WO2007148056A1

WO2007148056A1 - Optical frequency comb generator

Info

Publication number: WO2007148056A1
Application number: PCT/GB2007/002240
Authority: WO
Inventors: Pengbo Shen; Philip Anthony Davies; Nathan Joseph Gomes
Original assignee: University Of Kent
Priority date: 2006-06-22
Filing date: 2007-06-15
Publication date: 2007-12-27
Also published as: EP2036225A1; GB0612419D0

Abstract

An optical frequency comb generator for generating from first and second signal sources a plurality of signals with fixed frequency spacing derived from the second signal source frequency. The optical frequency comb generator comprises an optical ring device comprising an optical phase modulator, and a control circuit comprising a slow control portion and a fast control portion, wherein the slow and fast control portions are for compensating variations of a first frequency range and a second frequency range, respectively. The control circuit is arranged to compare a signal generated by the optical ring device with a reference signal and to generate a control signal in response to the comparison. The optical phase shifting components such as fibre stretchers, phase shifters and phase modulators can be arranged to modulate the phase of a signal in the optical ring device in response to the control signal generated by the control circuit.

Description

OPTICAL FREQUENCY COMB GENERATOR

Background of the invention

This invention relates to the field of communication systems, and more particularly to the field of optical frequency comb generators (OFCGs).

Interest in optical frequency comb generators (OFCGs) has grown for their use in a range of applications, for example from references/wavelengths for dense wavelength division multiplexed (DWDM) optical communication systems, to references/signals in microwave to THz signal generation and transmission systems.

OFCGs may employ different techniques for the generation of the comb line frequencies, such as high pulse powers combined with^, fibre nonlinear effects, free- space optical resonators and fibre resonators. Fibre-loop-based OFCGs can operate in a self-oscillation regime, wherein the comb generator is a source with simultaneous output at multiple optical frequencies. However, in this case, while the frequency difference between the comb line frequencies may be determined by a microwave source (within certain bandwidth limitations), the absolute optical frequencies of the lines are dependent on the cavity resonances set up by the fibre loop.

In many cases, it is preferable to have defined optical frequencies referenced to some "reference laser" source, as well as a well-defined frequency difference. For this, the fibre loop can be configured to permit multiple passes for modulation of the reference signal, and the generation of large numbers of modulation sidebands (generating the optical frequency comb), rather than for setting up oscillations at the resonant frequencies enhanced by the modulation. Clearly, in this case, matching of the resonant frequencies of the fibre loop to those of the modulation signal is still necessary.

So that the active fibre loop structure provides the advantages of flat gain with low ripples (due to limited back reflection), and gain independent of the input/output coupling ratio, the round-trip-gain in the loop needs to be close to but lower than unity. It is this form of all- or partially-fibre-based OFCG which is of significance to the invention presented hereinafter. However, many of the stabilisation techniques proposed are also applicable to self-oscillating, fibre ring OFCGs.

Until now, fibre-based comb generators have been implemented either with broad spectral sources which excite a number of fibre loop resonances within each of the optical comb lines (supermodes) or by using fibre stretchers as optical phase shifters, these providing the required magnitude in the phase change, albeit at low speeds.

The use of broader spectral sources is not desirable for many applications which require low phase-noise signal generation. As for fibre stretchers, they typically need to have a range of a few millimetres to be able to compensate the cavity length changes due to temperature variations. However, some ultra-stable "reference laser" sources, such as fibre lasers, possess higher frequency components in their noise spectra, which cannot be corrected by the slow fibre stretchers. The drift may lie within the short term linewidth of the lasers but may still have a significant frequency deviation. Such drift directly results in a changing optical phase matching error, and results in optical comb shape instability.

The direct coupling between the optical input and output of the OFCG also creates additional instability of the optical comb. The comb can be dominated by the input reference light, causing a strong central comb line which may have a different polarization compared to the other comb lines. ^;

It is, therefore desirable to realise an OFCG that, amongst other things, demonstrates improved levels of comb shape stability.

