter with the code sequence of the selected signal. Wh~n the output of the phase shifter is combined with the input signal in a P~ photodiode ~or opticdetection, the electrical output of the PIN diode represents the desired signal.More generally, these aIrangements of encoders and decoders can also be used for electrical systems. Since w avelength and frequency are related, it is 15 possible to refer to either. For convenience, and because the exemplary embodiment uses photonics, wavelength is used through this specificadon.
In accordunce with one aspect of the invention combined code and wavelength division modulation is achieved by a combination of a plurality of code division multiplexers acting as inputs to a wave division multiplexer. Each code20 division multiplexer comprises a plurality of phase shifters, each of which have a signal înput and a code sequence input. The output of these phase shifters is then combined in a passive optical coupler which is used as one of a plurality of inputs tO
a wave division multiplexer. The wave division multiplexer comprises a series ofwavelength converters for taking the outputs of the code division multiplexers, all of 2S which are at the same wavelength, and converting these outputs to the di~ferent wavelengths. The ou~puts of the wavelength converters are then c~mbined with a passive optical coupler.
In accordance with another embodiment of the invention, a switch comprises a combined code division and wavelength division multiplexe~ whose 30 output is connec~ed to the input of a plurality of the code division/wavelength division decoders.
In accordance with another aspect of the invention, a switch for extracting and inserting a plurality of signals into an optical transmission system is implemented as follows. The outputs are extracted USillg a plurality of the merged 3S decoders as described above. Inputs are added by generating standard outputs of the code division multiplexers and converting their wavelength to ones of the 13~3430 ..

wavelengths not used in the output of the switch. These outputs are then combined with inputs used in the output of the switch by a passive coupler. Advantageously, such an arrangement permits a large number of signals to be added and extracted at one point in an optical transmission system.
In accordance with another aspect of the invention, signals are maintained in the optical domain throughout the encoding process and through the decoding process - until the output of the demodulator which is an electrical output is produced by the decoder.
In accordance with one specific embodiment, the phase shifter is a lithium niobate device having electrical inputs representing the code sequences applied at the electrodes of a phase shifter in which a basic lithium niobate crystal has a titanium di~fused waveguide that passes through the crystal. The titanium at the surface is surrounded on the two sides by the electrocles containing the electrical signal that represents the code sequence. In accordance with one aspect oE the invention, the merged decoders can be wklely clistributecl to a number oE stations which receivc, a broadcast wiclebancl signal. Such an arrangement provides the capability for any station to extract the desired signal from the large number of signals being transmitted.
In an alternative embodiment oE the combined decoder, optical OUtp~lt is produced by using in series a tunable optical filter for selecting the wavelengEh, a wavelength converter for converting the selected wavelength to a standard wavelength, and a phase shifter, responsive to a selected orthogonal input sequence for selecting the desired signal.
Therefore, in accordance with the principles of this invention, a demultiplexer of a combined wavelength (frequency)/code division multiplex signal comprises a wavelength selector, for selecting the correct wavelength, which clrives a combining circuit for combirling a selected orthogonal sequence with the selected wavelength signal, and the output of the combiner is used to extract the desired output signal from the multiplex signal. A multiplexer comprises a plurality of code division multiplexers, each driving a wavelength converter for generating a signal at a different wavelength, and a means for combining the outputs of the wavelength converters to generate the multiplexed output signal.

3 a In accordance with one aspect of the invention there is provided a two-stage multiplexer for multiplexing a plurality of input s;gnals to generate an output signal comprising: a plurality of first means, each means responsive to a plurality of said input signals for generating an intermediate signal comprising a plurality of code division S multiplexed signals; and second means for combining the intermediate signals to generate the output signal, comprising a plurality of wavelengths, each ~;vavelength for carrying the intermediate signal of one of the first means; wherein the input signals comprise modulated optical input signals, and wherein each of the first means comprises: a plurality of means for phase shifting an optical signal, each means responsive to a modulatt,d 10 optical input signal and to one of a plurality of orthogonal code sequences for generating a modulated lightwave output signal, and third means comprising optical amplification means for combining the modulated lightwave output signals oE the plurality of phase shiEters for generating the intermetliate signal.
Brief ~escril~tion of the l)rawin~
:~5 FIG. 1 is a conceptual ~liagram of a code division switch;

