Optical frequency shifter and transmission system

[0002] This invention relates to optical frequency shifters and their applications.

[0003] It is desirable to have exactly spaced carrier frequencies for optical transmission systems. This invention provides for such exactly spaced carrier frequencies. It is also desirable to frequency shift previously modulated optical signals for the purpose of switching and retransmission.

[0004] The first object is a frequency shifter. It is a second object of the invention to create optical carrier signals spread at intervals of exactly ω_mwhere ω_mis a microwave frequency in the order of 5-50 GHz.

[0005] According to an aspect of the invention, there is provided an optical transmission system, comprising an input light guide and N frequency shifters connected to receive light from the input light guide and output light along N output light guides where N is 1 or more, wherein the light carried by each ith output light guide, where i=1, . . . , N, is translated in frequency from light entering the input light guide by iω_m, and where each frequency shifter contributes a frequency translation of ω_m.

[0006] Each of the N output light guides may be equipped with a modulator to impress an information signal on each output light guide. When the modulated outputs, each of different wavelength, are combined, the result is a wavelength division multiplexed optical signal.

[0007] In a further aspect of the invention, the optical transmission system is equipped with a cascade of N frequency shifters at the receiver to generate frequencies ω₀+iω_mwhich may be used for self-homodyne square law direct detection of the information signals.

[0008] In a further aspect of the invention, there is provided an optical frequency shifter for translating an input optical frequency by a given amount, which may be used for wavelength placement on outgoing fibers in an optical switch in a wavelength division multiplexing system.

[0009] These and other aspects of the invention are described in the detailed description of the invention and claimed in the claims that follow.

[0010] There will now be described preferred embodiments of the invention, with reference to the drawings, by way of illustration only and not with the intention of limiting the scope of the invention, in which like numerals denote like elements and in which:

[0011] FIG. 1A is a schematic representation of a first embodiment of a frequency translator according to the invention;

[0012] FIGS. 1B-D are phasor diagrams representing the manner of operation of the device of FIG. 1a;

[0013] FIG. 2 is a schematic representation of a second embodiment of a frequency translator according to the invention;

[0014] FIG. 3 is a schematic representation of first embodiment of a transmission system incorporating a frequency translator according to the invention; and

[0015] FIG. 4 is a schematic representation of receiver for use with the transmission system of FIG. 3;

[0016] FIG. 5 is a chart showing the frequency content of the signal in the transmission path of FIGS. 3 and 4; and

[0017] FIG. 6 is a schematic representation of a third embodiment of a frequency shifter according to the invention.

[0018] A Mach Zehnder modulator is a balanced bridge interferometer that allows amplitude modulation of the optical carrier. The Mach Zehnder modulator is known in the art in itself, and so only the characteristics important to the present invention are described here. The ideal modulator consists of an optical splitter coupled to two identical phase modulating waveguides forming the two arms of the modulator, which waveguides are in turn connected to an optical combiner. The refractive index of the waveguides is electronically modulated by application of a potential to electrodes either covering or adjacent to the waveguides. The basic input/output relation for the Mach Zehnder modulator is

[0019] E_out=E_in/2[exp(−jπv₁(t)/V_π)+exp(−jπv₂(t)V_π+φ_b)]

[0020] where v₁(t) or v₂(t) are the applied voltages.

[0021] Referring to FIG. 1A, there is shown an optical frequency translator which accepts as input light having a frequency ω₀and outputs light having a frequency ω₀+ω_m. The optical frequency translator of FIG. 1a, has an input light guide 10, a two way splitter 12, and a set of two dual path Mach Zehnder (MZ) modulators 14 in parallel which are each connected to receive light from the input light guide 10 via the splitter 12. Light output from the two MZ modulators is combined through combiner 16 and output along output light guide 18. Each of the MZ modulators has an associated voltage source 19 which supplies a modulating voltage to each MZ modulator 14. The voltage sources 19 have output voltages characterized such that modulated light appearing on the output light guide 18 is translated in frequency by a frequency ω_mfrom light input to the input light guide 10. An amplifier 17 is provided on the output light guide 18 to compensate for K₁.

