GB2250394A - Optical frequency synthesis - Google Patents

Abstract

Techniques for optical character generation and multiplexing schemes have been developed for using the potentially vast optical fibre bandwidth by utilising a single master laser from which the optical characters are simultaneously generated. A scheme is described which extends the idea of generating an optical comb of frequencies whereby individual optical channels are themselves synthesised from each frequency of the comb using an optical frequency translation loop. A system using both frequency and time division multiplexing schemes to realise a high capacity photonic MAN is described. <IMAGE>

Description

OPTICAL FREQUENCY SYNTHESIS The rapid advances in coherent lightwave technology have led to the development of sophisticated techniques for optical carrier generation which, together with suitable multiplexing schemes, will efficiently utilise the potentially vast optical fibre bandwidth. All these multichannel coherent optical schemes utilise a single master laser from which the optical carriers are simultaneously generated.

According to the present Invention there is provided an optical carrier generating system including a master laser, an optical phase modulator to generate a set of optical comb lines from the output of the laser and a carrier generation block for synthesing the transmitter and local oscillator frequencies from the comb lines, the carrier generation block comprising an optical control loop.

The present invention will now be described by way of example, with reference to the accompanying drawings, in which Figure 1 shows a diagrammatic representation of an ultrawideband optical lan; Figure 2 shows a diagrammatic representation of an optical phase-locked loop (OPLL) for use with the lan of Figure 1; Figures 3 to 9 show graphically the performance characteristics of the OPLL; Figure 10 shows a diagrammatic representation of a carrier generation block for use in the present invention; and Figure 11 shows a diagrammatic representation of an optical frequency locked loop (OFLL) for use with the present invention.

A scheme is described which extends the idea of generating an optical comb of frequencies whereby individual optical channels are themselves synthesised from each frequency of the comb by using an optical frequency translation loop. The potential for a high capacity FDM coherent network can then be visualised.

A system using both frequency and time division multiplexing schemes to realise a high capacity photonic MAN is described. The physical layer, shown in Figure 1, uses an optical phase modulator to generate a set of equally spaced optical comb lines from a narrow linewidth master laser. This optical comb is then distributed to each carrier generation block in the system.

The carrier generation block within the system is used to synthesise the transmitter and local oscillator optical frequencies from this optical frequency comb. This functional block should also effectively amplify the optical input comb line level and shift its frequency whilst still preserving its linewidth.

Two schemes based on using an optical control loop have been identified for the carrier generation block. The first scheme is based on an offset heterodyne optical phase-locked loop (OPLL) which phase locks a current controlled oscillator (CCO) laser to one of the incoming narrow linewidth comb lines offset by a predefined microwave frequency. The action of the loop is to shift the CCO laser optical frequency by that of the microwave offset and to reduce the phase noise of the CCO laser within the loop bandwidth. This allows the use of single-mode sources, such as DFB lasers with spectral llnewidths up to a few megahertz, to be used as the CCO element within the loop.

The second scheme uses a narrow linewidth CCO laser, based on an external cavity configuration and so, as no phase noise reduction is required, an optical frequency-locked loop (OFLL) can be used. This results in less stringent constructional requirements as loop propagation delay is no longer a significant problem.

Theoretical models and simulations are described for the OPLL together with practical requirements. Initial results are presented for the OFLL.

The FM-noise spectrum of a semiconductor laser consists of three main components. Typically, within the range of lMHz xfS 2GHz the Fourier frequency fluctuations show a white Gaussian behaviour and it is this which dominates the line shape and results in the familiar Lorentzian profile with a FWHM linewidth 6 f. This white noise is called Quantum FM noise as it is due to spontaneous emission and carrier density fluctuations within the laser cavity.

Above 2GHz, the spectrum exhibits a resonance peak at the laser relaxation oscillation frequency. This latter contribution is usually far above the system bandwidth and can be ignored.

Below a frequency of about 100KHz, a 1/f noise component in the FM-noise spectrum is dominant. The origin of this l/f noise, commonly called flicker noise, is due to laser temperature fluctuations and acoustic perturbations which are more intense at these low frequencies. The effect of this 1/f noise is to cause jitter of the laser spectrum in the frequency domain and, over a period of time, results in the drift of the centre frequency.

By considering these contributions to the laser linewidth it is possible to estimate the loop bandwidths required for each control loop. In the OPLL, the loop must reduce the phase noise of the CCO laser to that of the input comb line. The design specification for the comb line linewidth is 450kHz with a target specification of 100kHz. The free-running CCO linewidth lies between 10MHz and 60MHz. A substantial contribution to this linewidth will be the white noise part of the FM-spectrum.