.:; r~
I

,, 1313430 ~;IG. 2 is a dia~am of a photonic code division switch;
FIG. 3 is a diagram of a photonic code division multiple~er, FIG. 4 is a diagram of a photonic code division/wavelength division switch;
S FIG. 5 is an illustration of a phase shifter for use in a code division multiplexer and demultiplexer;
FIG. 6 is a diagram of a wavelength multiplexer;
FIGS. 7 and 8 are diagrams of code division/wavelength division demultiplexers; and FIG. ~ is a diagrarn of an addldrop multiplexer.
Detailed Description 1. Introduction . _ _ The use of Code Division Multiplexing (CDM,~ in a photonic switch has been described in T. K. Gustafson and P. W. Smith, Photon~c Switchin~ (book), Springer-Verlag, 1988, pp. 167-70. CDM can be used to make each input channel orthogonal to every other input channel so that the surn of all the orthogonalized input channels can be presented to a decoder at each output port of the switch. The actual output signal on each por~ can be selected to be any one of dle input signals by using the appropriate code for selection. This is possible because of the co~relalion 20 property of an orthogonal code family.
T

f ~i~j(t)dt = 1 for i = j = O for i ~ j ~i and ~j are members of the code family that are used to orthogonalize the input bits, and T is the duration of any code sequence and the duration of any input bit.
25 This is the means by ~vhich any input signal can be switched to any or all out3~uts (customers). ~erefore, the resultant switch is non-b~ocking with broadcast capability.
The basic s~ucture of a switch that uses an orthogonal code family is shown in FIG. 1. Members of a first g~oup of n multipliers 100 are each connected 30 to one of the inputs, input 1, input ~,...input n, and to a memb~r of the ordlogonal code sequence set, ~1 (t),~2(t),...,~n(t), respectively. The outputs of each of the n multipliers are connected to a summing circuit 102 whose output in ~urn is connected to a second group of multipliers 100, each of which has as one input the sum signal (i.e., the output of summing circuit 102) and an appropriate one of the code sequences for selecting the desir~d signal. 'Ihe output of e~ch second block 1~ is connected to an integrator 107 whose output represents the selected signal, i.e., the input associated with the coding sequence that is applied to ~e multiplier 100.
S In mathematical terms, the input signals are each multiplied by the different members of the orthogonal code family. Therefore, the orthogonalized inputs can be represented as ~j(t~ x input bits, where the input bits are a 1 or -1 for a duration of T, and ~i(t) = 1 or -1 for a duration l'ln. If the input is a baseband signal, the output is a baseband signal of n times the input bandwidth.
Therefore, an input to the summing circuit can be expressed as, mj(t)~j(t) (2) where mi(t) is the informaeion (~1 or -1) and ~j(t) is the orthogonal code for input channel i. The output from a multiplier on the right of FIG. 1 that is also fed with ~i(t) is:

ml (t)~l (t)~i(t) + .' ~ mi-l (t)q)i-l (t)~i(t) + mi ~t)~i2 (t) + mi+l (t)~hl (t)~i (t) + -All the unwanted cross terms with ~i (t)~j (t) will integrate to zero from equation 1, and the output from the integrator in FIG. 1 becomes:

~li(t)q)i2(t). (4) 20 If ~i(t) is a polar orthogonal code sequence, ~i2(t) is unity. Therefore, the decoder output becomes mi(t).
If the input entering one of the ~st blocks 100 of FIG. 1 is in optical form, it is already modulated typically as on-off keying or phase shift keying (PSK).
Since the ~1 (t) waveforms repeat for each information bit, the output from any of the 25 multipliers on the left is a PSK signal modulated at the rate determined by ~i(t).
This happens because ~ ) will either multiply the input by a +l or -1. A PSK signal can be represented by using an optical carrier cos~ct in equation 2; equation 2 becomes ~i(t)mi(t)cos~ct = ~coso~ct and equa~ion 4 becomes:

mi(t)~i2(t)Cos2oct = 2~ ) (1 + cos203ct~, ,., ~ ' ~

- " 1 31 3~30 sincç the coherent detection of the optical signal is being performed by injecting both the optical information signal and the optical carrier into a Posidvely doped-Intrinsic-Negadvely doped (PIN) photodiode which approximates a multiplier if the optical carrier is much larger than the optical information signal. Aft~r the output S integrator, m; (t)/2= +1/2. This is an electrical domain version of the desired polar input signal multiplied by one half. There are times when it is desLrable to stay in the optical domain. The following technique can be used !to accomplish this goal.
If the input is in optical form, i~ is already modulated typically as on-off keying (intensity moduladon) although phase shift keying (PSK) is pr~ferred for 10 coherent optical communications. In either case, a bandpass system rather than the baseband system of FIG. 1 is required. Since the ~i~t) wavefomls repeat for each bit, the output from any of the multipliers on the left of the new b~ndpass version of FI5. 1 is a PSK signal modulated at the rate determined by ~i(t). This happens because ~i(t) will either multiply the input by a ~1 or -1. For a PSK signal equation 15 2 becomes ~i(t)mi(t)cosc~ct = icosc~ct and equation 4 becomes, mi(t)~i2(t)cos~ct = mi(t)coscoct after the output bandpass integrator. 'l'his is a modulated version of the desired polaF
input. In actual optdcal implementadon, a phase shifter can be used to make the high frequency PSK op~cal signal under the control of the high frequency orthogonal 20 code that repeats within a given information bit interval. Further, the decoder can be another phase shifter followed by a narrow band opdcal filter if it is desired to maintain a continuous optical path between input and output.
This is illustrated in FIG. 2 which shows physical instead ~f mathematical elements. The phase shifters 101, the equivalent of the multipliers 100 25 of FIG. 1 are each con~rolled by a separate orthogonal sequence ~1 (t),~2~t),...,~n(t).
The ou~put of phase shifters 101 are connected to a passive combiner 103, comparable to the surnmer 102 of FIG. 1. The output of passive cornbiner 103 is connected to the input of passive splitter lOS whose outputs are the inputs to the second group of phase shifters 101. l~e outputs of each member of the second 30 group of phase shifters 101 is connected to a circuit 108 for providing opdcal selectivity and gain.
If it is required to add more inputs to this type of a code switch, it is necessary to increase the bandwidth requirements of the switch proporiionally.
Therefore, the switch size on the input becomes limited by the bandwidth of the ~-`` 1 31 3430 individual components that are available to implement the switch. The oueput size of the switch is limited by the losses associated with the combining circuit 103, and power splitting in the passive splitter 105.
One strategy for a switch design or tlansmission system design that 5 must accommodate a large number of input channels is to group the input channels into m groups of n channels. The total number of input channels that this switchcould handle is equal to n x m. A CD multiplexer is udlized for each group of n channels, and n is chosen to produce a high frequency operation consistent with the bandwidth of the technology that is used for implementadon. This approach makes 10 the speed requirement, and therefore the bandwidth, of the circuitry needed to implement a switch with n x m inputs equal to the r~uirement for an n input CD
multiplexer; a speed reduction by a factor of m compared to that of a CD multiplexer that must handle n x m inputs. A CD multiplexer 109, is shown in FIG. 3, and comprises a plurality of phase shifters 101 and a passive combiner 103.
FIG. 4 shows a complete n x m switch comprising m CD
multiplexers 109, each with n inputs as shown in FIG. 3; a wavelength d;vision (WD) multiplexer 201, discussed hereinafter with respect to FIG. 6; and a passive spl;tter 105 for distributing the output of WD multiplexer 201 to lc different combined wa~elength/code division demultiplexers 205, discussed hereinafter with20 respect to FIG. 7. The number of output ports, k, is limited by the fanout capability from the wavelength division multiplexer 201 and passive splitter 105. If the passive splitter cannot provide sufficient power to each output port, optical amplificadon is used to boost the power.
Wavelength division multiplexing (WDM) is the name given to 25 ~equency division multiplexing in the optical dornain and can also be used in the electrical domain since frequency and wavelength are related. There is great interest in WDM by the use of coherent lightwave communication. The main reason is that apotentially large number of channels can be multiplexed and subsequently demultiplexed (separated) because the high selectivity filtering requirements can be 30 accomplished in the elec~ical domain after the photodetector. This avoids thenecessity for high selectivity in ehe optical domain. However, if a switch or transmission system with a large number of inputs is ne~ed, there is a requirement for a large number of stable light sources to make each input orthogonal to every other input by translating each input channel to a different wavelength~ The switch 35 or transmission system outputs are selected by coherent detection of the optical signal. ~is requires a coherent carrier that is the proper frequency (wavelength) and ;