[0022] In the case of the four path structure of FIG. 1a, the voltages V₁-V₄may comprise the following voltages:

[0023] V₁=ε+Vπ/2 (1−K cos ω_mt)

[0024] V₂=ε−Vπ/2 (1−K cos ω_mt)

[0025] V₃=Vπ+Vπ/2 (−K sin ω_mt)

[0026] V₄=−Vπ/2 (−K sin ω_mt)

[0027] Vπ is the voltage required at one electrode of the MZ to change the optical output from full intensity to extinction. ε is a small bias voltage used to adjust the delay between the upper and lower dual path MZ modulators. In the phasor diagrams, FIG. 1B-1D, the voltages V₁-V₄cause the phase of the light to vary according to the phasors indicated by the respective voltages. For the optical signal, the inphase optical component I (FIG. 1B) is driven by cosω_mt (V₁and V₂) and the quadrature optical component Q is driven by sinω_mt (V₃and V₄). The resultant sum I+Q is shown in FIG. 1D. This combination of voltages can be shown to produce an output optical frequency ω₀+ω_mat the output light guide 18 as follows:

[0028] For input optical electric field

[0029] Ein=cos ω_ot=Re{e^jωt}

[0030] and for small values of the modulation coefficient K the output electric field is given by the following where K₁=πK/2

[0031] Eout=K₁cos ω_otcos ω_mt+K₁sin ω_otsin ω_mt

[0032] =K₁cos ω_ot(cos ω_mt+jsin ω_mt)

[0033] =K₁Re[e^jωt] (cos ω_mt+jsin ω_mt)}

[0034] =K₁Re[e^j(ωt.e^jωt]

[0035] =K₁Re{e^j(ω+ω)t}=K₁cos(ω_o+ω_m)t

[0036] The frequency shift can be +ω_mor −ω_mdepending on the polarity of bias voltages or microwave voltages on the electrodes.

[0037] An alternative structure is shown in FIG. 2, in which an optical frequency translator is formed with an input light guide 20, a three way splitter 22, and a three path modulator 24 in which each path is connected to receive light from the input light guide 20 via the splitter 22. Light output from the three path modulator 24 is combined through combiner 26 and output along output light guide 28. Each of the modulator paths has an associated voltage source 29 which supplies a modulating voltage to each modulator path. The voltage sources 29 have output voltages characterized such that modulated light appearing on the output light guide 28 is translated in frequency by a frequency ω_mfrom light input to the input light guide 20. An amplifier 27 is provided on the output light guide 18 to compensate for K₂. The device shown in FIGS. 1A and 2 is a frequency shifter (labelled FS in the drawings).

[0038] A selection of the phases of the input voltages applied by the voltage sources 29 to the MZ modulations 24 that results in the desired frequency translations is shown in the following equation:

[0039] E_o=Re{e^jωot[K₂cosω_mt+K₂(−1/2+j.866)cos(ω_mt+2π/3)+K₂(−1/2−j.866)cos(ω_mt+4π/3)]}

[0040] Several sets of multiple path modulators FS configured as shown in either of FIGS. 1 or 2 may be combined to form an optical transmission system. Each multiple path modulator preferably applies the same frequency shift ω_m, but it is possible to have different frequency shift in each succeeding modulator. The multiple path modulator can be termed as a frequency shifter. Such an optical transmission system is shown in FIG. 3 in which N frequency shifters 34 are connected to receive light from an input light guide 30 and output light along N output light guides 38. N may be 1 or more, but is preferably greater than 1. Each frequency shifter contributes a frequency translation of ω_m. Thus, the light carried by each ith output light guide, where i=1, . . . , N, is translated in frequency from light entering the input light guide by iω_m.

[0041] In the optical transmission system of FIG. 3, information may be carried by supplying dual arm MZ modulators 37 or other intensity and/or phase modulators on each output light guide 38. Each MZ modulator 37 is a conventional dual path MZ modulator for modulating an information signal m_i(t) onto light carried by the output light guide. In this way, N information signals m_i(t) where i=1, . . . , N may be carried by the optical transmission system. The output from the MZ modulators 37 is combined for transmission along an optical fiber 40 for long distance transmission. The combined output spectrum is illustrated in FIG. 5.

[0042] Exactly spaced carrier frequencies have several advantages. In non-linear systems, the main intermodulation frequencies or beat frequencies due to carrier components fall exactly on adjacent carrier components and do not interfere with the modulation components. If narrow spacing is used and the number of optical carrier frequencies is large, the transmission resembles discrete multione digital subscriber line transmission. An advantage is that dispersion compensation or equalization is not required up to 1000 km if each carrier handles only 2.4 Gb/s.