Therefore, the loop bandwidth of the OPLL will have to be much greater than the CCO free-running linewidth to achleve not only line-narrowing within the loop bandwidth but also a low phase noise level outside the bandwidth.

Considering the OFLL, the extremely wideband nature of the laser frequency noise spectrum would require a very large closed loop bandwidth to reduce the overall frequency devIations. This is not possible with the small signal to noise ratios encountered In the system. However, reduction of the l/f portion of the frequency noise spectrum will stabilise the centre frequency. Therefore, a loop bandwidth of around 100kHz will be required to track-out the l/f contribution of the tumble CC0 laser.

This section considers a transfer function analysis to determine the spectral density of the phase noise fluctuations of the line-narrowed CC0 laser at the output of the.loop and then extends the analysis to evaluate the phase noise variance at the output of the demodulator as a function of a finite loop propagation delay.

The basic elements of the OPLL are shown in Figure 2.

The model for a loop with a propagation delay 1Kd is based on a second order system where a lag-lead filter of the form shown below is used as the loop filter;

where e~SHr d is the Laplace transform of a time shifty d and -rl,1z2 are the filter time constants.

The input signals to the OPLL are the input comb lines and the CC0 laser. As the frequency fluctuations of a semiconductor laser can be described as white Gaussian noise in the spectral regime 1MHz up to several GHz, this corresponds to the Lorentzian profile for the optical field with a FWHM linewidth S f. The one-sided spectral density of the phase noise fluctuations can be expressed as

The model for the phase detector is derived as follows.

An optical coupler is used to add the fields of the optical comb line and the CC0 laser. Balanced detection ing a pair of photodiodes is used to detect the combined optical fields and the resulting intermediate frequency (IF) is amplified and fed to the phase detector.

The magnitude of the IF signal (Vp) delivered to the phase detector is then

where Ra fw o} is the photodetector amplifier transimpedance, Kpf0} is the photodetector frequency response function,Rp is the photodetector responsivity, Plo is the local oscillator power, P5 is the reference signal power, Z o is the angular frequency, #s and 810 are the phase noise spectral density fluctuations of the reference (comb line) and CCO laser optical fields respectively.

Assuming a linearised loop model the output voltage of the phase detector is given by Vd = Kd tos below (4) where Kd is the phase detector gain given by the amplitude of (3).

The CCO laser is modelled as a simple integrator. This is because current modulation of a semiconductor laser results in direct frequency modulation of the output spectrum; hence the time variation of the output phase is given by s lo Ko V (5) where s is the Laplacian complex variable1 V is the controlling voltage to the local oscillator and Ko is the CCO laser tuning sensitivity.

The model assumes that the FM response of the CCO laser is flat. However, the FM response can exhibit a phase reversa. 4n its characteristic. This is caused by two effects. At low frequencies, the FM response is related to the change in the refractive index of the cavity as a result of cavity heating. This has been an associated thermal time constant rth such that

At high frequencies, the FM response is related to the changes in the carrier density in the laser cavity causing a refractive index change due to the free plasma effect.

The frequency change due to these two terms is opposite in sign hence the possibility of a phase reversal. At low frequencies, the laser FM response has a 180 phase shift and approaches 0 asymptotically at high frequencies. Equalisation of this characteristic can be achieved using passive or active filter networks. Alternatively the use of multi-segment DFB lasers with flat inherent FM responses can be used.

To determine the values for the transfer functions, the following standard relationships for the loop natural frequency ( n) and damping factors ( 9) are used.

The loop coefficient were chosen so as to obtain a damping factor of 0.707 and loop natural frequency of 250MHz (equivalent to a loop 3d8 bandwidth of 500MHz).

Considering the ideal OPLL with zero delay, the resulting optical phase noise spectra are shown in Figures 3 and 4. For frequencies well within the loop bandwidth, the phase noise spectral density of the CC0 laser approaches that of the narrow input comb line1 indicating successful tracking of the phase noise of the CC0 by the loop. As the frequency approaches the loop natural frequency, phase noise peaking of the output spectrum is observed. Outside the loop bandwidth the phase noise fluctations approach that of the free-running CC0 laser as the loop can no longer track this noise.

An order of magnitude reduction in the phase noise spectral density (PNSD) is obtained by reducing the free-running CC0 linewidth from 60MHz to 10MHz. A reduction in the reference comb line linewidth from 450Hz to 100kHz does not cause a significant decrease in the PNSD. Therefore, substantial output phase noise reduction will be best achieved by using a CC0 laser with a narrower free-running linewidth.

It has been reported that a strong dependency of the laser linewidth on the l/f noise of the noise frequency spectrum may exist. Hence, the white noise contribution will effectively correspond to a lower linewidth. If the contribution to the CCO laser linewidth is dominated by l/f noise then the loop will track this and replace it with the low frequency components of the reference.