Summary of the invention

According to the invention, there is provided an optical frequency comb generator for generating from first and second signal sources a plurality of signals with fixed frequency spacing derived from the second signal source frequency, the optical frequency comb generator comprising: an optical ring device comprising an optical phase modulator; and a control circuit comprising a slow control portion and a fast control portion, the control circuit being arranged to compare a signal generated within the optical ring device with a reference signal derived from the second signal source and to generate a control signal in response to the comparison, wherein: optical phase shifting components such as fibre stretchers, phase shifters and phase modulators can be arranged to modulate the phase of a signal in the optical ring device in response to the control signal generated by the control circuit, the slow control portion is for compensating variations of a first frequency range in the generated plurality of signals; and the fast control portion is for compensating variations of a second higher frequency range in the generated plurality of signals.

Thus, the invention provides an OFCG that can be locked to an ultra-stable, narrow linewidth reference laser so that the coherence of comb lines generated by the OFCG may be preserved over considerable propagation distances.

The OFCG may further comprise an integrating unit arranged to integrate the control signal generated by the loop control circuit and to output the result of the integration. In this way, long term temperature drift can be detected and compensated for.

Further, the optical ring may be enclosed within a thermally insulated environment to minimise temperature effects on the loop (particularly effective length changes). This box may employ active/passive temperature control in order to provide a temperature stable environment for the OFCG fibre loop.

The OFCG may be implemented in an electromagnetic transmission and reception system. Also, the OFCG may be used in an imaging system. Of course, other potential applicants will be apparent to the skilled reader, such as it use fr measurements/metrology, transfers of frequency/time standards.

According to a second aspect of the invention, there is provided a method of generating from first and second signal sources a plurality of signals with fixed frequency spacing, the method comprising the steps of: comparing the generated plurality of signals with fixed frequency spacing with a reference signal derived from the second signal; generating a control signal in response to the comparison; modulating the phase of the plurality of signals, which can be in response to the generated control signal; compensating for variations of a first frequency range in the generated plurality of signals; and compensating for variations of a second higher frequency range in the generated plurality of signals.

Embodiments may provide a method of separating of the optical input and output to avoid potentially destructive interference in the output of the optical comb generator.

Embodiments may also provide a method for providing phase matching for the error correction loop over a wide operational frequency range without the need for incorporating additional long delay lines.

Brief description of the drawings

Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of an optical frequency comb generator (OFCG) according to a first embodiment of the invention;

Figure 2 shows a modification of the OFCG of Figure 1 , wherein a fast optical phase shifter (or fast fibre stretcher) is cascaded with the phase modulator to provide the fast correction;

Figure 3 is a schematic diagram of an optical frequency comb generator according to a second embodiment of the invention;

Figure 4 shows the method for matching the phases of the signals in the error correction loop according to the fourth aspect of the invention; Figure 5 shows an optical frequency comb generator according to an embodiment of the invention applied to a THz imaging system; and

Figure 6 shows an optical frequency comb generator according to an embodiment of the invention applied to a wireless communication system. Detailed description

This invention relates to improvements and the maintaining of stability in the generation of comb line frequencies by OFCGs which feature long cavity lengths (>0.3m). Hereinafter, methods are proposed for the implementation of a fast tracking loop with a loop-based OFCG, in addition to known slow tracking techniques. In this way, the invention provides an OFCG that can be locked to an ultra-stable, narrow linewidth reference laser so that the coherence of comb lines generated by the OFCG may be preserved over considerable propagation distances. An aspect of the invention is a dual coupler approach in the coupling of light into and out of the OFCG system, which provides improved stability, particular for the central comb line. Another aspect of the invention is a technique that maintains the locking of the OFCG system while the comb line spacing is tuned, without the need for adjusting the fibre loop length or the phase of the reference signal derived from the second signal.

An OFCG locked to an ultra-stable reference laser source, is capable of generating frequency lines with high absolute and relative (frequency difference) accuracy. Such an OFCG is of utmost importance in numerous technology fields, including millimetre- wave/THz communications.

Referring to Figure 1, an OFCG 100 according to a first embodiment of the invention is illustrated. The OFCG 100 is implemented with separate fast and slow tracking loops. Either a piezoelectric transducer (PZT) or optical phase shifter with large range can be used in the slow tracking loop.