[0043] Exactly spaced carrier frequencies allow for carrierless transmission if the receiver can determine the base optical frequency ω₀and the spacing ω_mbetween the carriers. As shown in FIG. 4, an optical receiver 42 at the end of the optical fiber 40 is conventionally connected to receiver light from the output light guides that has passed along the long distance transmission path 40. The optical receiver 42 may comprise N bandpass filters for isolating the sub-bands of information signals about each carrier frequency ω₀+iω_m, where i=1, . . . , N and ω₀is the frequency of light carried by the input light guide. However, as shown in FIG. 4, it is preferred that the sub-bands (clearly illustrated in FIG. 5) may be translated to baseband through self-homodyne direct detection using an added optical carrier ω₀+iω_madded at 43 and synthesized at the receiver using the base optical frequency ω₀plus some integer multiple of ω_m. The synthesizer is composed of frequency extractors 45 and 46 and frequency shifters 44. The added carrier is synthesized by firstly detecting the optical carrier ω₀at extractor 46 connected by coupler (splitter) 48 to optical fiber 40 and detecting the carrier spacing ω_min extractor 45 which is connected by splitter 47 to the optical fiber 40. Output from the extractor 45 is supplied as sin and cos control voltages with frequency ω_mto frequency shifters 44, which are each configured as shown in FIGS. 1A or FIG. 2. The output from extractor 46 is light with a frequency of ω₀, and this is supplied to the series of frequency shifters 44 as an input in the same manner as the inputs for the frequency shifters shown in FIGS. 1A or 2. The output from each successive frequency shifter 44 is ω₀shifted by successive integer multiples of ω_m. The output from each successive frequency shifter 44 is mixed with the signal from the optical fiber 40 at mixers 43, and then the information signals A, B etc are detected at detectors 49 by self-homodyne square law direct detection. The light from the optical fiber 40 is provided to the mixers 43 through splitters 41.

[0044] An advantage of reinserting carriers at the receiver is that the light level on the fiber can be considerably reduced, thus reducing non linear effects in transmission or allowing a higher transmission level which improves immunity to noise. Also the insertion at the receiver of large optical carrier at the desired channel frequency of each output eliminates the requirement for an optical filter to separate each output wavelength prior to detection (demodulation). This is especially useful in systems where the optical frequency spacing is very small and it is difficult to build stable narrowband optical filters.

[0045] This frequency shifter may also be used for wavelength placement on outgoing fibers in an optical switch in a WDM (wavelength division multiplexing) system. In this case, an optical carrier and all its associated modulation components can be shifted to a higher or lower wavelength.

[0046] In an alternative configuration, frequency shifting may be obtained by first forming a carrier with an optical single sideband, as for example disclosed in R. Olshansky, “Single sideband optical modulator for lightwave systems,” U.S. Pat. No. 5,301,058, 1994. The carrier is then cancelled through addition of unmodulated light at the frequency ω₀. another method of forming an optical single sideband uses an amplitude MZ modulator followed by a phase modulator. A configuration of amplitude and phase modulator for use in the present invention is shown in FIG. 6. Light with frequency ω₀is input along fiber 60 to an upper path 61 through splitter 62. Upper path 61 has a conventional dual arm Mach Zehnder modulator 63, whose two arms have applied voltages ±cosω_mt, followed by a phase modulator 66 to which is applied voltage sinω_mt. With this configuration, light appears along the upper path at 68 as a single sideband signal having a frequency of ω₀+ω_m. The upper path also contains a component ω₀, which requires cancellation as for example with the lower path 71 shown in FIG. 6. In this case, the light on the lower path 71 is provided to a dual arm Mach Zehnder 74 modulator to which is applied a suitable dc bias to adjust the amplitude of the light having frequency ω₀on path 71 so that it will match the amplitude of the light with frequency ω₀on path 61. Following the modulator 74 is a phase modulator 76 to which is applied a suitable voltage to phase shift the light on path 71 so that it cancels the component with frequency ω₀on path 61. The result is a single sideband signal with frequency ω₀+ω_m. A cascade of similar frequency shifters can also provide serial phase shifts to generate signals with frequency ω₀+iω_m, where i is an integer.

[0047] Dual arm MZ modulators, amplitude modulators, phase modulators, splitters, combiners, optical fibers, detectors, extractors, voltage sources for MZ modulators are all well known in the art in themselves, and need not be further described. In cases where an optical fiber is described, it will be understood that any suitable light guide may be used, as for example a waveguide.

[0048] In the embodiments shown in FIGS. 1A and 2, it is believed that a modulator with a minimum of three paths is required in order to provide progressive phase changes to the phase change provided by the modulator. The number of paths may be five or more. While the preferred embodiment described uses the upper side band and generates optical frequencies ω₀+iω_m, it is equally useful to use the lower sideband and generate optical frequencies ω₀−iω_m.

[0049] A person skilled in the art could make immaterial modifications to the invention described in this patent document without departing from the essence of the invention that is intended to be covered by the scope of the claims that follow.