The previous section considred ideal optical phase-locked loops where the propagation time for the phase error signal is considered negligible. In a practical system, a finite propagation will exist due to the finite bandwidth of the loop filter and the physical size of the optical and electrical components of the loop.

This section considers a non-negligible loop propagation delay and its effect on the performance of the OPLL.

The open-loop transfer function is given by

Substitution cf (1) into (11) gives

Using the Nyquist stability criteria, the condition for critical stability of the loop for a damping coefficient of 0.707 is given by nd < 0.768 (11) Therefore for a given loop delay, the maximum gain for critical loop stability is determined.

Using the transfer function model derived in section 3.1., a delay term has been implemented in the feedback loop of the system. These results are presented in Figures 5 and 6 and are described below.

Considering the stability criterion as derived for a lead-lag filter, Figure 5 shows the PNSD plot for a loop with 0.3ns delay (; n lCd = 0.471). An increase in the phase noise outside the loop bandwidth is observed. The Bode plot still indicates a stable loop however, the effect of the delay is quite dramatic in the phase response of the open-loop transfer function. As the loop delay is increased to 0.5ns (fi5 n7 d = 0.785, Figure 6), a strong resonance peak is observed in the spectral density plot. The Bode plot shows the loop reaching absolute as predicted by (13).

If the loop with the 0.suns delay is considered, then from (13) decreasing the loop natural frequency will enhance the overall stability at the expense of the phase noise reduction of the CC0 laser. This has been simulated for a loop gain one tenth of that used to derive Figure 6 and the results are shown in Figure 7.

The merits of implementing either a lag-lead filter (second - order loop) or a lag filter (modified first - order loop) in the loop have been compared and contrasted. Their conclusions show that due to the finite bardwidth of the loop amplifier reducing the phase margin of the system, it is preferable to use a modified first order loop when using lasers with linewidths greater than tens of kHz.

For a damping factor of 0.707, the maximum gain (km) is

where T1 is the filter time constant and frd is the total loop delay.

The system uses a DPSK modulation format with a gross bit rate of 300Mbit/s. The demodulator in the receiver is based on a different coherent principle and its output transfer function is given by T(s) s (1 - eST) (13) Hence the spectral density of the phase noise at the output of the demodulator (S0,demod) is given by SO,demod s 2S#,out (1 - cos(2# fT)] (14) The system performance is characterised in terms of the phase noise variance (PNV) which causes a certain penalty at a given bit error rate. For the system, a penalty of 0.5dB is specified at a bit error rate of 10-6 corresonding to a PNV of 0.042 at 300Mbit/s.

The PNV 2) 2) is evaluated by the integration, using an adaptive Simpson routine, of the spectral density at the output of the demodulator within the system bandwidth (B). Hence,

The phase noise variance is shown as a function of loop natural frequency in Figure 8 for the different linewidth parameters used in section 3.2. for a zero loop delay. A system bit rate of 300Mbit/s allows a 450kHz reference source to be used with a 1OMHt CC0 whilst meeting the PNV limit of 0.042.

The effect of a delay of 0.4ns is shown in Figure 9. The degradation in the PNV is noticeable for loops operating with low natural frequencies. At a natural frequency of 250MHz, the degradation is negligible for all the linewidth combinations considered. The PNV of 0.042 can still be achieved with a 1OMHz CCO laser and a 450kHz reference laser.

The conclusion from these results indicate that for an OPLL with a 500MHz loop bandwidth, it is possible to construct a stable loop with a 0.4ns loop delay using 1OMHz CC0 lasers. Any increase in this delay will degrade the performance significantly.

In order to construct a loop with a very short path length, it is necessary to consider the design of each component carefully. The key elements of the OPLL are the short path length isolator, its ancillary optics and the optical couplers.

Integrated optical technology could be used to fabricate the optical splltter and mtxer In order to achieve the minimum propagation delay through the couplers.

In considerating the layout of the waveguides, three main requirements have to be considered; minimum delay, minimum loss and compatibility with the input and output dimensions.

Commercial DFB lasers with 1OMHz free-running linewidths are not readily available at present and so the immediate demonstration of the OPLL is not possible. The recent commercial availability of a tunable narrow linewidth source means that it is possible to demonstrate the optical frequency - locked loop at an earlier stage and initial results are presented in the following section.

This section gives a detailed description of the carrier generation block and the acquisition procedure is outlined.

Preliminary results achieved for the OFLl using commercially available sources are discussed.