phase. This in turn requires a large number of tunable lasers that can be phase locked pr~erly or the multiplexing of a stable replica of each fre~quency (wavelength) on each output port for demodulation.
An advantage can be obtained by utilizing both CDM and WDM
5 techniques because both can share a common detector (PIN) and the number of precision lasers needed for modulation and detec~on can be reduced by a factor of n by combining the two techniques in a single switch, as illustrated in FIG. 4.
The output fiber from the WD multiplexer cames m wavelengths and each wavelength is modulated with n CDM input channels. There~ore, every output 10 is presented with all n x m inputs to the switch, and the switch has broadcast capability. Purther, the speed requirements of CDM and the numbe~ of precision lasers required for WDM are both moderated by combining or merging CDM and WDM. Before consideling the merged CD/WD architecture further, it is noted that the basic architecture illustrated in FIG. 4 can also be used to make a code 15 div;sion/space division (CD/SD) switch by replacing the WD multiplexer with anon-blocking space switch. Now the demultiplexer on the right of FrG. 4 is just for CD and the output could stay in optical form if desired.

2. Code DivisionlVVa~elen~th Div}sion Architecture The basic merged code/wavelength division architecture is illustrated in 20 FIG. 4. The code division multiplexer 109, illushated in detail in FIG. 3, can be implemented by using one member of an orthogonal code family to modulate each information bit by shifting the phase of the optical camer during the Wt interval to be in phase half of the time and 180 degrees out of phase during half of the bitin~erval time. This phase shift is controlled by one unique sequence from an 25 orthogonal code farnily for each input channel. The rcsultant signal will be inphase during the time when the code has logical zeros and out of phase when the code is producing logical ones; this is phase shift key (PSK) modulation. This results because of the polar nature of the q, signals; then two states are +1. When the incoming carrier is muldplied by +l, its phase is unaffected; when multiplied by -1, 30 a 180 phase shi~t is effected. Thus the phase shift circuit 101 of FIG. 3 is equivalent ~o the multiplier 100 shown in FIG. 1. Next, all the input signals that have been PSK
modulated in bl~k 101 a~e added together in a passive combiner 103 comparable tothe summer 102 of FIG. 1, to form the CD multiplexer 109.
The phase shifter can be realized by utillzing titanium diffused 35 waveguides in lithium niobate with metal electrodes on ~he surface near a portion of the titanium chaDnel. There is a change in the index of refraction of the material 1 31 3~30 g when an electrical signal is applied to the electrode, this produces the desired phase shift in the optical signal tha~ is propagating in the waveguide.
The device is illustrated in FIG. S which shows a lithium niobate phase shffler for phase shift keying the optical signals. A lithium niobate crystal 503 has a 5 narrow and shallow titanium indiffused waveguide 505 along its length. The waveguide is surrounded on two sides by electrodes 507 and 508 which are connected to the electrical modulating signal. The optical input signal is injected at one end of the waveguide and the ouq~ut appears at the othPr end.
The output ~>f the phas~ shifter is the desired phase shift keyed (PSK) 10 optical signal. Each orthogonal code word that controls the phase of the incoming signal lasts for exactly the duration of an information bit, and then repeats for the nex~ and all subsequent inforrnation bit time intervals. The bandwidth of each coded input channel is increased by a factor that equals the number of bits in the length of the orthogonal code family. However, all the coded input channels occupy the sarne 15 spectral density (frequency range). Further, all the coded channels may be added together and later be separated by correlating each coded channel with the appropriate rnember of the code family that was used for encoding, as suggested by equation 4. This will be performed on the output of the switch to select any input that is desired. However, the center bloek in PIG. 4, the wavelength 20 multiplexer 201, will be described firs~.
The function of the wavelength multiplexer 201, shown in detail in FIG. 6, is to shift the wavelength of the incoming channels from ~e value ~0, that is used for transrnission, probably 1.3 or 1.5 llm, to one of the selected values, ~1, ..., ~m- for wavelength multiplexing, using a plurality of wavelength 25 converters 501, addidvely combined by a passive combiner 103. The ou.tput of the WD multiplexer is a single optical signal that contains the sum of the outputs from the ~D multiplexers; each CD multiplexer output is translated to a predeterminedwavelengdl by the multiplexer.
The output signal from the WD multiplexer 2û1 in FIG. 4 is passed 30 through a passive splitter lOS that splits the signal into equally weighted parts, one for each demultiplexer 205. Each demultiplexer 205 has as inputs representing the wavelengt}ls ~ , and a selected code sequence, one of the k code sequences )(t),~j(2)(t),...,~ ,)(t). Each demultiplexer 205 drives an attached laser and electronic circuit 207 tO genera~e an optical output. The output circuitry combines 35 the demultiplexing of both the code division and the wavelength division processes in one detector.