An exemplary configuration of the amplified fibre loop OFCG is shown in Fig. 1. A single mode reference laser 1 (wavelength: X_REF) is incident, via a first polarization controller (PC) 2a, into the fibre loop in which an optical phase modulator 3, a fibre wound piezoelectric transducer (PZT) 4, first 5 and second 6 isolators (ISOs), an erbium-doped fibre (EDF) 7, a wavelength-division-multiplexing (WDM) coupler 8, an adjustable optical delay line 9, and a second PC 2b are connected. For example, the single mode reference laser 1 wavelength, λ_REF_> may be in the optical C-band of wavelength, approximately 1528-1564 nm, and a microwave oscillator with a frequency of approximately 10-20 GHz may be used to provide the reference frequency. However, it will be appreciated that other operating wavelengths or frequencies may be used, for example XR_EF may be in the optical L-band of wavelength (approximately 1570nm-1610nm).

The OFCG 100 emits an optical comb with exact frequency spacing of the spectral lines equal to an RF reference frequency (f_RF) applied to the optical phase modulator 3.

The single-mode reference laser 1 input is phase-modulated at the f_RF frequency spacing to produce multiple spectral lines. The optical loss in the loop is compensated using a pump laser 10 and the EDF 7. The pump laser 10 output is coupled to the EDF 7 using the WDM coupler 8.

Further spectral lines are produced¹ by phase modulation of the light with each recirculation through the loop. In other words, recirculation of modulated light through the phase modulator 3 increases the number and intensity of sidebands, producing a comb-like frequency spectrum.

The relatively long cavity featured in this type of the OFCG, or other types of OFCG with relatively long fibre delay lines, requires a high degree of matching of the reference laser frequency to the OFCG cavity mode frequencies. Improved tracking must be provided for this long cavity OFCG as detailed in the following paragraphs.

An optical signal in the loop is coupled to a photodetector 12 via a third ISO 14. The detected RF signal is amplified by a Low Noise Amplifier (LNA) 16 and input to a first input of a mixer 18. A second input of the mixer is connected to an amplified version of the RF reference signal, thereby providing the local oscillator (LO) input to the mixer. Thus, the mixer 18 is arranged to mix the loop output signal with the RF reference signal, the result of this mixing process being output from the mixer 18 as an intermediate frequency (IF) signal. It will be appreciated that the IF signal output by the mixer 18 is sensitive to the optical frequency difference between reference signal (provided to the mixer 18 as the LO signal) and the detected optical resonance of the cavity (provided to the mixer as the RF signal). The IF signal generated by the mixer 18, therefore, enables loop length control.

It is noted the amplified version of the RF reference signal is provided to the local oscillator (LO) input of the mixer via a phase shifter 24. This phase shifter 24 is arranged to be controlled by an externally applied phase shifter control (phase bias) signal which can be used to adjust the phase of the LO signal for maximum response in the homodyne detector.

The driving voltage of the PZT 4 is generated by a loop control circuit 20 using the IF signal, an output of the loop control circuit being applied to the PZT 4 via a High Voltage Amplifier (HVA) 22. For example, the loop control circuit 20 may be arranged to low pass filter the IF signal to produce a drive control signal for the PZT. In this way, the PZT 4 is arranged to stabilize the loop length, compensating for fibre length changes due to temperature fluctuations in a negative feedback manner.

Accordingly, this tracking loop compensates a range of effective fibre loop length variations over longer periods, which is critical for fibre loops of longer lengths. It is preferable that cavity length of the fibre is as short as is practically sensible. Reduction in the length of the cavity makes the fibre cavity less sensitive to the drift of the reference laser in the higher frequency noise regime. With shorter fibre loops (a few metres or less) the range of the variation will decrease (proportionately) and there is a possibility, in a strictly controlled environment, that the slow loop could be discarded or become less significant. This enables the use of small range devices such as a phase modulator or small range phase shifter.

Reducing the length of the fibre loop in this way poses difficulties in design, requiring the use of specially doped fibres. Furthermore, the reduction in the cavity length also means that the supported operation frequencies of the OFCG are coarsely populated. Therefore techniques to enable locking with a longer cavity and tolerating greater drift in the reference laser are still desired. For a long cavity, to compensate for the temperature changes, an optical delay device with large stroke is necessary, such as a PZT based fibre stretcher with large range. However, it is difficult for one device to provide both a large range and fast speed at the same time. Thus, an aspect of the invention employs two tracking loop, one accommodating the large range, namely the slow tracking, and one accommodating the need for speed, namely the fast tracking.