The carrier generation block is shown in Figure 10. The comb line reference input is combined with the output of the electronically controlled tunable CC0 laser and applied to the photodetector to produce a signal at the difference frequency between the two optical signals. This intermediate frequency is amplified and applied to a double balanced mixer together with a signal from the offset frequency generator. A bandpass filter (BPF) extracts the difference frequency. This signal is square law detected and fed to the frequency shifter controller for use in the acquisition scheme. The signal also feeds a delay line frequency discriminator. When the signal is within the frequency range for correct frequency discriminator operation and lock is desired, the discriminator output is connected to the amplifier by the loop controller.A sign inverter is set by the controller to give the correct tuning direction for upper or lower sideband mixing at the photodetector.

The tunable CCO laser is controlled by temperature and current control signals. The current control signal from the loop is monitored by the loop controller so that the temperature tuning can be used to ensure that the laser current remains within the range for acceptable operation.

Lock of the loop is confirmed by an auxiliary mixer, coupled to the discriminator output, which generates a low frequency signal when the loop is locked. This signal is processed by a low pass filter with a cut-off frequency equal to the maximum permissible frequency error and then detected. The presence of the detected signal is monitored by the loop controller and transmitted via the system Interface to the Access Control Unit of the system.

The centre frequency of the discriminator is chosen to be 350MHz, and the linear part of the characteristic gives a pull-in range of +140MHz. The IF input to the discriminator is filtered by bandpass filter with a 225-475MHz bandwidth. The allocation plan requires transmitter channels spaced by 1.2GHz and 2.4GHz. The intermediate frequency is 600MHz.

At the start of the acquisition procedure, the tunable CC0 laser frequency is set to below that of the lowest comb line. The offset frequency generator is set to a convenient value and the tunable laser frequency is ramped up using temperature and current tuning. An output is obtained from the detector connected to the discriminator input each time the difference frequency between a comb line and the laser output produces an IF. From the initial ramp, a look-up table of CC0 laser frequency against temperature/current inputs is formed.

Selection of a channel through the interface causes the offset frequency generator to be set and the CC0 laser frequency to be pretuned using the look-up table. The detector indicates that the controlled laser frequency is within the pull-in range of the loop and the loop is closed. Lock is confirmed by the auxiliary mixer and detector.

The experimental arrangement for the OFLL is shown in Figure 11. Commercially available tunable narrow line width lasers with a measured FWHM linewidth of around 100kHz were used as both the reference and the CC0 lasers.

Each laser has 40d8 of optical Isolation on its output and the optical signals were combined In a 3dB coupler via manual polarisation controllers. The quality of the optical spectrum was monitored using a Fabry-Perot interferometer. The resulting beat signal was detected, using a PIN photodetector and amplified by 60dB using three broadband amplifiers. The resulting signal was fed to the delay-line discriminator. The loop was closed through the loop filter and current modulator to the laser. The IF spectrum was monitored on a spectrum analyser, and the optical spectrum on a scanning Fabry-Perot interferometer (FSR g 2GHz).

Tuning of the laser was achieved by current and fine piezo tuning. The measured current tuning of the laser was 15MHz/mA.

The discriminator delay time was chosen to give a pull-in bandwidth of +140MHz. The loop filter time constant was chosen to give a loop bandwidth of 40kHz.

Successful locking was achieved for periods up to one hour. During these locked periods, offset locking was demonstrated for offset frequencies up to 1GHz thereby demonstrating the principle of the synthesis of optical channels.

The design concepts for the realisation of a wideband optical phase-locked loop have been discussed and the technological challenges outlined. The rapid developments in monolithic integration of lasers and photodetectors will present a relatively simple solution to many of the problems encountered. An interim solution will be to use narrow linewidth DFB lasers, with typically a few MHz linewidths, as they become available.

The use of the OPLL in the system is certainly more attractive than using an OFLL as the former gives an absolute frequency offset as well as the capability to use relatively broad linewidth sources as the CCO elements in the loop. However, the OFLL is a simpler system to construct, and provided the frequency error is small the penalty incurred will not be large.

The use of the frequency synthesiser has been discussed in the context of a system to demonstrate the dynamic tunability of both the transmitter and receiver sub-systems, thereby achieving a high capacity photonic MAN.

Claims (4)

1. An optical carrier generating system including a master laser, an optical phase modulator to generate a set of optical comb lines from the output of the laser and a carrier generation block for synthesing the transmitter and local oscillator frequencies from the comb lines, the carrier generation block comprising an optical control loop.

2. An optical carrier generating system as claimed in Claim 1, wherein the optical control loop is an offset heterodyne optical phase-locked loop (OPLL).

3. An optical carrier generating system as claimed in Claim 1, wherein the optical control loop is an optical frequency-locked loop (OFLL).

4. An optical carrier generating system substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.