.
The circuit of block 205 is shown in FIG. 7. The circuit includes m optical modulators 701 each with an input of one of the wavelengths ~l, ..., ~m and each having an attached control signal so tbat only the selected one of these optical modulators is turned on. These optical modulators are lithium niobate optical S modulators in the exemplary embodiment of the invention. The output of these optical modulators is combined in a passive coupler 103 which is connected to a phase shifter 101 whose other input is the selected code sequence. The output of this phase shif~er is then connected to the PIN detector 707 which combines this signal with the combined optical input signal to generate dle selected elecerical oulput.
10 This electrical signal now feeds block 207 (FIG. 4) which contains the appropriate and well-known elec~onics to filter, amplify, and drive a laser di~e also contained in block 207.
A passive splitter 105 may not provide sufflcient energy to drive the required number of demultiplexers. In that case, an optical amplifier would be 15 required at the input to passive splitter 105. A traveling wave or Fabry-Perot optical amplifier can provide the required amplification. This is described in G. Eisenstein et al.: "Measurements of the Gain Spectrum of Near-Traveling Wave and Fabry-Perot Semiconductor Optical Amplifiers at 1.5 ~m, " International Journal of Electronics, vol. 60, no. 1 ,1986, pp. 1 13- 121 .
A PIN or avalanche photo diode (APD) fO7 (PIG. 7) is used to perfonn the coherent detection needed for demultiplexing both multiplexing schemes (bothCD and WD) simultaneously. This is accomplished by selecting a p~rtion of the same source that is used in ~he waveleng~ multiplexing process illustrated in FIG. S
as a second signal to be fed into the PIN. Since the diode produces an output 25 electrical signal that is proportional to the input optical power input, it is acting as a square law device. The detector output, E(t), is given by the following expression.
~2 E(t) = l[A + m(t)]cOs~ctJ

[A2 + 2Am(t) + m(t)2~
( 1 + ~os2cl)0 t) 30 A is the optical calTier amplitude and m(t) is the information signal. The lowpass filter output yu(t) is ~313430 y (t) = A2 ¦1 ~ 2m(t) ~ m(t)2 ]

It is obvious that there is a distortion term present in the conventional fo~n of optical homodyne detection that is not present in the pure produc~ detector that is nonnally used for elec~ical homodyne detection. However9 if the optical carrier is made large S enough, m(t)/A~l, compared to the optical infonnation signal, this distortion ~erm can be made negligiMe. Therefore, the optical canier needs to be sufficiently larger than the infQrmation signal to ensure reliable digital detection. Also, if the LPF is AC coupled, the output is given by the following.