The DC-driving voltage of the optical phase modulator 3 is generated by the loop control circuit though the bias-T. Thus, the optical phase modulator 3, acting as a fast optical phase shifter, can be used to modify the optical frequency resonances of the fibre loop in order to maintain the frequency of one of the resonances on the reference laser frequency. In other words, the DC-bias of the optical phase modulator 3 may be used to correct the effective length of the fibre loop. The small ranged phase modulator is faster and more linear in phase response compared to a PZT. This helps perform the automatic locking to the reference laser.

As an alternative to using the DC bias of the phase modulator, a separate small ranged fast optical phase shifter or fast fibre > stretcher can be adopted, in cascade with the optical phase modulator, to provide the fast correction. This is illustrated in Figure. 2.

Reducing the length of the cavity may make the cavity less sensitive to the drift of the reference laser in the higher frequency noise regime. Thus, a short cavity length for the fibre ring may means that any relatively fast frequency drift in the master laser only requires a limited correction in the cavity length. This enables the use of small range, fast correction devices such as a phase modulator or special phase shifter. This can help to perform the automatic locking to the master laser.

Temperature changes in the cavity and longer term frequency drift in the reference laser require a large stroke correction system. This is provided by the slow tracking loop.

In other words, the combination of slow and fast tracking means that the optical phase modulator may compensate for minor fluctuations, whereas larger corrections are achieved through the slow tracking arrangement. In this example, the range of frequencies compensated by the slow control loop arrangement preferably range from DC (0 Hz) to 50 Hz, and more preferably DC (0 Hz) to 20 Hz. Because the slow control loop is being used to compensate for temperature variations and slow laser wavelength drift, the variations may not have fixed or defined period. However, the frequency of such variations can be simply approximated, wherein monotonic variations (those which consistently increase or decrease in value) are said be DC (0 Hz) variations.

For the fast control loop arrangement, the frequency range is preferably from 20 Hz to 1 KHz, and more preferably 20Hz to 250Hz. Variations within such ranges, referred to as jitter, may be problematic and should be compensated. Reduced fibre loop length can also help in this respect by reducing the phase adjustment necessary to compensate the jitter. The fast control loop is used to compensate for laser jitter that is not typically included in the specifications for narrow linewidth lasers. Typically, laser spectra are measured over approximately lms. Thus, any jitter occurring at frequencies less than 1 kHz does not appear in the linewidth, although they can occur with large frequency deviation.

It is important to note that the invention also compensates for low frequency and high frequency laser noise. Low frequency noise can be compensated for by the slow tacking arrangement, whereas higher frequency laser frequency deviations can be compensated for by the fast tracking arrangement.

Accordingly, the invention provides an additional fast tracking loop which improves the locking to reference laser sources with certain types of phase noise characteristics, such as fibre lasers. This results in a stable optical comb spectral output, low super- mode noise, and minimises the phase noise through better resonance matching.

It will be appreciated that very high temperature stability is desirable. For this reason, in the embodiment of Figure 1, all of the components of the optical fibre loop are enclosed in a thermally insulated environment to minimise temperature effects on the loop (particularly effective length changes). This box may employ active/passive temperature control to provide a temperature stable environment for the OCFG fibre loop.

Figure 3 illustrates a preferred embodiment of the invention in which the OFCG 200 is arranged such that the tracking is carried out through a fast correction loop, namely the optical phase modulator 3 (or a fast phase shifter as detailed in Figure 2) and a slow loop which feeds back the detected signal thermally by a TEC or heater placed inside the optical box, or to the adjustable optical delay line 9.

It is desirable that the phase modulator 3 (or the cascaded, independent fast phase shifter/fast fibre stretcher) has sufficient bandwidth and range to track the phase error effectively. For this reason, the optical phase modulator 3 is selected such that it is able to operate with large DC voltages, and the loop length is minimised so that the effective length changes remain within the controllable limits of the phase modulator 3. The slow but large error is further integrated out into a slow control signal, and fed into the adjustable delay line or to a thermal control loop in a feedback manner. Such a thermal control loop has many varieties, but the general idea is to' compensate most of the large length changes, therefore maintaining the DC bias of the phase modulator in a range close to zero. It is therefore necessary to use the slow error signal as the feedback signal instead of a temperature sensor. The details of the thermal control loop are apparent to the experienced engineers. For example, it can be operated with a deadband, in relay/heater mode or with a Thermal Electrical Cooler. Overall, the equivalent gain in the slow loop must be greater than in the phase modulator, but without overshooting too much: this will limit the residual DC bias drift on the phase modulator. In this arrangement, the slow loop has a much narrower bandwidth, typically less than 1 Hz.