yo-Am(t) ~, 10 FIG. 6 is a block diagrarn of wavelength division multiplexer 201 of FIG. 4. Each of a plura}ity of s;gnals at wavelength ~0 is connected to a wavelength converter 501 to generate a signal at one of the wavelengths ~ m, which ate cornbined in combiner 103. Wavelength division multiplexers are known in the prior art and described, for example, in I. Bar-Joseph et al:
15 "Self-electro-optic Effect Device and Modulation Converter in InGaAs/LrlP Multiple Quantum Wells", Applied Physics Letter, vol 52, no. 1, 4 January 1988, page 53.
These wave division multiplexers comprise rneans for changing the wavelength of a modulated signal such as an integrated circuit with two multiple quantum well (MQW) diodes 601 connected in series, one of the diodes comprising matçrial 20 responsiw to an input wavelength, the other diode comprising material responsive to the desired output wavelength.
FIG. 8 shows an alternative embodiment of a CD/WD
demultiplexer 800, operating in the opdcal domain. If demultiplexer B00 is used instead of demultiplexer 205 and laser and elect~onics 207, then ehe switch of PIG. 4 25 operates in a fashion to provide a continuous optical path from input to ou~ut.
Demuldplexer 800 comprises a ~nable optical filter 801, operative under electrical control, for selecdng a wavelength. This wavelength is transmitted to ~ wavelength converter 501 whose output is a light signal of the standard wavelength ~. If the wavelength converter 501 cannot be made sensitive to all outputs of the tunable 30 filter 801, then a group of wavelength converters 5û1 can be placed in parallel and the appropriate converser output gated for transmission to phase shifter 101. The output of wavelength converter 501 is tnen transmitted to phase shifter 101, ~ 3 1 3430 controlled by an electrical signal representing the selected orthogonal sequence ~i.
The OUtpllt of phase shifter 1()1 then is a light wave of the standard wavelength ~0, modulated only by the selected signal. Tunable optical filters are well known in the art and are discussed, for example, in S. R. Mallinson: "Wavelength-Selective Filters 5 for Single-Mode Fiber WDM Systems Using Fabry-Perot Interferometers", Applied Optics, vol. 26, no. 3, February 1987, pages 430-436; F. Heismann et al.: "Narrow Band Double-Pass Wavelength Filter With Broad Tuning Range", Integrated and Guided Wave Optics Conference Record, Santa Fe, New Mexico9 March 28-30, 1988, pages 103-106; and W. V. Sorin et al.: "THB4 Tunable and Single-Mode Fiber10 Reflective C;ating Filter", Inte~rated Optics and Optical ~iber Communications Conference, Reno, Nevada, January 19-22, 1987, page 194. Alternatively, the relative positions of the wavelength converter and phase shifter may be reversed; the arrangement shown in FIG. 8 offers the advantage of permitting a phase shifter to work at one wavelength.
The switch architecture des-~ribed above merges CDM with WDM to moderate the more difficult requirements of each approach when a large switch ortransmission system is re~quired. A potential application for an optical switch in the local environment is likely in the 1990's since broadband ISDN (B-ISDN) is likely to become a reality in the early or rnid 1990's. Such a system requires individual 20 video channels with a n~inimum data rate o~ 45 Mb/s for present TV signals and data rates as large as 400 Mb/s are expected for high definition TV (HDTV) signals. It is likely that a rate of about 140 Mb/s will be used as a common rate for both the USA
and Europe. These systems will use switches that may require approximately lO0 inputs to be compedtive with CATV systems. Further, it is likely that an enhanced 25 level of TV quality (preferably high definition quality if the cost is not too high) will be required to show product improvement for E~-ISDN over the present CATV.
These inputs should be switched to hlmdreds and perhaps thousands of outputs (customers). If 16 channels are CD muldplexed together on the input of aswitch illustrated in FIG. 4, and six wavelengths are used in the WD multiplexer, the 30 switch can accept 96 input channels. These inputs can be sent to any of hundreds and potentially thousands of outputs if the fanout capability of the technology used will pelm~t.
Instead of making one code switch that will accept 96 inputs and require processing at 96 times the data rate of the input channels (as high as 96 x 140 Mb/s -35 13.4 Gb/s,~, the resultant upper lirnit for data rate is only 16 x 140 = 2.24 Gb/s.Alternatively, if a WD switch were used for all 96 inputs, it would require 1 3 1 3~30 96 wavelengths, multiplexed lasers OT tunable phase-locked lasers. However, thisswitch only requires 6 wavelengths.
The output ports of the merged switch are almost the same complexity as if a CD or WD switch were implemented and the input ~ports only requirç simple 5 phase shifters, a rela~vely simple device. Since the center stage only requires 6 wavelengths in a WD multiplexer, it is simpler than a pure WD switch. The result is a switch that could be implemented if the fanout requirements can be met. Thefanout requirements equal the number of switch outputs (customers).
The aIrangement of FIG. 4 can be used as a switch with a plurality of lû inputs to each of the CD multiplexers 109 and a plurality of outputs, one from each of the combined demultiplexers 205. For applications such as cable television, the output of the WD demultiplexer 201 is connected to a fiber for distribution and each of the demultiplexers 205 are connected to a tap on that cable.
For add-drop multiplexing applications in which a plurality of signals 15 are withdrawn at a switch and another plurality of signals are injected into the optical signal at that switch, the configuration of FIG. 9 may be used. In this example, an incoming light signal has components at wavelengths ~ 3, ~4 and ~5.
Additional optical inputs 1 l,...,ln and 21,...,2n are multiplexed in CD
multiplexers 109 of FI&. 1. The outputs of the two CD multiplexers 109 of FIG. 920 are each connected to one of two wavelength converters 501 which have outputs at wavelengths ~4 and ~5 respectively. Three filters 901 are used to preserve the 2 and ~3 components of the incoming light signal. The outputs of these filters 901 and the two CD multiplexers 109, are then com~ined in passive coupler 103 to produce a lightwave output at wavelengths ~ 2. ~3, ~4 and ~5 .
25 To extract signals from the incoming lightwave, the incoming lightwave is connected to passive splitter 105 which is connected to a plurality of CD/WD
demultiplexers 205 to generate output signals l,...,p. This is similar to the method of extracting signals described with respect to FIG. 4. This arrangement then permits another source to generate signals conveyed at the ~ 2, and ~3 wavelengths, and 30 the source of FIG. 9 to generate signals conveyed at the ~4 and ~5 wavelengths. The optical circui~y of the multiplexer arrangement of FIG. 9 ex~acts input signals calTied at the ~ 2 and ~3 wavelengths and injects new optical signals at ~4 and ~5 wavelengths. The light may be carried, for example, over a photonic ring interconnecting the various signal sources and destinations.