The loop components of this preferred embodiment are similar to those used in the embodiment of Figure 1, with the adjustable optical delay line 9 being used to (manually or automatically) initially tune the microwave resonances of the loop prior to the locking. However, in the preferred embodiment of Figure 3, the slow large range PZT 4 has been removed. Consequently, it is preferable that the total loop length is minimised and, for example, a high gain per unit length EDF 7b is used to compensate optical loss.

It will also be appreciated that is preferable to reduce the lengths of fibres interconnecting the loop components. However, in this regard, it is noted that the fibre loop length defines the optical resonance frequencies, and therefore the permitted comb line frequencies. If high resolution is required in the definition of these frequencies

(i.e. a small step size between possible comb line frequencies), then decreasing the fibre loop length is contrary to such a requirement. The preferred implementation of an OFCG incorporating the principles of the invention will, therefore, vary according to practical requirements.

The embodiment of Figure 3 also comprises secondary thermal insulation (such as an outer housing) of the fibre loop and its constituent components. Further, alternative embodiments may provide for the removal of heat generating components from the thermally insulated loop (this includes removing the optical pump sources for optical amplifiers, RF amplifiers, and electronics, generally, from the thermally insulated enclosure).

Preferably, the secondary thermal insulation is temperature-controlled. This may provide some small temperature fluctuations with time, as the feedback loop corrects itself, but such changes will be very small. In such an arrangement, the inner thermal insulation box may be insulated to significantly dampen any temperature fluctuations from the outer housing. This avoids changes that may occur due to finite delays in feedback control. Of course, if the environment in which the OFCG is located is temperature-stabilised itself, the secondary thermal insulation may not be necessary.

As the range of the fast phase shifter or phase modulator is limited, very long term temperature drift may eventually result in the effective length change exceeding the tracking range. It is therefore desirable to integrate the tracking signal (e.g with time constant >10 s) so that excessive correction, which could result in the modulator operating near its working limits, is indicated. Such a provision is incorporated into the embodiment of Figure 3 through the inclusion of a slow integrator 11. In Figure 3, the integrated signal is provided to an additional temperature control circuit with dead band protection. Such a control circuit may then be used to adjust the control loop operating conditions, thus maintaining the long term stability of the comb generator.

For the configurations of Figure 1 and 3, the performance of the OFCG can be further improved by optimising the sequence of components in the loop, the coupling points for the insertion of the reference laser signal into the loop, and the extraction of the comb signal from the loop. This enables the removal of the interference between the central comb line and the reference laser line, thereby making the phase of the central comb line unambiguous. Furthermore, removal of the unwanted reference laser lightwave in its undesired polarisation prevents mixing with the comb and improves the polarisation and intensity stability of the central comb line.

In the illustrated embodiments, two optical couplers are used to minimise the destructive interference between the optical reference input and optical comb output. Such a configuration, with the output coupler being placed before the input coupler in particular, separates the output from the input optical signals, and avoids destructive interference between them. This improves the stability of the comb, and maintains the comb shape as the output no longer contains contamination from the input light. Here, "before" the input coupler, refers to the direction of the light circulating in the comb generator and with reference to the output of the optical amplifier; it does not necessarily mean that the two couplers are directly connected, although this is preferred. The insertion of other optical components, such as polarisation controller or delay lines, between the two couplers is also acceptable.

An aspect of the invention is a method to preserve a proper phase relationship between the RF and LO signals for the error detecting microwave circuit across a wide frequency tuning band using short optical/microwave lengths. This is done by optimization of the sequence of the components in the loop, and the use of delay lines to match the delay accordingly. In order to maintain an efficient phase relation for the error detection across the tuning band, the microwave phase shifter can be replaced by delay lines. Such delay lines can be either in the microwave or the optical path, and in the RF or the LO distribution within the error detection circuit, as shown in Figure 4. The delay line is used to enable wideband phase matching by matching the propagation delay of the RF and LO paths. This reduces the need for incorporating or adjusting the phase of the said microwave phase shifter, therefore allowing fast frequency adjustment of the OFCG without adjustment of the system. The RF path refers to the transmission from the splitting point of the power divider, to the phase modulator, then along the direction of the light propagation to the coupler where the error detection signal is taken, then to the error detection photodetector, all the way to the RF mixer. The LO path refers to the transmission from the power divider to the RF mixer. Therefore, when the frequency is being adjusted, the RF and LO signals see fixed delays in their transmission before reaching the mixer, and therefore their phase relationship is approximately maintained regardless of frequency.

Owning to the length of the fibre in the RF path, the delay matching method could be difficult to manage. This is where the aspect of the invention lies. The first invented method separates the two couplers. The coupler from which the error signal is taken (this can be the input coupler, or it can be tapped from the output coupler) is placed following the optical phase modulator. This keeps the RF and LO paths short.

The second invented method has the error signal coupler before the optical phase modulator. Utilizing the fact that the operating frequency is resonant in the fibre loop, a negative propagation concept holds for the remainder of the path in the loop. The propagation delay from the optical phase modulator to the said coupler along the direction of the light propagation is equivalent to a negative propagation delay from the optical phase modulator to the said coupler in the reverse direction. Therefore, the propagation delay matching is reduced to the matching in relatively short paths. The

LO path added to the path between the phase modulator and the said coupler in a reverse direction must equal the sum of the path from the power splitter to the optical phase modulator and the path from the optical coupler to the mixer. Fig. 3 details the related paths.

It should be pointed out that in order to make the path match, delay devices can be inserted into different positions in the setup for the same results.

For completeness, a description of the components that can be used to implement an OFCG according to embodiments of the invention will now be presented.

Optical amplifier

The optical amplifier 7 is used to compensate for the loss in the cavity (fibre loop), thus maintaining a strong resonance, and allows freedom in the choice of the coupling ratios for the optical coupler(s) to be used. In principle, any optical amplifier operating at the desired optical wavelength can be used. However, the Erbium-Doped Fibre Amplifier (EDFA) is favoured due to its wide bandwidth and spectral flatness. Gain ripples must be avoided to prevent self-oscillation ' in the OFCG which could easily occur when operating with close to unity loop gain (when trying to achieve wide comb bandwidths around a reference laser). Therefore, Erbium-Doped Waveguide Amplifiers (EDWAs) and Semiconductor Optical Amplifiers (SOAs) need to meet low gain ripple specifications in order to be used in the system.

Since its is preferable to minimise the optical length of the EDFA, special glass fibre amplifiers and heavily doped fibre amplifiers may be used to achieve high gain per unit length.

Optical Phase Modulator. RF Drive arid Fast Tracking Loop

The optical phase modulator 3 generates the sidebands as the light circulates in the fibre loop cavity. This transfers the RF reference frequency into the comb line spacing. It is always modulated by the source signal from the RF reference. ' As the modulation frequency is much higher than the required correction speed, the phase modulator drive signal may additionally comprise a DC signal resulting from the optical phase error detection . The phase modulator then acts as the device for the fast tracking.

As an alternative to using the DC bias of the phase modulator, a fast optical phase shifter or fibre stretcher can be used for the fast tracking. These devices have to be short range types, typically less than 1 mm in order to provide the speed required.

Fibre Stretcher, Optical Delay Line, Large range Optical Phase Shifter and the Slow Tracking Loop

A slow fibre stretcher/ phase shifter with large stroke or temperature controlled optical delay line 9 can be used to provide the slow correction for the tracking loop. These components need to be polarisation stable at their output if the OFCG is to provide the required spectral stability; this is due to presence of polarisation dependent components in the fibre loop (phase modulator and its associated polariser). A polarisation maintaining fibre stretcher/delay line can be adopted, but only if good polarisation alignment at its input can be achieved. The range required of these optical devices depends on the environmental temperature stability. Typically, a range of greater than 1 mm is required in order to provide sufficient range to accommodate the temperature drift.

Optical Fibre Couplers

The inclusion of the optical amplifier means that the coupling ratio of the coupler(s) no longer determines the strength of the resonance of the loop. One coupler or multiple couplers (a tree) can be used. The dual coupler configuration removes the destructive interference between the optical reference input and the optical comb output. The invented dual coupler configuration further separates the output from the input. This maintains the phase, polarisation and spectral stability of the optical comb output, as well as maintaining a stable power level at the central comb line Microwave Signal Synthesiser

It is preferable that an external or internal microwave reference used with the OFCG demonstrates low phase noise and high stability.