It is to be understood that the above description is only of one p~efeIred embodiment of the invention. Numerous other a~rangements may be devised by one skilled in the art without departing from the spint and scope of ~e inven~on. The invention is thus lirnited only as defined in the accompanying claims.

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Claims (17)

Claims

1. A two-stage multiplexer for multiplexing a plurality of input signals to generate an output signal comprising:
a plurality of first means, each means responsive to a plurality of said input signals for generating an intermediate signal comprising a plurality of code division multiplexed signals; and second means for combining the intermediate signals to generate the output signal, comprising a plurality of wavelengths, each wavelength for carrying the intermediate signal of one of the first means;
wherein the input signals comprise modulated optical input signals, and wherein each of the first means comprises:
a plurality of means for phase shifting an optical signal, each means responsive to a modulated optical input signal and to one of a plurality of orthogonal code sequences for generating a modulated lightwave output signal, and third means comprising optical amplification means for combining the modulated lightwave output signals of the plurality of phase shifters for generating the intermediate signal.

2. A two-stage multiplexer for multiplexing a plurality of input signals to generate an output signal comprising:
a plurality of first means, each means responsive to a plurality of said input signals for generating an intermediate signal comprising a plurality of code division multiplexed signals; and second means for combining the intermediate signals to generate the output signal, comprising a plurality of wavelengths, each wavelength for carrying the intermediate signal of one of the first means;
wherein the second means comprises:
a plurality of third means, each for converting an incoming lightwave of a first wavelength to an outgoing lightwave of one of a plurality of second wavelengths; and fourth means comprising optical amplification means for combining, the outgoing lightwaves of the plurality of third means for generating the output signal.

3. A signal demultiplexer for extracting an output signal from a multiplexed signal, comprising:

first means for selecting one of a plurality of wavelengths and for generating a first intermediate signal of the one selected wavelength;
second means for combining the first intermediate signal with one of a plurality of orthogonal code sequences to generate a second intermediate signal; and third means for extracting the output signal by combining the second intermediate signal and the multiplexed signal;
wherein the third means comprises a PIN photodiode responsive to the second intermediate signal for extracting the output signal from the multiplexedsignal.

4. A photonic signal demultiplexer for extracting a modulated optical signal at a preselected wavelength from a multiplexed optical signal, comprising:
first means responsive to the multiplexed signal for generating a first intermediate signal by selecting one of a plurality of wavelengths of the multiplexed signal;
second means responsive to the first intermediate signal for converting the first intermediate signal to a second intermediate signal at the preselectedwavelength; and third means, responsive to the second intermediate signal and to one of a plurality of orthogonal code sequences for generating the modulated optical signal;
wherein the second means comprises an integrated circuit with two multiple quantum well (MQW) diodes connected in series, one of the diodes comprising material responsive to the wavelength of the first intermediate signal, the other diode comprising materialresponsive to the preselected wavelength.

5. A photonic switch comprising:
a two-stage multiplexer for multiplexing a plurality of input signals to generate a multiplexed signal; and a plurality of demultiplexers each for extracting an output signal from the multiplexed signal;
each of said demultiplexers comprising:
first means for selecting one of a plurality of wavelengths and generating a first intermediate signal of the selected wavelength;

second means for combining the first intermediate signal with one of a plurality of orthogonal code sequences to generate a second intermediate signal; and third means for extracting the output signal by combining the second intermediate signal and the multiplexed signal;
said two-stage multiplexer comprising:
a plurality of fourth means, each responsive to a plurality of said input signals for generating a third intermediate signal comprising a plurality of code division multiplexed signals; and fifth means for combining the third intermediate signals to generate the multiplexed signal comprising a plurality of wavelengths, each wavelength for carrying the signal of one of the fourth means.