Reference Laser Source

A narrow linewidth laser (<2kHz), possibly locked to a known lightwave reference, is preferable. The drift of this reference laser frequency determines the tracking range required in order to maintain a stable OFCG output.

As is clear from the description above, there are a number of applications for an OFCG according to the invention. An OFCG locked to an ultra-stable reference laser source, is capable of generating frequency lines with high absolute and relative (frequency difference) accuracy. Such an OFCG is useful in many technology areas, including: coherent millimetre- wave/THz imaging and analysis; millimetre-wave/THz network analysis; remote delivery of millimetre-wave/THz reference signals over fibre; and in coherent optical communications (particularly with remote optical local oscillator delivery).

The use of an OFCG according to the invention in applications such as those mentioned above will now be described in more detail.

Figure 5 shows an OFCG according to an embodiment of the invention applied to a THz imaging system.

In the imaging system of Figure 5, a stable reference laser 310 defines one stable operating point of the system. This laser output and the output of a microwave source 312 is supplied to an OCFG 314 according to an embodiment of the invention. The OFCG 314 generates a number of frequency components separated by the 15GHz microwave frequency, and thus generates a coherent optical comb. There may be of the order of hundreds of these frequency components ("comb lines"), so that differences in frequency of the order of THz exist between pairs of the comb lines. A comb line selection unit 316 selects pairs of these comb lines to generate desired frequencies from them, using a photomixer 318, which in turn drives a transmitter 320. The comb line selection unit 316 thus combines comb lines together in a manner such that the difference frequencies generated are the frequencies needed for the imaging illumination and the local oscillator. Internal connections of the signals are by means of low loss optical fibres 317. In Figure 5, a first pair V₁ and V₂ is shown and a second pair v₃ and V₄.

A received signal at receiver 322 is supplied to an electrical mixer 324 which is used to down-convert the frequency to frequencies which can then be processed more easily. For this down conversion operation, the comb line selection unit is also used to generate a local oscillator signal (LO) by mixing of a pair of comb lines in photomixer 326. The millimetre/sub-millimetre wavelength signals can be detected by different photomixers, as shown.

Comb lines generated from the OFCG can be used to generate spectrally pure signals in the terahertz frequency band. A pair of comb lines is used to produce one such signal by their detection at a high speed photomixer. Some of the signals are used for transmission, with others being used as local oscillator (LO) signals for coherent reception of the former. Employing distribution by low loss optical fibre 317, many remote detectors can be powered by one source, working at the same or different millimetre- wavelength to THz frequencies. The noise correlation between all comb lines results in ultra-low phase noise signals being generated, and in low noise coherent reception.

Figure 6 shows an OFCG according to embodiment of the invention applied to the field of wireless communications. The comb lines to be used for the transmission of data have data modulated onto them by optical modulators (OM). An un-modulated comb line is sent along with the many modulated ones. At the transmit photomixer, these modulated comb lines are mixed with the unmodulated one and produce a set of electrical (millimetre/sub-millimetre-wave) signals. In the example shown, three comb lines V₁ v₂ and V₃ are combined with one local oscillator comb line VL_O to generate three THz output signals fi, f₂ and f₃. The chromatic dispersion between the reference lightwave Vw and the data-carrying comb line is converted as a fixed phase difference on the detected signals. Therefore, dispersion only affects the signal within the bandwidth of the data signals.

To assist further the detection of the uplink signal, a millimetre- wave LO signal can be also transmitted, and this is shown in the lower part of Figure 6.

Thus, the top and bottom parts of Figure 6 show different alternative transmission techniques. In the lower part of Figure 4, V₃' is not modulated - when optically heterodyned with V_LO it produces the millimetre-wave LO signal fu>

Reception is not shown in Figure 6. In the case where the local oscillator signal is transmitted, this reception would be in a similar manner to Figure 5.

Although particular embodiments of the invention have been described above, various modifications will be apparent to those skilled in the art.

Claims

1. An optical frequency comb generator for generating from first and second signal sources a plurality of signals with fixed frequency spacing derived from the second signal source frequency, the optical frequency comb generator comprising: an optical ring device comprising an optical phase modulator; and a control circuit comprising a slow control portion and a fast control portion, the control circuit being arranged to compare a signal generated within the optical ring device with a reference signal derived from the second signal source and to generate a control signal in response to the comparison, wherein: optical phase shifting components such as fibre stretchers, phase shifters and phase modulators, can be arranged to modulate the phase of a signal in the optical ring device in response to the control signal generated by the control circuit, the slow control portion is for compensating variations of a first frequency range in the generated plurality of signals; and the fast control portion is for compensating variations of a second higher frequency range in the generated plurality of signals. <

2. An optical frequency comb generator according to claim I₅ wherein the first frequency range may typically be within OHz - 20Hz and the second frequency range typically within 20Hz - 1 KHz., if the first range is corrected by a large stroke fibre stretcher and the second range is corrected by a fast phase shifter, phase modulator or wherein the first frequency range may typically be below IHz and the second frequency range up to 1 kHz, if the first range is corrected by temperature control and the second range corrected by a fast fibre stretcher (small range), phase shifter or phase modulator.

3. An optical frequency comb generator according to claim 1 or 2, further comprising an integrating unit arranged to integrate the control signal generated by the loop control circuit and to output the result of the integration.

4. An optical frequency comb generator according to claim 1, 2 or 3, wherein the optical ring is enclosed within a thermally insulated environment

5. An optical frequency comb generator according to any preceding claim, wherein the optical ring device comprises at least one optical coupler, a first input port being supplied with the first signal source output and a first output port providing the output of the comb generator, wherein a second input port and a second output port are coupled together by the optical ring, which comprises an optical amplifier, the optical phase modulator and an isolator.

6. An optical frequency comb generator in a ring configuration comprising two optical couplers, wherein the first, input coupler is dedicated to the input of the first signal source and the second, output coupler is dedicated to the output of the comb, in which the said output coupler precedes the said input coupler in the direction of the signal light transmission with reference to the output of the optical amplifier.

7. An optical frequency comb generator according to any of the preceding claims, wherein RF and LO phases are matched by using optical or microwave delay line(s), to give a high efficiency for the error detection, across a range of operation frequencies.

8. An optical frequency comb generator according to claim 7, wherein the coupler which supplies the optical signal for the error detection is placed after the optical phase modulator, and the RF and LO path delays are matched.

9. An optical frequency comb generator according to claim 7, wherein the coupler which supplies the optical signal for the error detection is placed before the optical phase modulator, and wherein the propagation delay of the RF path and LO path is not equal, but the delay difference between the two is equal to the round-trip delay of the ring cavity.

10. The optical frequency comb generator of claim 9, wherein the LO path delay plus the delay between the coupler which supplies the signal for the error detection and the phase modulator is substantially equal to the sum of the delays from the power splitter to the phase modulator and from the coupler to the mixer.

11. An optical frequency comb generator according to any preceding claim, wherein the optical ring device further comprises a pump laser source, and the fibre ring further comprises an adjustable optical delay line, a wavelength-division- multiplexing coupler, and a second isolator, wherein the pump laser source output is supplied to an input of the wavelength-division-multiplexing coupler.

12. An optical frequency comb generator according to any preceding claim wherein the optical ring is a fibre ring containing single mode fibre and/or polarization maintaining fibre.

13. An optical frequency comb generator according to any preceding claim wherein the first signal source comprises a laser.

14. An optical frequency comb generator according to any preceding claim wherein the second signal source comprises a microwave reference source.

15. An electromagnetic transmission and reception system comprising an optical frequency comb generator according to any preceding claim.

16. An imaging system comprising an optical frequency comb generator according to any of claims 1 to 14.

17. A method of generating from first and second signal sources a plurality of signals with fixed frequency spacing, the method comprising the steps of: comparing the generated plurality of signals with fixed frequency spacing with a reference signal derived from the second signal; generating a control signal in response to the comparison; modulating the phase of the plurality of signals in response to the generated control signal; compensating for variations of a first frequency range in the generated plurality of signals; and compensating for variations of a second higher frequency range in the generated plurality of signals.

18. A method as claimed in claim 16, wherein the first frequency range is typically within OHz - 20Hz and the second frequency range is typically within 20Hz - HCHz.

19. A method as claimed in claim 16 or 17, further comprising the step of integrating the generated control signal.

20. A method as claimed in claim 16, 17 or 18, used within an optical frequency comb generator.

21. A method as claimed in any of claims 16 to 19, wherein the fixed frequency spacing corresponds to a microwave frequency.