6. A photonic signal demultiplexer for extracting an output signal from a multiplexed photonic input signal, comprising:
first means for selecting one of a plurality of wavelengths and for generating a first intermediate signal of the one selected wavelength;
second means for combining the first intermediate signal with one of a plurality of orthogonal code sequences to generate a second intermediate signal; and third means for extracting the output signal by combining the second intermediate signal and the multiplexed signal;
wherein the first means comprises a plurality of lithium niobate optical modulators each responsive to an optical signal of a different one of the plurality of wavelengths for generating an output signal; and a passive optical coupler for combining the output signals from the optical modulators for generating the first intermediate signal;
wherein each orthogonal code sequence comprises a plurality of 1's and a plurality of -1's, and the second means comprises a phase shifter for an optical signal, the phase shifter comprising a lithium niobate crystal with a titanium indiffused wave guide along its length and metallic electrodes connected to the one of a plurality of orthogonal code sequences;
wherein the third means comprises a PIN photodiode responsive to the second intermediate signal for extracting the output signal from the multiplexedsignal.

7. A signal demultiplexer for extracting an output signal from a multiplexed signal, comprising:
first means for selecting one of a plurality of wavelengths and for generating a first intermediate signal of the one selected wavelength;
second means for combining the first intermediate signal with one of a plurality of orthogonal code sequences to generate a second intermediate signal; and third means for extracting the output signal by combining the second intermediate signal and the multiplexed signal;
wherein the third means comprises an avalanche photo diode (APD) responsive to the second intermediate signal for extracting the output signal from the multiplexed signal.

8. A two-stage multiplexer for multiplexing a plurality of input signals to generate an output signal comprising:
a plurality of first means, each means responsive to a plurality of said input signals for generating an intermediate signal comprising a plurality of code division multiplexed signals; and second means for combining the intermediate signals to generate the output signal, comprising a plurality of wavelengths, each wavelength for carrying the intermediate signal of one of the first means;
wherein the second means comprises:
a plurality of third means, each for converting an incoming lightwave of a first wavelength to an outgoing lightwave of one of a plurality of second wavelengths; and fourth means for combining, the outgoing lightwaves of the plurality of third means for generating the output signal;
wherein each of the third means comprises an integrated circuit with two multiple quantum well (MQW) diodes connected in series, one of the diodes comprising material responsive to the wavelength of the first intermediate signal, the other diode comprising material responsive to the preselected wavelength.

9. Multiplexer means for deleting output signals from an incoming lightwave signal and adding input signals to an outgoing lightwave signal, comprising:

first means responsive to a plurality of input signals for generating a first intermediate signal comprising a plurality of code division multiplexed signals at a first wavelength;
second means responsive to the first intermediate signal for generating a second intermediate signal at a second wavelength;
third means responsive to the incoming lightwave signal for filtering selected wavelengths to generate third intermediate signals, and fourth means for combining the second and third intermediate signals for generating the outgoing lightwave signal.

10. The multiplexer means of claim 9, wherein the input signals comprise modulated optical input signals, and wherein the first means comprises:a plurality of means for phase shifting optical signals, each means responsive to a modulated optical input signal and to one of a plurality of orthogonal code sequences for generating a modulated lightwave output signal; and fifth means for combining the modulated lightwave output signals of the plurality of phase shifters for generating the first intermediate signal.

11. The multiplexer means of claim 10 wherein each orthogonal code sequence comprises a plurality of l's and a plurality of -l's, and each of the means for phase shifting comprises a phase shifter for an optical signal comprising a crystal with an indiffused wave guide along its length and metallic electrodes connected to electrical code division multiplex input signals.

12. The multiplexer means of claim 11 wherein the crystal is a lithium niobate crystal.

13. The multiplexer means of claim 11 wherein the metallic indiffused wave guide comprises titanium as an indiffusion material.

14. The multiplexer means of claim 9, further comprising:
another first means for generating another first intermediate signal at the first wavelength; and another second means, responsive to the another first intermediate signal for generating another second intermediate signal at a third wavelength different from the second wavelength;

wherein the fourth means combines the another second intermediate signal with the second and third intermediate signals for generating the outgoing lightwave signal.

15. The multiplexer means of claim 9 wherein the second means comprises:
an integrated circuit with two multiple quantum well (MQW) diodes connected in series, one of the diodes comprising material responsive to the first wavelength, the other diode comprising material responsive to the second wavelength.

16. The multiplexer means of claim 9 wherein the fourth means comprise passive optical combining means for additively combining the modulated signal togenerate the outgoing lightwave signal.

17. The multiplexer means of claim 9 wherein the fourth means comprise optical amplification means for generating the outgoing lightwave signal.